NCERT Class 10 Science Chapter 12 Activities — Magnetic Effects of Electric Current
NCERT Class 10 Science — Magnetic Effects of Electric Current, Activities 12.1 to 12.7 on pages 195 to 202.
Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.
The seven activities follow the chapter’s argument exactly, and in a strict order that is worth keeping:
- Activity 12.1 establishes the fact the whole chapter rests on — a current produces a magnetic effect.
- Activities 12.2 and 12.3 step back to magnets themselves, mapping field lines with iron filings and then with a compass, to establish what a field is before asking what a current does to one.
- Activities 12.4 and 12.5 return to the current-carrying wire and work out the shape, direction and strength of its field.
- Activity 12.6 changes the shape of the conductor from a straight wire to a coil.
- Activity 12.7 turns the whole thing around: if a current exerts a force on a magnet, a magnet must exert a force on a current.
Key insight. The chapter’s logic runs in one direction and then reverses. Activities 12.1 to 12.6 all ask what does a current do to a magnet? Activity 12.7 asks the opposite — and the answer, that the magnet pushes back on the wire, is the basis of every electric motor. Ampère’s argument for it was simply Newton’s third law, and it is worth saying so aloud before doing 12.7.
Activity 12.1
Place a straight thick copper wire between points X and Y in a circuit as in Fig. 12.1, with the wire perpendicular to the plane of the paper. Lay a small compass horizontally near the wire and note where its needle points. Now close the key and watch the needle.
What you should observe. The needle swings the moment the current flows, and returns when it stops.
The compass needle is deflected. In the chapter’s words, “the electric current through the copper wire has produced a magnetic effect”.
Why it is the first activity in the chapter. Everything else depends on this one fact, and it is genuinely surprising — there is no magnet anywhere in the apparatus, only a wire and a battery. A needle that moves when a switch is closed and returns when it opens rules out every explanation except the current itself.
What goes wrong. Too little current, or a compass too far from the wire. The effect is weak and needs a thick wire close to the needle. It is also worth turning the compass so the needle lies parallel to the wire before switching on, so any deflection is obvious against a known starting position.
Activity 12.2
Fix white paper to a drawing board, put a bar magnet at its centre, sprinkle iron filings uniformly around it — a salt sprinkler works well — and tap the board gently.
What you should observe. The filings arrange themselves into curves running from one pole to the other.
The filings line up along curved lines running from one pole to the other — the magnetic field lines. They are crowded near the poles and sparse further away.
Why the tapping matters. Friction holds each filing where it landed. A gentle tap frees them for a moment so they can turn into line with the field, and without it the pattern never appears. Tap several times, lightly — a hard knock scatters them.
What the crowding means. The chapter states the rule directly: “the relative strength of the magnetic field is shown by the degree of closeness of the field lines”. Where they bunch, the field is strong — which is why they bunch at the poles.
What goes wrong. Too many filings. A thick layer clumps into ridges and hides the pattern; a thin, even dusting shows it clearly. Use a sprinkler rather than pouring from the tin, and keep the filings off the magnet itself — they are very hard to get off again.
Activity 12.3
Put a bar magnet on paper on a drawing board and mark its outline. Place a small compass near the north pole and see how it behaves. Mark the positions of the compass needle’s two ends, move the compass so its south end sits where the north end was, and repeat — joining the marks as you go until you reach the south pole of the magnet. Repeat from other starting points.
What you should observe. The compass traces out the same curves the filings drew, and now they have a direction.
The lines run from the north pole to the south pole outside the magnet (and from south to north inside it, so field lines are closed curves). The south pole of the needle points towards the north pole of the magnet.
No two field lines ever cross.
Why do this after Activity 12.2 rather than instead of it? The filings show the shape of the field but not its direction — a filing is a tiny magnet with no arrow on it. The compass adds the direction, because it has a north pole that turns to point along the field. The two activities together give the complete picture, and neither does alone.
Why field lines cannot cross — the chapter’s own argument, and a good one. If two lines crossed, then at the point of intersection the compass needle would have to point in two directions at once, which it plainly cannot. That is a much better answer than “because they just do not”, and it is the kind of reasoning examination questions reward.
What goes wrong. Losing track of which end of the needle is which. Mark the north end with a dab of paint or a small sticker before starting; a traced line with the arrows the wrong way round is worse than no line.
Activity 12.4
Connect a long straight copper wire in series with two or three 1.5 V cells and a plug key, as in Fig. 12.5(a). Lay the wire parallel to and above a compass needle. Close the key and note which way the north pole of the needle moves. Then reverse the cell connections so the current flows the other way, and watch again.
What you should observe. Reversing the current reverses the deflection.
With the current flowing north to south, the north pole of the needle moves towards the east. Reverse the current and the needle deflects the opposite way — so the direction of the magnetic field depends on the direction of the current.
Why the reversal is the whole activity. The first half only repeats Activity 12.1. What is new is that the field has a direction tied to the current’s direction — which is what makes a right-hand rule possible at all. Do not skip the second half to save time; it is the half that matters.
What goes wrong. Reversing the cells but leaving the compass where it was nudged during the first run, so the two deflections are measured from different starting points. Let the needle settle before each reading.
