Magnetic Effects of Electric Current: Class 10 Important Questions with Answers

Case-based questions (4 marks each)

(i) Name the device. (1) (ii) Why is soft iron used for the core? (1) (iii) Give two ways to lift heavier loads, and say what happens to the poles if the current is reversed. (2)

(i) What does the deflection show? (1) (ii) What happens if the current is reversed? (1) (iii) What happens to the deflection if (a) the current is doubled (b) the compass is moved far below the wire? Name the rule that gives the field direction. (2)

(i) Name the rule that predicts this. (1) (ii) When is the force maximum? (1) (iii) Predict the motion if (a) the magnet is turned upside down (b) both the current and the field are reversed. (2)

(i) What happened at the frayed cord? (1) (ii) Which device cut off the power, and how? (1) (iii) Explain how an earth wire would have protected the neighbour, and give the colour code of the three wires. (2)

20 MCQs (1 mark each)

1. Who discovered that a current deflects a compass?
   (a) Ampère (b) Oersted (c) Faraday (d) Ohm
2. Outside a bar magnet field lines go from
   (a) S to N (b) N to S (c) centre outward (d) nowhere
3. Strength of field is shown by
   (a) colour (b) length of lines (c) closeness of lines (d) arrows
4. Field lines around a straight wire are
   (a) parallel lines (b) radial (c) concentric circles (d) ellipses
5. Field near a wire, moving away from it:
   (a) increases (b) decreases (c) same (d) zero
6. Right-hand thumb rule gives direction of
   (a) current (b) force (c) magnetic field (d) motion
7. Field at centre of a loop appears as
   (a) circles (b) straight lines (c) zero (d) spirals
8. Field of n turns compared with one turn:
   (a) same (b) n times (c) 1/n (d) n²
9. Field inside a long solenoid is
   (a) zero (b) uniform (c) larger at ends (d) circular
10. An electromagnet core is made of
    (a) steel (b) copper (c) soft iron (d) aluminium
11. Seen from one end, current is anticlockwise; that end is
    (a) N (b) S (c) neutral (d) cannot say
12. Force on a conductor is maximum when angle between I and B is
    (a) 0° (b) 45° (c) 90° (d) 180°
13. In Fleming's left-hand rule, the forefinger shows
    (a) current (b) field (c) force (d) motion
14. A proton moving in a field — which stays constant?
    (a) velocity (b) momentum (c) speed (d) direction
15. Electron moving east is equivalent to current towards
    (a) east (b) west (c) north (d) south
16. The live wire colour is
    (a) green (b) black (c) red (d) blue
17. P.d. between live and neutral in India:
    (a) 110 V (b) 220 V (c) 440 V (d) 50 V
18. A geyser should be on the circuit of rating
    (a) 1 A (b) 5 A (c) 15 A (d) 50 A
19. Earth wire is mainly for
    (a) saving power (b) safety from shock (c) increasing current (d) lighting
20. Short circuit means
    (a) live touches neutral (b) fuse is new (c) low voltage (d) switch off

Answer key

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: Field lines are closed curves. R: Inside a magnet they run from S to N. → (a)
2. A: The field inside a solenoid is uniform. R: Field lines inside a solenoid are parallel straight lines. → (a)
3. A: The field due to a straight wire is the same at all distances. R: Field lines around it are concentric circles. → (d)
4. A: An electromagnet is a temporary magnet. R: Soft iron loses its magnetism when the current stops. → (a)
5. A: A charged particle moving along the field lines experiences no force. R: The force is maximum when motion is perpendicular to the field. → (b)
6. A: The speed of an electron moving in a magnetic field changes. R: The magnetic force is perpendicular to the velocity. → (d)
7. A: Fuse is connected in the neutral wire. R: Fuse melts on high current. → (d)
8. A: Metallic appliances are earthed. R: The earth wire gives a low-resistance path for leakage current. → (a)
9. A: Household appliances are connected in parallel. R: Each then gets 220 V and has its own switch. → (a)
10. A: A solenoid behaves like a bar magnet. R: A solenoid always has a soft-iron core. → (c)

Give reasons (2 marks each)

1. Two field lines never cross.

At the crossing the field would have two directions, and a compass cannot point two ways. 2

2. A soft-iron core is used in an electromagnet, not steel.

Soft iron gets strongly magnetised and loses its magnetism quickly when the current stops; steel would stay magnetised. 2

3. The switch and fuse are put in the live wire.

Then, when the switch is off or the fuse melts, the appliance is disconnected from the 220 V supply and is safe to touch. 2

4. Too many appliances on one socket are dangerous.

The total current drawn becomes very large (overloading); the wires overheat and can catch fire. 2

5. The field at the centre of a coil increases with the number of turns.

The current flows the same way in every turn, so the fields of the turns add up. 2