Come and sit down for a minute. If somebody has told you that this is the chapter where physics suddenly turns difficult, please ignore them. Magnetic Effects of Electric Current is honestly one of the friendliest chapters in the whole Class 10 Science book, because nearly everything in it is something you can see, hear or feel. You have watched a fan blade spin, heard a doorbell buzz, and pushed an MCB back up after the lights went off at home. This chapter simply explains what was going on behind all of that.
Here is the single sentence that holds the entire chapter together: a moving charge produces a magnetic field, and a magnetic field pushes back on a moving charge. That is it. Every rule you will learn, every diagram you will draw and every time you are asked to hold your hand up in a slightly odd position — all of it is just a method for working out the direction of that field or that push. Once you genuinely believe those two ideas, the rest is only practice.
We will go slowly. I will build each idea up from zero, paint the picture in words, and then work through model answers exactly the way an examiner wants to read them — because in this chapter marks are usually lost not because a student did not know, but because the answer was missing one small line. So do not rush. Read a section, look away, try to re-tell it to yourself in your own words, and only then move on.
What You’ll Learn
- Magnets, Poles and the Idea of a Magnetic Field
- Magnetic Field Lines and Their Properties
- Magnetic Field Due to a Straight Current-Carrying Conductor
- The Right-Hand Thumb Rule
- Magnetic Field Due to a Circular Loop or Coil
- Magnetic Field Due to a Solenoid
- Electromagnets Compared With Bar Magnets
- Force on a Current-Carrying Conductor in a Magnetic Field
- Fleming’s Left-Hand Rule, Step by Step
- Electric Motor
- Electromagnetic Induction
- Electric Generator and Fleming’s Right-Hand Rule
- Direct Current and Alternating Current
- Domestic Electric Circuits
- Short Circuit, Overloading, Fuse, MCB and Earthing
- The Three Hand Rules Compared
- Activities You Should Be Able to Describe
- Quick Revision Sheet
- Practice Worksheet — 10 Questions
Your Game Plan
Here is the order I would work in if I were sitting beside you. Please do not jump straight to the worksheet — the questions there assume you have already read the sections.
- Get the picture first. Read the field lines and straight-conductor sections and actually sketch the diagrams on paper. Physics diagrams live in your hand, not on a screen.
- Drill the two hand rules until they are automatic. Right-Hand Thumb Rule for finding a field; Fleming’s Left-Hand Rule for finding a force. Do them out loud with your real hand, ten times, in different orientations.
- Do the conceptual comparisons. Solenoid versus bar magnet, AC versus DC, fuse versus MCB. Examiners love these because they are quick to mark.
- Learn the domestic wiring section like a checklist. Wire colours, 220 V, 50 Hz, the 5 A and 15 A circuits, earthing, short circuit, overloading. This is the most predictable scoring block in the whole chapter.
- Study the motor, induction and generator sections carefully. Practise the construction, working, energy conversion and the correct Fleming rule for each situation.
- Attempt all ten worksheet questions with the answers hidden. Write your answer down first, then reveal. Reading an answer feels like learning; it is not.
Magnets, Poles and the Idea of a Magnetic Field
Start with the thing you already know. A bar magnet has two ends that behave differently. Hang it freely from a thread and one end always swings round to point roughly towards the geographic north — we call that the north-seeking pole, or simply the north pole (N). The other end is the south pole (S). Bring two magnets close and you feel the famous rule in your fingers: like poles repel, unlike poles attract.
Now the important part, and it is the part students skip. That attraction and repulsion happens without the magnets touching. So something must be filling the space between them, carrying the influence across. That something is the magnetic field.
Why does it work like that? Think of how you feel warmth near a cooking fire. You do not have to touch the flame; the region around it is changed, and your skin detects that change. A magnetic field is the same sort of idea — the space around a magnet is genuinely altered, and anything magnetically sensitive placed in that space notices. A small compass needle is our detector. Place it anywhere near a magnet and it swings until it settles; the direction its north pole points is, by definition, the direction of the magnetic field at that point.
One more foundation stone. The Earth itself behaves like a gigantic weak magnet, which is why a compass works at all even with no other magnet nearby. That is worth remembering, because in a laboratory activity your compass will always be feeling the Earth’s field as well as whatever you are testing.
Model answer, marked up:
• The compass needle is itself a tiny magnet. It experiences a force from the magnetic field of the bar magnet at that point. (1 mark — the reason)
• It rotates until its north pole points along the direction of the resultant magnetic field at that point, and then stays there because the turning effect on it has become zero. (1 mark — the conclusion)
Tutor’s note: notice how the answer names the cause first and the result second. If you only write “it points north”, you have described one special case, not the physics, and you will be given at most half the marks.
Magnetic Field Lines and Their Properties
A field exists at every single point in space, which is impossible to draw. So we cheat, beautifully: we draw magnetic field lines. A field line is simply the path a free north pole would follow if it were released in the field — a line drawn so that the direction of the field at any point on it is along the tangent to the line at that point.
You can see them for real in two classic ways. Sprinkle iron filings on a sheet of paper resting on a bar magnet and tap the paper gently; each filing becomes a tiny magnet and lines up, and the whole pattern of curves appears in front of you. Or move a small plotting compass a step at a time around the magnet, marking the needle’s direction with pencil dots, then join the dots into a smooth curve.
1. Direction: outside a magnet they run from the north pole to the south pole; inside the magnet they run from south to north, so every field line is a closed continuous loop.
2. They never intersect. If two lines crossed, the compass at that crossing point would have to point in two directions at once, which is impossible.
3. Crowding shows strength. Where the lines are close together the field is strong (that is why they bunch near the poles); where they are far apart the field is weak.
4. Tangent gives direction: the direction of the field at any point is the tangent to the field line at that point.
Why does it work like that? Property 3 is not a rule someone invented — it falls out of the drawing convention. We agree to draw more lines through a region where the field is stronger. So “lines close together” and “field strong” are two ways of saying the same thing. And property 2 follows straight from the definition of a field line as the direction a compass points: one point, one direction, so one line.
Model answer, marked up:
• The direction of the magnetic field at a point is given by the tangent to the field line at that point. (1 mark — state the definition you are about to use)
• If two field lines crossed, there would be two tangents, and therefore two different directions of the magnetic field, at the same point. A compass needle placed there cannot point in two directions at once, so this is impossible. Hence field lines never cross. (1 mark — the contradiction and the conclusion)
Tutor’s note: this is a two-line answer worth two marks — among the best value-for-effort answers in the paper. Learn the structure: definition → contradiction → conclusion.
Model answer:
• The field is stronger at P. (1 mark)
• Justification: near the pole the field lines are crowded very close together, and the closeness of field lines represents the strength of the field. At Q the lines are widely spaced, so the field there is weak. (1 mark)
• Direction at P: draw the tangent to the field line passing through P; the direction of the field is along this tangent, in the sense shown by the arrowhead (outward, away from the north pole). Equivalently, place a small compass at P — the way its north pole points is the field direction. (1 mark)
Do not move past this section until the phrase “close lines mean strong field, tangent gives direction” feels completely comfortable. Everything that follows is built on it.