Activity 12.5
Take a 12 V battery, a variable resistance or rheostat, an ammeter (0–5 A), a plug key, connecting wires and a long straight thick copper wire. Push the thick wire vertically through the centre of a fixed horizontal cardboard sheet. Sprinkle iron filings on the cardboard, pass a large current, and tap the sheet gently. Then vary the current, and try a compass at different distances.
What you should observe. The filings form rings, not the bar-magnet pattern of Activity 12.2.
The field lines around a straight current-carrying wire are concentric circles centred on the wire. They are crowded close to the wire and further apart with distance.
- Increase the current → the deflection increases: the field is stronger.
- Move the compass further away → the deflection decreases: the field is weaker.
Why circles and not a north and a south pole. There is no pole anywhere. The field simply encircles the current, which is why the right-hand thumb rule works the way it does — thumb along the current, curled fingers along the field.
Why the two variations are worth doing separately. They establish the two dependences the chapter needs: field strength grows with current, and falls with distance. Both are stated in as many words on page 199, and both are examination staples.
What goes wrong. Not enough current. This activity needs amperes, not milliamperes, which is why the chapter specifies a 12 V battery and a rheostat — the rheostat is there to let you push the current up safely and bring it back down. Do not leave a large current flowing: the wire gets hot.
Activity 12.6
Push a circular coil of many turns through two holes in a rectangular cardboard sheet, normal to its plane. Connect the coil in series with a battery, a key and a rheostat, as in Fig. 12.9. Sprinkle iron filings uniformly on the cardboard, close the key, and tap gently.
What you should observe. Where the wire passes through the card, the circles are there again — but now they add up.
Each turn produces its own concentric circles, and at the centre of the coil the arcs from every turn are in the same direction, so the field there is nearly straight and much stronger.
With n turns, the field is n times that of a single turn.
Why the turns add. The chapter’s reasoning is simple and worth reproducing: the current in each turn has the same direction, so the field due to each turn points the same way at the centre and the contributions simply sum. That is why a coil is the practical way to make a strong field from a modest current — and why an electromagnet is wound rather than straight.
What goes wrong. Too few turns to see the effect, or a coil whose turns are spread out rather than bunched. The pattern at the centre only looks straight if the turns are close together.
Activity 12.7
Suspend a small aluminium rod AB, about 5 cm long, horizontally from a stand using two connecting wires. Place a strong horse-shoe magnet so that the rod lies between its poles with the field directed upwards. Connect the rod in series with a battery and a key. Close the key and watch the rod. Then reverse the current and watch again.
What you should observe. The rod moves — pushed by nothing visibly touching it.
The rod is displaced, moving perpendicular to both the current and the field. Reverse the current and it moves the other way. The displacement is largest when the current is at right angles to the field.
Fleming’s left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. Forefinger = Field, Centre finger = Current, Thumb = Thrust (force).
Why this activity reverses the chapter’s question. Everything up to here showed a current affecting a magnet. Ampère’s argument was that the magnet must therefore push back on the wire with an equal and opposite force — Newton’s third law — and this activity is that prediction tested. It works, and the consequence is the electric motor.
Why “at right angles” matters. The force is greatest when current and field are perpendicular, and there is no force at all when they are parallel. That is why the rod is set between the poles with the field directed upwards and the current running along the rod — the geometry is chosen to make the force as large as it can be.
What goes wrong. A rod suspended too stiffly. The force is small, so the rod must hang freely on flexible wires; thick or springy leads hold it in place and nothing appears to happen. A light aluminium rod and thin flexible connections are both specified for this reason. Use a genuinely strong magnet, and keep the current on only briefly.
Common mistakes
- Expecting a bar-magnet pattern around a straight wire. It is concentric circles with no poles at all — Activity 12.5 against Activity 12.2 is the contrast that makes this stick.
- Drawing field lines that cross. They never do, and the reason is worth knowing: a compass at the crossing point would have to point two ways at once.
- Forgetting the field lines inside the magnet. They run from south to north inside, which is what makes them closed curves rather than lines that begin and end at the poles.
- Skipping the reversal in Activities 12.4 and 12.7. The reversal is the result. Without it, neither activity shows that direction depends on the direction of the current.
- Using a milliampere current in Activity 12.5. The pattern needs amperes. A weak current gives a faint, ambiguous set of rings and looks like a failed experiment.
- Confusing Fleming’s left-hand rule with the right-hand one. Left hand for the force on a current-carrying conductor in a field, which is what Activity 12.7 is about. The right-hand thumb rule is for the direction of the field around a current, from Activity 12.5.
- Pouring iron filings too thickly in Activities 12.2, 12.5 and 12.6. A thin, even dusting and a few gentle taps is the whole technique, and it is the difference between a clear pattern and a heap.
Practise next
- The end-of-chapter Exercises for this chapter, where the field patterns of Activities 12.5 and 12.6 return as questions on solenoids, and Activity 12.7 as questions on the electric motor and Fleming’s left-hand rule.
- The in-text questions of this chapter, thirteen across five boxes, which check the field-line rules and the right-hand thumb rule before the Exercises do.
- The activities of Chapter 11, where the current used throughout this chapter is set up and measured, and where the series and parallel circuits behind these apparatus diagrams are explained.
- Chapter 11, Electricity, for the heating effects of the same current whose magnetic effects are the subject here.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.