NCERT Solutions Class 10 Science Chapter 12 In-text Questions — Magnetic Effects of Electric Current
NCERT Class 10 Science — Magnetic Effects of Electric Current, the five in-text QUESTIONS boxes on pages 196, 200, 201, 203 and 205. All 13 questions solved.
Also here. Solutions to the end-of-chapter questions for this chapter and the activities are on their own pages.
Chapter 12 checks in five times, and the boxes track the chapter’s argument exactly: first that a magnet has a field, then what its field lines mean, then the field a current makes, then the force a field puts on a current, and finally the safety of the mains circuit. Three rules carry nearly all of it:
- Right-hand thumb rule — hold the conductor in the right hand with the thumb along the current, and the curled fingers give the direction of the field lines around it.
- Fleming’s left-hand rule — forefinger along the field, middle finger along the current, and the thumb gives the force on the conductor.
- A magnetic force is always perpendicular to the velocity — so it can change the direction of a moving charge, never its speed.
Key insight. Direction questions in this chapter are never guesswork, because each rule names which hand and which three quantities. The one habit worth building is to write down what you are given and what you want in the rule’s own order — field, current, force — before you twist your hand. Both of the multiple-choice questions in these boxes fall out immediately once that is done.
Question, page 196. The chapter’s first box, holding a single question — which is why its banner reads QUESTION rather than QUESTIONS.
Page 196, Question 1
Why does a compass needle get deflected when brought near a bar magnet?
Solution. The compass needle is not a passive pointer: it is itself a tiny bar magnet, free to turn, with its own north and south poles. That is the fact the question turns on.
A bar magnet has a magnetic field in the region around it. When the needle is brought into that field, the field pulls on the needle’s two poles in opposite directions — attracting its south pole towards the magnet’s north pole and repelling its north pole — and that pair of opposite forces twists the needle round. It settles pointing along the field line through its position.
Because a compass needle is itself a small magnet. The bar magnet’s magnetic field exerts forces on the needle’s north and south poles in opposite directions, and that turning effect rotates the needle until it lies along the field line at that point.
Questions, page 200. After Section 12.1, on magnetic field lines.
Page 200, Question 1
Draw magnetic field lines around a bar magnet.
Solution. The lines have to show three things to be right: that they run out of the north pole and into the south pole outside the magnet, that each one is a closed curve continuing from south to north inside the magnet, and that they crowd together near the poles where the field is strongest.
The field lines are drawn above: outside the magnet they run from the north pole to the south pole, inside it they continue from south to north, so that every line is a closed curve. They are closest together at the poles, where the field is strongest, and no two of them cross.
Page 200, Question 2
List the properties of magnetic field lines.
Solution. Each property is there to say something about the field itself, so it is worth pairing them up rather than memorising a list.
- A field line is drawn so that the tangent to it at any point gives the direction of the magnetic field at that point — the direction a compass needle placed there would point.
- Outside a magnet the lines run from the north pole to the south pole; inside, from south to north. So every field line is a closed, continuous curve with no beginning and no end.
- The closer together the lines, the stronger the field. They are most crowded at the poles.
- No two field lines ever intersect.
Page 200, Question 3
Why don’t two magnetic field lines intersect each other?
Solution. Suppose, for a moment, that two field lines did cross. At the crossing point each line would have its own tangent, and the tangent gives the direction of the field. So the field at that one point would have two different directions at once.
That is impossible: a compass needle placed at the point has to settle in one direction, not two. The contradiction is the proof — the lines cannot cross.
Because the field can only have one direction at any one point. If two lines crossed, the tangents to the two lines at the crossing point would give two different directions for the field there, and a compass needle placed at that point would have to point two ways at once — which is impossible.
Questions, pages 201 and 202. After Section 12.2.4, on the field of a circular loop and of a solenoid. This box runs across the page break — its third question is printed on page 202.
Page 201, Question 1
Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Solution. Take the loop flat on the table and look down on it from above, which is how the current is described as clockwise.