Magnetic Field Due to a Straight Current-Carrying Conductor
Before this chapter, revise electric circuits and their components if cells, switches and closed circuits feel rusty. After the notes, test the complete subject with the Class 10 Science practice paper for 2026–27, and use the Light: Reflection and Refraction notes for more diagram-direction practice.
This is the discovery that started everything. Set up a straight copper wire passing vertically through a horizontal sheet of card, connect it to a cell through a switch, and place a small compass on the card near the wire. With the switch open, the compass points north as usual. Close the switch, and the needle swings away. Open it again, and the needle returns. The current is producing a magnetic field.
Now sprinkle iron filings on the card, tap it, and look at the pattern. You do not get straight lines radiating outwards like spokes. You get concentric circles centred on the wire, tightly packed close to the wire and spreading further apart as you move outwards.
• The field is directly proportional to the current I — double the current, double the field strength at a given point.
• The field is inversely proportional to the distance r from the conductor — move twice as far away and the field strength halves.
So the field is strongest hugging the wire and dies away as you move outwards.
Why does it work like that? Ask yourself: is there anything special about one side of a straight wire compared with another? No — the wire looks identical from every direction around it. So the field pattern cannot favour any direction; it must be perfectly symmetric about the wire. Circles are the only shape with that symmetry. And since the same field is spread over a bigger circle as you move outwards, its strength must thin out with distance. The physics is doing exactly what the geometry demands.
Worked reasoning:
• (a) Field is directly proportional to current. The current has gone from 2 A to 6 A, a factor of 6 ÷ 2 = 3. So the field strength becomes three times its original value, that is 3B. (1 mark)
• (b) Field is inversely proportional to distance. Distance has gone from 4 cm to 8 cm, a factor of 8 ÷ 4 = 2. So the field strength becomes half, that is B ÷ 2. (1 mark)
• (c) The magnitude is unchanged, because neither the current value nor the distance has changed. But the direction of the field reverses — the concentric circles now carry arrowheads pointing the opposite way round. (1 mark)
Tutor’s note: part (c) is the one students rush. “No change” is wrong; “magnitude same, direction reversed” is right.
The Right-Hand Thumb Rule
We now know the field lines are circles. But which way round do the arrows go — clockwise or anticlockwise? For that we need a rule, and it is a lovely one because you carry the apparatus with you everywhere.
Two things to be strict about. First, it is the right hand — using the left hand here is the single most common self-inflicted error in this chapter. Second, it is conventional current (positive to negative outside the cell), not electron flow.
You will constantly need the two symbols used for “into the page” and “out of the page”. Picture an arrow flying away from you: you see its feathered tail, drawn as a cross (×), meaning into the page. Picture an arrow flying towards you: you see its point, drawn as a dot, meaning out of the page. Tail equals cross equals in; tip equals dot equals out.
Step-by-step application of the Right-Hand Thumb Rule:
• Step 1 — grip the wire. Hold your right hand as if gripping the vertical wire, with the thumb pointing straight up, matching the current direction.
• Step 2 — read the fingers. Now look down at your hand from above. Your fingers curl round in the anticlockwise sense.
• Step 3 — conclusion for the field. Seen from above, the circular field lines around the wire run anticlockwise. (1 mark)
• Step 4 — apply it at the given point. At a point due east of the wire, an anticlockwise circle is heading towards the north at that instant. So the compass needle’s north pole points north, that is, tangentially to the circle in the anticlockwise sense. (1 mark)
Tutor’s note: always finish by saying where you are standing (“seen from above”). Clockwise from above is anticlockwise from below — the same field, described from the other side. Examiners will not accept an unqualified “clockwise”.
Worked reasoning:
• Step 1. “Into the page” is drawn as a cross (×) at the wire’s position — the tail feathers of an arrow going away from you.
• Step 2. Point your right thumb away from you, into the page. Watch your fingers: they curl clockwise as you look at the back of your hand.
• Step 3. Therefore the field lines are concentric circles centred on the wire, with arrowheads running clockwise. (1 mark for circles, 1 mark for the correct sense)
• Step 4 — the detail that earns the diagram mark. Draw the innermost circles close together and the outer ones increasingly far apart, to show that the field weakens with distance.
Handy shortcut to memorise: current into the page → field clockwise; current out of the page → field anticlockwise. Check it with your own right hand now rather than trusting my word for it.
Magnetic Field Due to a Circular Loop or Coil
Take that straight wire and bend it round into a circle. What happens to the field? Think about it before reading on: at every point along the wire, the field lines are still circles wrapping around the wire itself. But now, because the wire has been curved, all those little wrapping circles are forced to pass through the middle of the loop in the same direction. They add up.
So near the wire the lines are still nearly circular, but as you approach the centre of the loop they straighten out and become almost a straight line perpendicular to the plane of the loop. At the very centre, the field is uniform and points straight through the loop.
• Current: field ∝ I. More current, stronger field.
• Radius: field ∝ 1/r. A smaller loop gives a stronger field at its centre, because every part of the wire is closer to the centre.
• Number of turns: for a coil of n closely wound turns, the field at the centre is n times that of a single turn, because each turn contributes its own field in the same direction and the fields add.
Why does it work like that? This is the “everyone pushing the same way” effect. Ten people shoving a stalled car all in the same direction move it ten times as effectively as one person. The turns of a coil are those ten people, and the field at the centre is the car. Note the wording carefully: the turns must be closely wound and carry the same current in the same sense, otherwise they would partly cancel rather than add.
For direction, you use the same Right-Hand Thumb Rule, applied to a small piece of the loop. Or, more conveniently for a coil: curl the fingers of your right hand in the direction the current circulates around the loop, and your outstretched thumb points along the field through the centre — that face of the loop behaves as a north pole.
Worked reasoning:
• (a) Curl the fingers of your right hand in the direction of the current — anticlockwise as seen from above. Your thumb then points upwards, out of the table towards you. So the magnetic field at the centre is directed vertically upwards, and the upper face of the coil behaves as the north pole. The lower face is the south pole. (2 marks)
• (b) The field at the centre is proportional to the number of turns. Going from 25 turns to 5 turns, the ratio is 5 ÷ 25 = 1/5. So the field at the centre becomes one-fifth of its earlier value. (1 mark)
Tutor’s note: “anticlockwise seen from above gives north on top” is worth committing to muscle memory, but always re-derive it with your hand in the exam — it takes three seconds and removes all doubt.
Magnetic Field Due to a Solenoid
Now stretch the idea one step further. Instead of one circular loop, wind many circular turns of insulated copper wire side by side into the shape of a cylinder, like the spring of a ballpoint pen. That is a solenoid.
Pass a current through it and something rather wonderful happens. Every turn produces its own field, and inside the cylinder all those fields line up and reinforce one another. The result is that inside the solenoid the field is uniform and parallel — the field lines are straight, evenly spaced and all pointing the same way, which means the field has the same strength and direction at every point inside. Outside, the lines swing round from one end of the cylinder to the other.
To find which end is north, use the right hand again: curl your fingers in the direction of the current in the windings, and your thumb points to the north pole of the solenoid. There is also a neat back-up: look at one end of the solenoid face-on. If the current appears to flow anticlockwise, that end is the North pole; if it appears clockwise, that end is the South pole. Remember it with the letters — you can draw the arrowheads of an aNticlockwise arrow onto the letter N, and a clockwise arrow onto the letter S.
The strength of a solenoid’s field depends on four things: the current through it, the number of turns per unit length (a tightly wound coil beats a loosely wound one), the length of the solenoid, and above all the material of the core placed inside it.
Worked reasoning:
• (a) Current appears clockwise at end B, so end B is the south pole. A solenoid, like any magnet, must have opposite poles at its two ends, so end A is the north pole. (1 mark)
• (b) The bar magnet’s north pole faces the solenoid’s south pole at B. Unlike poles attract, so the suspended magnet is attracted towards end B. (1 mark)
• (c) Any one of: insert a soft iron core inside the solenoid; increase the number of turns; wind the turns more closely together. (1 mark)
Tutor’s note: part (a) trips people up because they answer only for the end they were told about. Always state both poles — it costs you four extra words and secures the mark.
Electromagnets Compared With Bar Magnets
Slide a rod of soft iron inside a solenoid and switch the current on. The magnetic field becomes dramatically stronger — strong enough to lift things. That combination, a solenoid wound on a soft iron core, is an electromagnet.
Why does the core help? Soft iron is a ferromagnetic material. Placed in the solenoid’s field it becomes strongly magnetised itself, and its own magnetism adds to the solenoid’s. The word “soft” here has nothing to do with hardness you could dent with a fingernail — it means magnetically soft: iron magnetises very easily and, crucially, loses almost all its magnetism the instant the current is switched off. That controllability is the whole point.
You have already used electromagnets today without noticing. The scrap-yard crane that picks up a car and drops it on command; the electric bell in your school corridor; the relay inside a car’s starter circuit; the coil that pulls the door of an automatic gate. Each one needs magnetism that can be turned on and off — something a permanent magnet can never do.
| Point of comparison | Electromagnet (soft iron core) | Permanent bar magnet (steel) |
|---|---|---|
| Source of magnetism | Electric current flowing in the surrounding coil | Permanently aligned magnetism within the material itself |
| Can it be switched off? | Yes — open the switch and the magnetism practically vanishes | No — it is magnetic all the time |
| Strength | Very strong; can be increased by raising the current or adding turns | Comparatively weak and fixed at manufacture |
| Can the poles be reversed? | Yes — reverse the current direction and N and S swap over | No — the poles are fixed |
| Usual core material | Soft iron — magnetises easily, demagnetises easily | Steel — hard to magnetise, but retains magnetism |
| Typical use | Scrap-yard crane, electric bell, relay, loudspeaker, motor | Compass needle, fridge magnet, magnetic door catch |
Model answer:
• Increase the number of turns (say from 40 to 120). Each turn contributes its own magnetic field in the same direction, so more turns means a stronger resultant field inside the coil. (1 mark)
• Increase the current, for example by using two cells in series instead of one. The magnetic field of a coil is directly proportional to the current, so tripling the current roughly triples the field. (1 mark)
• Wind the turns closer together / use a thicker soft iron core. More turns per unit length raises the field, and a good ferromagnetic core becomes strongly magnetised itself and adds its magnetism to that of the coil. (1 mark)
What NOT to write: “use a bigger nail” on its own earns nothing — the examiner needs the physical reason attached to every suggestion. One sentence of reason per suggestion is the habit to build.
Force on a Current-Carrying Conductor in a Magnetic Field
So far, current has been making magnetic fields. Now we flip it round. If a magnetic field can push on a magnet, and a current-carrying wire behaves like a magnet, then a magnetic field should push on a current-carrying wire. It does — and you can see it move.
The demonstration is simple. Suspend a short length of stiff aluminium wire horizontally between the poles of a strong horseshoe magnet, so the wire lies across the gap. Connect it to a battery through a switch. The moment you close the switch, the wire jumps — it is pushed either up or down out of the gap. Reverse the current and it jumps the other way. Reverse the magnet instead and it also jumps the other way. Reverse both, and it goes back to the original direction.
When is the force maximum, and when is it zero? This is the part that separates a full-mark answer from a half-mark one, so let us be precise about it.
Why does it work like that? Here is the picture I find helpful. The current wraps its own circular field around the wire. When the wire sits across the magnet’s field, its circles add to the magnet’s field on one side of the wire and oppose it on the other. So one side becomes crowded with field lines and the other side becomes sparse. Field lines behave a bit like stretched elastic bands that want to shorten and that push each other apart sideways — so the crowded side pushes the wire towards the sparse side. That is the force. If instead the wire lies along the field, its own circles wrap symmetrically around the field direction, nothing gets crowded on one side more than the other, and there is nothing to push the wire anywhere. Force zero.
Fleming’s Left-Hand Rule, Step by Step
Three quantities here are mutually perpendicular — the field, the current and the force — and we happen to have three mutually perpendicular fingers. That is the entire idea behind the rule.
• Forefinger → direction of the magnetic Field (from N to S)
• Central (middle) finger → direction of the Current (conventional current)
• Thumb → direction of the Thrust, that is, the force or the resulting motion of the conductor
The memory hook that has rescued thousands of students: Fore–Field, Central–Current, Thumb–Thrust. Notice each pair starts with the same letters. And remember the overall pairing: left hand for the force on a wire; right hand for the field made by a wire.
Here is my method for using it under exam pressure without getting into a tangle. Set two, read the third. You will always be given two of the three directions. Line up those two fingers with what you are given — do it physically, turn your wrist, twist your arm, it does not matter how silly you look — and then simply read off where the third finger ends up pointing.
Step-by-step:
• Step 1 — identify what you are given. Field = into the page (drawn as crosses). Current = towards the right. Unknown = force.
• Step 2 — set the forefinger. Point the forefinger of your left hand away from you, straight into the page.
• Step 3 — set the central finger. Keeping the forefinger there, turn your wrist until the middle finger points to the right, matching the current.
• Step 4 — read the thumb. The thumb now points vertically upwards in the plane of the page.
• Conclusion: the force on the conductor is directed upwards, in the plane of the page and perpendicular to both the current and the field. (1 mark for the direction, 1 mark for correctly naming and applying Fleming’s Left-Hand Rule)
Useful cross-check: if either the current alone or the field alone is reversed, the force reverses to point downwards. If both are reversed, the force stays upwards.
Worked reasoning:
• Step 1 — get the field direction. Outside a magnet, field lines run from N to S. N is on the left and S on the right, so the field in the gap points from left to right.
• Step 2 — apply Fleming’s Left-Hand Rule. Forefinger points to the right (field). Middle finger points vertically downwards (current). The thumb then points out of the page, towards the observer.