Apply the right-hand rule to the loop: curl the fingers of the right hand along the direction of the current — clockwise as seen from above — and the extended thumb gives the direction of the field through the loop. With the fingers curling clockwise from above, the thumb points downwards, into the table.
Every field line is closed, so what goes down through the inside of the loop must come back up somewhere. It returns outside the loop, so there the field points upwards, out of the table.
Inside the loop the field is directed vertically downwards, into the table; outside the loop it is directed upwards, out of the table. (Reverse the current and both directions reverse.)
Page 201, Question 2
The magnetic field in a given region is uniform. Draw a diagram to represent it.
Solution. Uniform means the field has the same magnitude and the same direction at every point of the region. Both parts have to show in the drawing: same direction means the lines are parallel, and same magnitude means they are equally spaced, since crowding would mean a stronger field somewhere.
A uniform field is drawn as parallel straight lines, equally spaced, all with arrows in the same direction — as above. The parallel lines show that the direction is the same everywhere, and the equal spacing that the strength is the same everywhere. The field inside a long current-carrying solenoid is the standard example.
Page 202, Question 3
Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current
Solution. The chapter answers this directly with Fig. 12.10: inside a current-carrying solenoid the field lines are parallel straight lines. By the previous question, parallel equally spaced lines are exactly what a uniform field looks like — so the field has the same magnitude and direction at every point inside.
Option (a) is wrong because the field inside is in fact strong — it is what magnetises a soft iron core into an electromagnet. Options (b) and (c) contradict the parallel straight lines shown.
(d) is the same at all points. The field lines inside a long solenoid are parallel straight lines, so the field there is uniform.
Questions, pages 203 and 204. After Section 12.3, on the force on a current-carrying conductor and Fleming’s left-hand rule. This box also runs across a page break.
Page 203, Question 1
Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
Solution. The whole question rests on one property of the magnetic force: it always acts at right angles to the velocity, as Fleming’s left-hand rule shows — the thumb is perpendicular to the middle finger.
A force perpendicular to the motion does no work on the particle, so its kinetic energy, and therefore its speed, cannot change. Its mass is not something a magnetic field can alter at all. But the force does change the direction of motion, and velocity is speed together with direction, so the velocity changes. Momentum is mass times velocity, so it changes too — in direction, though not in magnitude.
(c) velocity and (d) momentum. The magnetic force is always perpendicular to the velocity, so it changes the direction of motion but not the speed; mass and speed are therefore unchanged, while velocity and momentum change direction.
Page 204, Question 2
In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
Solution. In that activity the aluminium rod AB hangs in the field of a horse-shoe magnet and is pushed sideways when a current passes through it. The displacement is a measure of the force on the rod, and that force grows with each of the three quantities named.
(i) More current means more charge moving through the field each second, so the force is larger and the rod is displaced further.
(ii) A stronger magnet means a stronger field acting on the same current, so again a larger force and a larger displacement.
(iii) A longer rod means a greater length of current-carrying conductor lying inside the field, so more of it is being pushed — a larger force, and a larger displacement.
The displacement increases in all three cases. The force on a current-carrying conductor in a magnetic field grows with the current, with the strength of the field, and with the length of the conductor in the field — so (i), (ii) and (iii) each make the rod move further.
Page 204, Question 3
A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
Solution. Set the rule up carefully. The particle is positively charged, so the direction of conventional current is the same as its direction of motion: towards the west. The deflection tells us the force: towards the north. What is wanted is the field.
Fleming’s left-hand rule links the three: forefinger along the field, middle finger along the current, thumb along the force. Point the middle finger west and the thumb north, and the forefinger — held perpendicular to both — points vertically upward.
The same conclusion follows from the vector form without any twisting of fingers. Take east as $+x$, north as $+y$ and up as $+z$. Then the velocity is $-\hat{x}$, and
$$\vec{F} = q\,\vec{v} \times \vec{B} = q\,(-\hat{x}) \times (B_x\hat{x} + B_y\hat{y} + B_z\hat{z}) = q\,(B_z\hat{y} – B_y\hat{z})$$
For the force to point north, along $+\hat{y}$, we need $B_z$ positive — that is, the field points upward.