• (a) The wire is pushed outwards, towards you — that is, horizontally out of the gap in the direction the observer is standing. (1 mark)
• (b) Interchanging the battery terminals reverses the current to vertically upwards. With the field unchanged, the force reverses: the wire is now pushed away from the observer, into the page. (1 mark)
• (c) Reversing the magnet as well reverses the field too. Two reversals cancel, so the force returns to its original direction — out of the page, towards the observer. (1 mark)
Tutor’s note: the “two reversals cancel” idea is examined again and again. Reverse one thing and the force flips; reverse both and it does not.
Worked reasoning:
• (a) Forefinger of the left hand → east (field). Middle finger → north (current). The thumb then points vertically downwards. So the rod is pushed downwards, towards the bench. (2 marks)
Check: if the current were reversed to flow towards the south, the force would be vertically upwards and the rod would tend to lift off the bench.
• (b) Now the current is towards the east and the field is also towards the east — the current is parallel to the magnetic field. In this situation the force on the conductor is zero, because the force is maximum only when the current is perpendicular to the field and falls to nothing when the two are along the same line. The rod stays exactly where it is. (1 mark)
Tutor’s note: in part (b) you cannot even set up Fleming’s Left-Hand Rule — the forefinger and the middle finger would have to point the same way, which is impossible for perpendicular fingers. That impossibility is the physics: no perpendicular direction exists, so no force exists.
1. Using the right hand for Fleming’s Left-Hand Rule — you get exactly the opposite answer, so a careless slip costs the whole question.
2. Taking the field direction as S to N. Outside a magnet, field lines go N to S. Get that wrong and everything after it is wrong.
3. Using the direction of electron flow instead of conventional current. Always use conventional current for the middle finger.
Electric Motor
An electric motor is a device that converts electrical energy into mechanical energy. It is the direct, practical child of the previous section: if a magnetic field pushes on a current-carrying wire, then arrange the wire cleverly and that push can be made to go round and round.
How the turning happens. Picture a rectangular coil of insulated wire, ABCD, placed between the poles of a magnet so that the field runs across it. Current enters at one side, travels along AB, round through BC and CD, and out again. Now here is the crucial observation: in arm AB the current flows one way, and in the opposite arm CD it flows the other way. Same field, opposite currents — so by Fleming’s Left-Hand Rule the two arms feel forces in opposite directions. One arm is pushed up while the other is pushed down. A pair of equal and opposite forces acting at different points is exactly what makes something rotate.
The problem, and the split-ring solution. After the coil has turned through half a revolution, arm AB now sits where CD used to be. If nothing changed, the force on it would now oppose the rotation and the coil would simply rock back and forth. The fix is a split ring commutator — a metal ring cut into two halves, each half connected to one end of the coil, pressing against fixed carbon brushes. At the exact moment the coil passes the vertical position, the two halves swap brushes, which reverses the direction of current in the coil. The force on each arm therefore keeps pushing the same way round, and rotation continues smoothly in one direction.
• Armature coil — the rotating coil that carries the current and experiences the forces; usually wound on a soft iron core to strengthen the field.
• Magnet (field magnet) — provides the uniform magnetic field.
• Split ring commutator — reverses the current in the coil every half rotation, so the coil keeps turning the same way.
• Brushes — fixed carbon contacts that pass current from the battery to the rotating commutator.
Commercial motors are made stronger by using many turns, a soft iron core and a powerful electromagnet instead of a permanent magnet.
Model answer:
• An electric motor converts electrical energy into mechanical (kinetic) energy. (1 mark)
• Function of the split ring: it acts as a commutator, reversing the direction of the current through the coil after every half rotation. Because the current reverses at the same moment the arms change sides, the force on each arm continues to act in the same rotational sense, so the coil goes on rotating continuously in one direction. (1 mark)
• With a continuous ring, the current in the coil would never reverse. After half a turn the forces would act so as to oppose further rotation, and the coil would merely oscillate back and forth about the vertical position instead of rotating steadily. (1 mark)
Electromagnetic Induction
We have seen that current makes magnetism. Now for the beautiful reverse question: can magnetism make current? The answer is yes — but only under one condition, and that condition is the whole idea.
Connect a coil of wire to a sensitive galvanometer, with no battery anywhere in the circuit. Hold a bar magnet still near the coil: the galvanometer reads zero. Now push the magnet quickly towards the coil: the needle kicks to one side. Hold it still inside: back to zero. Pull it out: the needle kicks the other way. Move it faster: a bigger kick. Keep the magnet still and move the coil instead: you get a deflection again.
Why does it work like that? Think of it as the mirror image of the force on a wire. When you push a conductor across a magnetic field, you are dragging its free electrons sideways through that field. A charge moving through a magnetic field feels a force — so the electrons are pushed along the wire, piling up at one end. That build-up is a potential difference, and if the circuit is closed, a current flows. No motion, no sideways push, no current. That is exactly why holding the magnet still gives you nothing.
You can induce a current without moving anything at all, too. Place a second coil near the first, and simply switch the current in the first coil on or off. At the instant of switching, the magnetic field through the second coil changes, and the galvanometer connected to it flicks. Leave the switch closed and steady, and the deflection dies away to nothing — because a steady current gives a steady field, and a steady field induces nothing.
Electric Generator and Fleming’s Right-Hand Rule
An electric generator (or dynamo) does the exact opposite job to a motor: it converts mechanical energy into electrical energy. Its construction looks almost identical to a motor — a coil, a magnet, brushes — but instead of feeding current in to get rotation, you supply rotation to get current out.
How it works. A rectangular coil ABCD is rotated mechanically between the poles of a magnet. As arm AB sweeps upwards through the field while arm CD sweeps downwards, each arm cuts across the field lines and an induced current is set up in it. Because the two arms move in opposite directions, the induced currents in them push the same way round the loop, and a current flows through the external circuit. Half a rotation later the arms have swapped over, so the induced current in the external circuit reverses direction. Current that periodically reverses like this is alternating current, and a generator with two continuous slip rings delivers exactly that — it is an AC generator. Swap the slip rings for a split ring commutator and the output is rectified into one direction only: a DC generator.
Worked reasoning:
• (a) Here the motion is causing the current, so we use Fleming’s Right-Hand Rule. (If a current had been supplied and we were asked for the resulting motion, it would be the Left-Hand Rule.) (1 mark)
• (b) Right hand: forefinger points east (field), thumb points vertically upwards (motion). The middle finger then points towards the north. So the induced current in the rod flows from its south end towards its north end. (1 mark)
• (c) Any one of: move the rod faster; use a stronger magnet; use a longer conductor within the field; if a coil is used, increase the number of turns. All of these increase the rate at which field lines are cut. (1 mark)
Direct Current and Alternating Current
Back to fully examinable ground — and this section is a reliable source of easy marks.
Direct current (DC) flows steadily in one direction only. Cells, batteries and the power bank in your bag all supply DC. Alternating current (AC) periodically reverses its direction, over and over. The electricity arriving at your home through the mains supply is AC.
Think of a corridor. DC is a queue of students all walking steadily towards the canteen. AC is the same students walking three steps forward, three steps back, three forward, three back — going nowhere on average, but every one of them is definitely moving, and that movement still delivers energy.
• Frequency f = 50 Hz means the current completes 50 full cycles every second.
• Time period T = 1/f = 1/50 = 0.02 s for one complete cycle.
• In one complete cycle the current reverses direction twice. So it changes direction 100 times per second, that is, once every 1/100 s = 0.01 s.
| Point of comparison | Direct Current (DC) | Alternating Current (AC) |
|---|---|---|
| Direction of flow | Always the same — it never reverses | Reverses periodically, again and again |
| Frequency | Zero — there are no cycles | 50 Hz in India (60 Hz in some other countries) |
| Usual source | Cell, battery, DC generator, solar panel | AC generator at a power station |
| Transmission over long distances | Wasteful — large energy loss as heat in the cables | Efficient — the voltage can be stepped up for transmission, so far less energy is lost |
| Practical range | Only a short distance without heavy loss | Can be carried economically over hundreds of kilometres |
| Where you meet it | Torch, mobile phone, remote control, inverter output side | Household mains sockets, fans, mixers, air conditioners |
Working, line by line:
• (a) Time period T = 1 ÷ f = 1 ÷ 50 = 0.02 s (that is, 1/50 of a second, or 20 milliseconds). (1 mark)
• (b) In one complete cycle the current reverses direction twice. So the interval between successive reversals is T ÷ 2 = 0.02 ÷ 2 = 0.01 s, that is 1/100 s. (1 mark)
• (c) 3 minutes = 3 × 60 = 180 s.
Number of complete cycles = f × t = 50 × 180 = 9000 cycles.
Number of reversals = 2 × 9000 = 18 000 reversals. (1 mark)
Tutor’s note: “changes direction every 1/100 s” is a stock board answer — but only if you can also say why it is half the period. Two reversals per cycle: one when the current swings from positive to negative, one when it swings back.
Working:
• (a) T = 1 ÷ f = 1 ÷ 60 = 0.0167 s (to 4 d.p.), that is 1/60 s. (1 mark)
• (b) Reversal interval = T ÷ 2 = 1 ÷ 120 = 0.0083 s (to 4 d.p.), that is 1/120 s. (1 mark)
• (c) Reversals per second at 60 Hz = 2 × 60 = 120.
Reversals per second at 50 Hz = 2 × 50 = 100.
Difference = 120 − 100 = 20 more reversals per second. (1 mark)
Tutor’s note: the higher the frequency, the shorter the period — they are reciprocals of one another, so as one grows the other shrinks. Sanity-check every such answer against that relationship before you write it down.
Domestic Electric Circuits
This is the section where physics walks straight into your own home, and it is the most predictable scoring block in the chapter. Learn it as a checklist and you will not lose a mark here.
How the supply reaches you. Electricity arrives at an Indian house through a pole or an underground cable as a 220 V, 50 Hz AC supply carried on two insulated wires — the live wire and the neutral wire. They pass first through the electricity board’s main fuse, then through the energy meter (the kWh meter you see near the door), and then to the house’s main switch and distribution box. A third wire, the earth wire, is connected locally to a metal plate buried deep in the ground near the house.
| Wire | Usual insulation colour | Its job in the circuit | What goes wrong if it fails or is missing |
|---|---|---|---|
| Live | Red, or brown | Carries the supply at high potential (about 220 V) into the appliance | This is the dangerous wire. Touching it gives a severe shock. If its insulation wears through and it touches the neutral, a short circuit occurs. |
| Neutral | Black, or blue | Completes the circuit, returning the current; it is at (or very near) zero potential | If it breaks, the circuit is incomplete and the appliance simply will not work, even though the live wire remains dangerous. |
| Earth | Green, or green with a yellow stripe | A safety wire connected to a metal plate buried in the ground; it is joined to the metal body of heavy appliances | Without it, if the live wire touches the metal body the whole casing becomes live and anyone touching it receives a dangerous shock. |
• a 5 A circuit for light-current appliances — bulbs, tube lights, ceiling fans, television;
• a 15 A circuit for heavy-current appliances — geyser, air conditioner, electric iron, water pump.
Within each circuit, all the appliances are connected in parallel, so that every one of them receives the full 220 V and each can be switched on or off independently of the others. The switch and the fuse are always placed in the live wire, so that switching off truly disconnects the dangerous side.
Why in parallel and not in series? Two solid reasons, and an examiner usually wants both. First, each appliance then gets the full supply voltage of 220 V, which is what it is designed for. Second, each appliance can be operated independently — if one bulb fuses, the rest of the house does not go dark, because the other branches are still complete.
Short Circuit, Overloading, Fuse, MCB and Earthing
Two different things go wrong in household wiring, and students mix them up constantly. Get the distinction crisp and this becomes free marks.
• Short circuit: the live wire and the neutral wire come into direct contact — usually because the insulation has worn away or a joint has failed. The current then bypasses the appliance and takes a path of almost no resistance. Since I = V/R and R has become tiny, the current becomes enormous, the wires heat violently and a fire can start.
• Overloading: the wires are intact, but too many appliances are switched on at once in one circuit (or the supply voltage rises accidentally). The total current drawn exceeds what the wiring is designed to carry, so the cables overheat.
The clue words: “insulation damaged / wires touching” points to a short circuit; “too many appliances on one socket” points to overloading.
The fuse. A fuse is a short piece of wire made of an alloy with a high resistance and a low melting point, sealed in a porcelain or glass holder and connected in series in the live wire. When the current exceeds the fuse’s rating, the heat produced (recall H = I2Rt from the previous chapter) melts the fuse wire, the circuit breaks and the rest of the wiring and the appliance are protected. A fuse is a deliberate weak link — you sacrifice a fifty-paisa wire to save a five-thousand-rupee appliance.
The MCB. A Miniature Circuit Breaker does the same job as a fuse but electromagnetically, and this is where our chapter joins up: the excessive current passes through a small coil, the coil’s magnetic field becomes strong enough to pull a latch, and a switch trips open. Nothing melts. Once the fault is fixed you simply push the switch back up, which is why almost every modern Indian home has MCBs in the distribution box rather than rewireable fuses.
Earthing. The earth wire is connected to the metal body of heavy appliances such as a geyser, a refrigerator or an electric iron, and its other end goes to a metal plate buried deep in moist ground. Normally no current flows in it at all. But if the live wire ever touches the metal casing, the earth wire offers a path of very low resistance straight to the ground, so a large current flows harmlessly away instead of through anyone who touches the appliance — and that surge usually blows the fuse or trips the MCB as well, cutting off the supply. Earthing keeps the potential of the metal body at zero.