(d) upward. With the current (a positive charge moving) towards the west and the force towards the north, Fleming’s left-hand rule puts the field vertically upward.
Questions, page 205. The last box, after Section 12.4 on domestic electric circuits.
Page 205, Question 1
Name two safety measures commonly used in electric circuits and appliances.
Solution. The two standard measures guard against different dangers, which is why both are needed.
An electric fuse protects the circuit and the appliances. It is a short piece of wire of known melting point, placed in series with the live wire; if the current rises above the rated value, Joule heating melts it and the circuit breaks before anything else is damaged.
Earthing protects the person. The metallic body of an appliance is connected by the earth wire to a metal plate buried in the ground. If the live wire touches the body, the leaking current runs to earth along that low-resistance path instead of through anyone who touches the appliance.
(i) An electric fuse of proper rating, connected in series with the live wire, which melts and breaks the circuit if the current exceeds the safe value. (ii) Earthing, in which the metallic body of an appliance is connected to the earth wire, giving any leakage current a low-resistance path to the ground instead of through the user. (A miniature circuit breaker, MCB, does the fuse’s job reusably.)
Page 205, Question 2
An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Solution. Find the current the oven would draw and compare it with what the circuit is rated to carry.
$$I = \frac{P}{V} = \frac{2000\ \text{W}}{220\ \text{V}} = 9.09\ \text{A}$$
The circuit is rated at 5 A, and the oven demands over 9 A — nearly twice as much. The wiring is not built to carry that current: it would heat up badly. This is overloading, and it is exactly what the fuse is there to catch. The fuse in that circuit melts, breaking the circuit, and the oven does not run.
$$I = \frac{P}{V} = \frac{2000}{220} = 9.09\ \text{A}$$
This is far more than the circuit’s 5 A rating, so the circuit is overloaded: the wires would overheat, and the fuse melts and breaks the circuit, cutting off the supply. The oven should be run only on a circuit of higher rating, such as a 15 A one.
Page 205, Question 3
What precaution should be taken to avoid the overloading of domestic electric circuits?
Solution. Overloading means drawing more current than the wiring is rated for, so every precaution is a way of keeping the total current down — or of catching it when it rises.
- Do not connect too many appliances to one socket or to a single circuit at the same time.
- Do not run several high-power appliances together on a circuit of low current rating; put the heavy ones on the 15 A circuit.
- Use a fuse or MCB of the proper rating in each circuit, so an excess current breaks it.
- Use wires of adequate current rating and good-quality, properly insulated fittings, since damaged insulation causes short-circuiting.
- Do not use faulty appliances, where a fault can suddenly draw a very large current.
Common mistakes
- Answering the compass question with “because a magnet attracts iron”. The needle turns rather than being dragged in, and that turning is the evidence that it is itself a magnet with two poles being pushed in opposite directions.
- Drawing field lines that stop at the magnet’s surface. Every field line is a closed curve, continuing through the magnet from south to north. Lines that simply end make it look as though the field has a source, which it does not.
- Choosing only (c) in the proton question. The question says there may be more than one correct answer, and momentum is mass times velocity, so it changes whenever the velocity does. Leaving out (d) loses the mark.
- Saying the proton’s speed changes because there is a force on it. A force perpendicular to the motion changes only the direction. That is why a charged particle in a uniform magnetic field moves in a circle at constant speed.
- Using the right hand for Fleming’s rule. The rule for the force on a current is the left-hand rule; the right hand is for the thumb rule about the direction of the field around a wire, and for the generator rule. Mixing them up reverses every answer.
- Answering the oven question without computing the current. “It will overload” is only half the answer; the comparison of 9.09 A against the 5 A rating is the explanation the question asks for.
Practise next
- The end-of-chapter Exercises for this chapter, where Questions 6 and 7 ask for the same two direction rules and Question 3 for the properties of field lines.
- Chapter 11, Electricity — its in-text questions set up the fuse, the power rating and the current these last three questions depend on.
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.