Working:
• (a) Power P = V × I, so I = P ÷ V
I = 2000 ÷ 220 = 9.09 A (to 2 d.p.). (1 mark)
• (b) The fuse rating must be greater than the normal working current, or the fuse would melt during ordinary use; but it should be the smallest such rating, so that it still responds quickly to a genuine fault. Working current is 9.09 A, so 1 A, 3 A and 5 A are all too small. The smallest suitable rating is the 10 A fuse. (1 mark)
• (c) The 15 A power circuit. A 5 A circuit can carry at most P = V × I = 220 × 5 = 1100 W, and the geyser alone needs 2000 W, which is well beyond that. The 15 A circuit can carry 220 × 15 = 3300 W, comfortably more than 2000 W. (1 mark)
Tutor’s note: always write the formula, then substitute, then give the answer with its unit. Marks in numericals are awarded for those three separate steps, not just for the final figure.
Working:
• (a) Total power = 2000 + 1000 + 1200 = 4200 W
Total current I = P ÷ V = 4200 ÷ 220 = 19.09 A (to 2 d.p.). (1 mark)
• (b) The circuit is rated for only 15 A, which corresponds to a maximum safe power of 220 × 15 = 3300 W. The demand of 4200 W (19.09 A) is greater than this, so the circuit is indeed overloaded. (1 mark)
• (c) The fuse in the live wire of that circuit (or the MCB, in a modern installation) will operate. The fuse wire heats up because of the excessive current and melts, breaking the circuit; an MCB instead trips its switch open magnetically. Either way the supply is cut off before the household wiring can overheat and cause a fire. (1 mark)
Tutor’s note: notice the two equivalent ways to justify part (b) — compare currents (19.09 A against 15 A) or compare powers (4200 W against 3300 W). Show one clearly; do not do half of each.
Working:
• (a) Total power = 120 + (3 × 75) + (4 × 9) + 40
= 120 + 225 + 36 + 40 = 421 W
I = P ÷ V = 421 ÷ 220 = 1.91 A (to 2 d.p.), which is comfortably under the 5 A rating. (2 marks)
• (b) With the iron added, total power = 421 + 1000 = 1421 W
New current = 1421 ÷ 220 = 6.46 A (to 2 d.p.)
This exceeds the 5 A rating of the circuit, so it would be unsafe — the circuit would be overloaded and its fuse would blow. The iron is a heavy-current appliance and must be connected to the 15 A circuit instead. (1 mark)
Extra check for confidence: the maximum power a 5 A, 220 V circuit can carry is 220 × 5 = 1100 W. Of that, 421 W is already in use, leaving only 1100 − 421 = 679 W spare — nowhere near enough for a 1000 W iron.
The Three Hand Rules Compared
Three rules, three hands raised in three different shapes — no wonder students get them muddled. Here they are side by side. Read the middle column first, because “what it finds” is the question you will actually be answering in the exam.
| Rule | What it finds | What you must already know | Which hand and how to hold it | Typical question |
|---|---|---|---|---|
| Right-Hand Thumb Rule | Direction of the magnetic field produced by a current | Direction of the current in the conductor | Right hand. Grip the wire, thumb along the current; curled fingers give the field | “A wire carries current into the page. Sketch the field pattern.” |
| Fleming’s Left-Hand Rule | Direction of the force (thrust) on a current-carrying conductor | Direction of the field and of the current | Left hand. Forefinger = Field, Central finger = Current, Thumb = Thrust, all mutually perpendicular | “In which direction will the rod between the magnet poles move?” |
| Fleming’s Right-Hand Rule | Direction of the induced current in a moving conductor | Direction of the field and of the motion | Right hand. Forefinger = Field, Thumb = Motion, Central finger = Induced current | “A rod is pulled across a magnetic field. Which way does the induced current flow?” |
• Current → movement (a motor situation): LEFT hand.
• Movement → current (a generator situation): RIGHT hand.
• Only a current, and you want the field it makes: RIGHT hand thumb rule.
Memory line: the left hand does the pushing, the right hand does the producing.
(i) A vertical wire carries a current downwards; you must draw the magnetic field around it.
(ii) A wire carrying current lies across a magnetic field; you must predict which way it jumps.
(iii) A wire is pushed downwards through a magnetic field; you must find which way current flows in it.
Model answer:
• (i) Right-Hand Thumb Rule. Current is known and the field is wanted. Thumb of the right hand points downwards along the current; the curled fingers give the sense of the circular field lines. (1 mark)
• (ii) Fleming’s Left-Hand Rule. Field and current are known and the force is wanted. Forefinger along the field, central finger along the current, thumb gives the thrust. (1 mark)
• (iii) Fleming’s Right-Hand Rule. Field and motion are known and the induced current is wanted. Forefinger along the field, thumb along the motion, central finger gives the induced current. (1 mark)
Tutor’s note: a question that simply asks “which rule?” is testing exactly one thing — whether you can tell a cause from an effect. Slow down for two seconds and identify what is causing what.
Activities You Should Be Able to Describe
There are no listed board practical numericals for this chapter — the practical work here is about plotting field patterns and observing deflections. But descriptions of these activities appear regularly as three- and five-mark written questions, so learn to narrate each one properly: apparatus, what you do, what you observe, what you conclude.
Activity 1 — Plotting the field of a bar magnet. Place a bar magnet at the centre of a sheet of white paper and draw round its outline. Put a small plotting compass near the north pole and mark two pencil dots, one at each end of the needle. Move the compass so its tail sits on the dot the head just occupied, and mark the new head position. Repeat all the way round until you reach the south pole, then join the dots into a smooth curve and add an arrowhead pointing from N to S. Repeat from several starting points. Conclusion: the field lines emerge from the north pole, curve round and enter the south pole; they are crowded near the poles and never cross.
Activity 2 — Iron filings pattern. Rest a sheet of card on a bar magnet, sprinkle iron filings thinly and evenly over it, and tap the card gently. Observation: the filings arrange themselves into curved chains showing the field pattern, densest at the poles. Why: each filing is magnetised by the field and turns to lie along the local field direction, and the tapping frees them from friction so they can settle. Sprinkle too many and the pattern turns into a blob, so use a light hand.
Activity 3 — Deflection of a compass near a current-carrying wire. Fix a straight wire so it runs north–south above a compass, and connect it to a cell, a switch and a rheostat. Note the needle’s rest position with the switch open. Close the switch: the needle deflects. Reverse the connections at the cell: it deflects the opposite way. Increase the current with the rheostat: the deflection grows. Move the compass further from the wire: the deflection shrinks. Conclusion: a current produces a magnetic field whose direction depends on the current direction, whose strength grows with current and falls with distance.
Activity 4 — Force on a conductor. Suspend a stiff aluminium rod horizontally on two flexible connecting wires in the gap of a strong horseshoe magnet, and connect it through a switch to a battery. Close the switch and watch the rod swing. Reverse the current: it swings the other way. Turn the magnet round: it swings the other way again. Rotate the rod so it lies along the field: it does not move at all. Conclusion: a current-carrying conductor in a magnetic field experiences a force whose direction is given by Fleming’s Left-Hand Rule and which is zero when current and field are parallel.
Quick Revision Sheet
Read this the night before the exam, and again in the ten minutes before you walk in. If any line here makes you hesitate, go back and re-read that section — it is a signal, not a failure.
- Magnetic field — the region where a magnet’s influence is felt; a vector quantity; SI unit tesla (T).
- Field lines — N to S outside the magnet, S to N inside, always closed loops; never cross; crowded lines mean a strong field; direction is the tangent.
- Straight conductor — concentric circular field lines; field ∝ I, field ∝ 1/r.
- Right-Hand Thumb Rule — thumb along the current, curled fingers along the field. Current into the page gives a clockwise field; out of the page gives anticlockwise.
- Circular coil — field at the centre is perpendicular to the plane of the coil; ∝ I, ∝ n, and ∝ 1/r (a smaller coil gives a stronger central field).
- Solenoid — uniform field inside; outside behaves exactly like a bar magnet. Looking at one end, anticlockwise current means North, clockwise means South.
- Electromagnet — solenoid plus a soft iron core; strong, switchable, reversible. Soft iron because it magnetises strongly and demagnetises instantly.
- Force on a conductor — maximum when current is perpendicular to the field, zero when parallel to it; direction from Fleming’s Left-Hand Rule (Forefinger–Field, Central–Current, Thumb–Thrust).
- AC and DC — India: 220 V, 50 Hz; T = 1/50 = 0.02 s; direction reverses every 1/100 s, that is 100 times a second. AC’s advantage is efficient long-distance transmission.
- Domestic wiring — live (red or brown), neutral (black or blue), earth (green, or green with yellow); separate 5 A and 15 A circuits; appliances in parallel; switch and fuse always in the live wire.
- Faults and protection — short circuit means live touching neutral; overloading means too many appliances at once. A fuse melts; an MCB trips magnetically; earthing sends fault current safely to the ground.
- Official 2026–27 reading material: electric motor, electromagnetic induction, electric generator and Fleming’s Right-Hand Rule are included for current course support.
Practice Worksheet — 10 Questions
Here is the part that actually builds marks. Please do it properly: cover the answers, write your attempt in your notebook first, and only then click to reveal. Reading an answer feels like learning — it is not. Writing one is.
Q1. State any three properties of magnetic field lines. Explain clearly why two field lines can never intersect each other. (3 marks)
Any three properties:
(i) Outside a magnet, magnetic field lines run from the north pole to the south pole; inside the magnet they run from south to north, so every line is a closed continuous loop.
(ii) The direction of the magnetic field at any point is given by the tangent drawn to the field line at that point.
(iii) Field lines that are crowded close together represent a strong magnetic field, and lines that are widely spaced represent a weak field. That is why they bunch up near the poles.
(1½ marks — half a mark for each property)
Why they cannot intersect: if two field lines crossed at a point, two different tangents could be drawn at that point, meaning the magnetic field there would have two different directions at the same instant. A compass needle placed at that point cannot point in two directions at once, so this is impossible. Hence magnetic field lines never intersect. (1½ marks)
Q2. A long straight wire is stretched horizontally and carries a conventional current from west to east. A small plotting compass is placed first directly above the wire and then directly below it. In each position, state the direction in which the north pole of the compass needle points, and name the rule you used. (3 marks)
Rule used: the Right-Hand Thumb Rule — grip the wire in the right hand with the thumb pointing along the conventional current; the curled fingers then give the direction of the circular magnetic field lines. (1 mark)
Compass placed above the wire: point the right thumb towards the east. The fingers passing over the top of the wire are heading towards the south. So the field above the wire points south, and the needle’s north pole points towards the south. (1 mark)
Compass placed below the wire: the same circular field lines pass under the wire in the opposite sense, so the field there points north and the needle’s north pole points towards the north. (1 mark)
Sense check: the two answers must be opposite — a point above and a point below lie on opposite sides of the same circular field line, so the field there must run in opposite directions. If your two answers came out the same, you have made an error somewhere.
Q3. The mains supply in an Indian home is 220 V, 50 Hz alternating current. (a) What is meant by 50 Hz here? (b) Calculate the time period of this supply. (c) How many complete cycles does the current make, and how many times does it reverse its direction, in 0.5 s? (3 marks)
(a) A frequency of 50 Hz means the alternating current completes 50 complete cycles in every second. (1 mark)
(b) Time period T = 1 ÷ f
T = 1 ÷ 50 = 0.02 s (that is, 1/50 s or 20 ms). (1 mark)
(c) Number of complete cycles in 0.5 s = f × t = 50 × 0.5 = 25 cycles.
The current reverses direction twice in each cycle, so number of reversals = 2 × 25 = 50 reversals. (1 mark)
Extra point worth adding: since there are 100 reversals per second, the interval between successive reversals is 1/100 s = 0.01 s — a standard board-answer line.
Q4. A circular coil of 30 closely wound turns of insulated copper wire lies flat on a table and carries a current. Viewed from directly above, the current flows clockwise. (a) Which face of the coil acts as a north pole? (b) State how the magnetic field at the centre of the coil would change if the number of turns were doubled to 60 with the current unchanged. (c) State how it would change if a coil of the same 30 turns but half the radius were used. (3 marks)
(a) Curl the fingers of the right hand in the direction of the current — clockwise when seen from above. The extended thumb then points downwards, into the table. So the magnetic field at the centre is directed vertically downwards, which makes the lower face of the coil the north pole; the upper face is the south pole. (1 mark)
(b) The magnetic field at the centre of a coil is directly proportional to the number of turns, because each turn contributes its own field in the same direction and the fields add. Doubling the turns from 30 to 60 makes the field at the centre twice as strong. (1 mark)
(c) The field at the centre is inversely proportional to the radius of the coil. Halving the radius therefore doubles the field at the centre, because every part of the current-carrying wire is now closer to the centre. (1 mark)
Q5. (a) Draw in words the shape of the magnetic field inside and outside a current-carrying solenoid, and state one important way in which a solenoid differs from a bar magnet. (b) A soft iron rod is now inserted into the solenoid. Name the device formed and give two reasons why soft iron is preferred to steel for this core. (3 marks)
(a) Inside the solenoid the field lines are straight, parallel and evenly spaced along its axis, which means the field there is uniform — the same in magnitude and direction at every interior point. Outside, the field lines emerge from one end, curve round and enter the other end, giving a pattern identical to that of a bar magnet, with one end acting as a north pole and the other as a south pole. (1½ marks)
Key difference: a solenoid’s polarity and strength can be changed — reverse the current and the poles swap over; increase the current and the field gets stronger. A bar magnet’s poles and strength are fixed. (You could also say that a solenoid has a strong uniform field inside it, which a bar magnet cannot offer.)
(b) The device formed is an electromagnet. (½ mark)
Reasons for soft iron: (i) soft iron is magnetised very strongly by the field of the solenoid, so it makes the electromagnet far more powerful; (ii) soft iron loses almost all of its magnetism as soon as the current is switched off, so the electromagnet can be turned on and off at will. Steel would retain its magnetism and the device could not be switched off. (1 mark)
Q6. A straight horizontal rod carrying a conventional current towards the north is placed in a uniform magnetic field that points vertically downwards. (a) Find the direction of the force acting on the rod, naming the rule you use. (b) What happens to the force if the current is reversed to flow towards the south? (c) The rod is now turned so that the current flows vertically downwards, along the field. What force acts on it, and why? (4 marks)
(a) Use Fleming’s Left-Hand Rule: forefinger along the field, central finger along the current, thumb gives the thrust, all three mutually perpendicular. Point the forefinger of the left hand vertically downwards (field) and the central finger towards the north (current). The thumb then points towards the west. So the rod experiences a force directed towards the west. (2 marks)
(b) Reversing the current reverses the force while leaving its magnitude unchanged. The rod is now pushed towards the east. (1 mark)
(c) The force is zero. The force on a current-carrying conductor is maximum when the current is perpendicular to the magnetic field and falls to zero when the current is parallel (or antiparallel) to it. Here the current and the field both point vertically downwards, so they are parallel and no force acts. (1 mark)
Tutor’s note: in part (c) you physically cannot set up Fleming’s Left-Hand Rule, because the forefinger and central finger would have to point the same way. That impossibility is itself the clue that the force is zero.
Q7. A room heater is rated 2200 W, 220 V. (a) Calculate the current it draws. (b) Fuse wires of ratings 3 A, 5 A, 10 A and 15 A are available. Which should be fitted, and why is the next lower rating unsuitable? (c) Show by calculation whether this heater could safely be run on the 5 A lighting circuit. (4 marks)
(a) Power P = V × I, so I = P ÷ V
I = 2200 ÷ 220 = 10 A. (1 mark)
(b) A fuse must have a rating greater than the normal working current of the appliance. The working current is exactly 10 A, so a 10 A fuse would be running right at its limit and would melt during ordinary use, cutting off the heater for no good reason. The correct choice from the list is therefore the 15 A fuse. (The 3 A and 5 A fuses are far too small and would blow instantly.) (2 marks)
(c) Maximum power a 5 A, 220 V circuit can carry:
P = V × I = 220 × 5 = 1100 W
The heater alone needs 2200 W, which is double that limit, so it cannot be run on the 5 A lighting circuit. It must be connected to the 15 A power circuit, whose capacity is 220 × 15 = 3300 W. (1 mark)
Q8. Describe the domestic electric circuit of an Indian home. Name the three wires with their usual colours and state the function of each. Also explain (i) why appliances are connected in parallel, and (ii) why the fuse and the switch are placed in the live wire. (5 marks)
The supply: electricity reaches an Indian home as a 220 V, 50 Hz alternating supply through two insulated wires, the live and the neutral, passing through the main fuse, the energy meter and the main switch before reaching the distribution box. The house is then divided into separate circuits, typically a 5 A circuit for lights, fans and the television and a 15 A circuit for heavy appliances such as the geyser, air conditioner and iron. (1 mark)
The three wires: (2 marks)
• Live wire — red or brown insulation. Carries the supply at high potential (about 220 V) to the appliance. This is the dangerous wire.
• Neutral wire — black or blue insulation. Completes the circuit and returns the current; it is at nearly zero potential.
• Earth wire — green, or green with a yellow stripe. A safety wire connected to a metal plate buried in the ground and joined to the metal body of heavy appliances.
(i) Why parallel: so that every appliance receives the full 220 V for which it is designed, and so that each appliance can be switched on or off independently — if one bulb fuses, the others keep working because their branches are still complete. (1 mark)
(ii) Why in the live wire: if the fuse or the switch were placed in the neutral wire, then even after the fuse blew or the switch was opened, the appliance would still remain connected to the live wire at 220 V and would be dangerous to touch. Placing them in the live wire ensures the appliance is genuinely disconnected from the high-potential supply. (1 mark)
Q9. Distinguish clearly between a short circuit and overloading, giving one cause of each. Explain how earthing protects a person using a metal-bodied appliance, and state one advantage of an MCB over a rewireable fuse. (4 marks)
Short circuit: occurs when the live wire and the neutral wire come into direct contact, so the current bypasses the appliance and flows through a path of almost no resistance. Since I = V/R and R has become extremely small, the current becomes very large and the wires may overheat and catch fire. Cause: damaged or worn-out insulation on the wiring, or a faulty joint. (1 mark)
Overloading: occurs when too many appliances are switched on simultaneously on the same circuit, so the total current drawn exceeds the safe current-carrying capacity of the wiring. The wires then overheat even though nothing is actually touching. Cause: plugging several high-power appliances into one socket, or an accidental rise in the supply voltage. (1 mark)
How earthing protects: the earth wire connects the metal body of the appliance to a metal plate buried deep in moist ground. If the live wire accidentally touches the metal casing, the earth wire provides a path of very low resistance to the ground, so the large fault current flows harmlessly into the earth instead of through the body of anyone touching the appliance. The casing is thus kept at zero potential, and the surge of current also blows the fuse or trips the MCB, disconnecting the supply. (1 mark)
Advantage of an MCB: an MCB does not have to be replaced — once the fault has been cleared, the tripped switch is simply pushed back up to restore the supply, whereas a blown fuse wire must be replaced by hand. It also responds faster and more reliably at its rated current. (1 mark)
Q10. A rectangular coil PQRS is placed between the poles of a magnet and connected to a battery through a split ring commutator. Explain why the coil rotates continuously, and state what the split ring contributes. Name the energy conversion involved. (4 marks)
Why the coil rotates: when current flows round the coil, arm PQ carries current in one direction and the opposite arm RS carries it in the opposite direction, while both lie in the same magnetic field. By Fleming’s Left-Hand Rule the two arms therefore experience forces in opposite directions — one arm is pushed up while the other is pushed down. Two equal and opposite forces acting at different points produce a turning effect, so the coil rotates about its axis. (2 marks)
What the split ring contributes: after every half rotation the two halves of the split ring change contact from one brush to the other. This reverses the direction of the current in the coil at exactly the moment the arms swap sides, so the force on each arm continues to push in the same rotational sense and the coil keeps turning steadily in one direction. Without it — that is, with a continuous ring — the coil would only oscillate back and forth about the vertical position. (1 mark)
Energy conversion: electrical energy is converted into mechanical (kinetic) energy. (1 mark)
Scope check: CBSE’s official 2026–27 Class X Science reading material includes the electric motor, so learn this explanation as current course content.
One Last Word Before You Close This Page
If some of this still feels slippery, that is completely normal — magnetism is the first topic in your course where direction matters as much as magnitude, and directions take a little while to settle in the mind. The fix is not a longer study session tonight. The fix is a shorter one tomorrow, and again the day after.
So here is all I ask of you. Come back tomorrow and get one more question right than you managed today. Curl your right hand around an imaginary wire once more than you did yesterday. Add one more line to a model answer than you wrote last time. That is the whole method — small, honest, daily improvement, repeated until the day of the exam arrives and you find, quite calmly, that you already know this chapter. You have got this. Now go and try Q1 again without looking.