CBSE Class 10 Physics Magnetic Effects of Electric Current MCQs Set 11

Answer: (b) Oersted

Explanation:
1. Oersted observed that a compass needle was deflected by a current-carrying wire.

Teacher's Note:
1. Faraday discovered induction.

Answer: (c) \( 6.3 \times 10^{-4}\ \text{T} \)

Explanation:
1. Turns per metre \( n = 100/0.5 = 200 \).
2. \( B = \mu_0 nI = 4\pi \times 10^{-7} \times 200 \times 2.5 = 6.28 \times 10^{-4} \) T.

Teacher's Note:
1. Convert length to metres first.

Answer: (c) may be attractive or repulsive

Explanation:
1. Unlike poles attract and like poles repel, so magnetic forces can be of either kind.

Teacher's Note:
1. Same rule as for electric charges.

Answer: (d) steel

Explanation:
1. Steel retains its magnetism once magnetised, while soft iron loses it quickly.
2. So permanent magnets are made of steel or alloys such as alnico.

Teacher's Note:
1. Soft iron is used for electromagnets.

Answer: (b) electrical to mechanical energy

Explanation:
1. A motor takes electrical energy and makes a coil rotate, giving mechanical energy.

Teacher's Note:
1. A generator does the reverse.

Answer: (b) Iron

Explanation:
1. Iron, nickel and cobalt are strongly magnetic; mercury, gold and silver are not.

Teacher's Note:
1. Steel is made of iron.

Answer: (c) 200 V

Explanation:
1. \( e = N\,\Delta\Phi/\Delta t = 100 \times 0.2/0.1 = 200 \) V.

Teacher's Note:
1. Take the magnitude.

Answer: (a) A-S, B-P, C-Q, D-R

Explanation:
1. The formula 2I/r with mu0/4pi gives the field of a long wire (A-S).
2. F = Bqv sin theta is the force on a moving charge (B-P).
3. The direction of force on a current-carrying conductor comes from Fleming's left-hand rule (C-Q).
4. The radius mv/qB is for circular motion (D-R).

Teacher's Note:
1. Learn each law with its formula.

Answer: (a) live wire

Explanation:
1. The fuse must be on the live wire so that when it melts the appliance is cut off from the high potential.

Teacher's Note:
1. The earth wire never carries a fuse.

Answer: (b) it has low melting point

Explanation:
1. A fuse must melt quickly when the current becomes too large.
2. Tin-lead alloy has a low melting point, so it melts and breaks the circuit.

Teacher's Note:
1. The fuse protects appliances from overload and short circuit.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Oersted's experiment showed a compass needle deflecting near a current-carrying wire, which proves the assertion.
2. So the reason explains the assertion.

Teacher's Note:
1. Both statements are true and the second explains the first.

Answer: (a) generator

Explanation:
1. A generator converts mechanical energy into electrical energy by electromagnetic induction.

Teacher's Note:
1. A motor does the opposite.

Answer: (c) 100

Explanation:
1. In each cycle the current reverses direction twice.
2. 50 cycles per second give 100 reversals per second.

Teacher's Note:
1. Direction changes = 2 x frequency.

Answer: (b) uniform and equal to \( \mu_0 nI \)

Explanation:
1. Well inside a long solenoid the field is uniform with \( B = \mu_0 nI \), where n is the number of turns per unit length.
2. Only at the ends does it fall to half, \( \mu_0 nI/2 \).

Teacher's Note:
1. n is turns per metre, not total turns.

Answer: (c) A and B

Explanation:
1. Field lines form continuous closed loops, so A is true and C is false.
2. Two lines cannot cross, because the field would then have two directions at one point, so B is true.

Teacher's Note:
1. Outside a magnet they go from N to S and inside from S to N.

Answer: (b) geographic north-south direction

Explanation:
1. The needle aligns with the Earth's magnetic field, which lies nearly along the geographic north-south line.

Teacher's Note:
1. That is how a magnetic compass works.

Answer: (b) \( 4.3 \times 10^{-4}\ \text{N} \)

Explanation:
1. \( F = BIl\sin\theta = 5 \times 10^{-4} \times 2 \times 0.5 \times 0.866 \)
2. \( F = 4.33 \times 10^{-4} \) N.

Teacher's Note:
1. Use the angle between the wire and the field.

Answer: (d) weber

Explanation:
1. Flux = B x A. The SI unit of flux is the weber (Wb); gauss is a unit of field, and dyne is a unit of force.

Teacher's Note:
1. 1 Wb = 1 T x 1 m squared.

Answer: (b) \( 6.3 \times 10^{-4}\ \text{T} \)

Explanation:
1. \( B = \dfrac{n\mu_0 I}{2r} = \dfrac{100 \times 4\pi \times 10^{-7} \times 4}{2 \times 0.1} \)
2. \( B = 6.28 \times 10^{-4} \) T.

Teacher's Note:
1. The centre field is proportional to turns and current and inversely to radius.

Answer: (d) no deflection

Explanation:
1. Induced current needs a changing magnetic flux.
2. A stationary magnet gives constant flux, so there is no induced current.

Teacher's Note:
1. Pushing the magnet in or pulling it out gives momentary deflections in opposite directions.

Answer: (a) 1.6 x 10^-3 Wb and zero

Explanation:
1. Area = 0.040 x 0.050 = 2 x 10^-3 square metre.
2. Horizontal plane: the vertical field is normal to it, so flux = 0.80 x 2 x 10^-3 = 1.6 x 10^-3 Wb.
3. Vertical plane: the field is parallel to the plane, so flux = 0.

Teacher's Note:
1. Flux is B times the area perpendicular to B.

Answer: (a) the magnetic field lines

Explanation:
1. Each filing becomes a tiny magnet and lines up along the magnetic field, tracing the magnetic field lines.

Teacher's Note:
1. Source of the pattern: the magnet's own field.

Answer: (d) 800 V

Explanation:
1. Change in flux per turn \( = (0.60 - 0.20) \times 0.20 = 0.08 \) Wb.
2. \( e = N\,\Delta\Phi/\Delta t = 100 \times 0.08/0.01 = 800 \) V.

Teacher's Note:
1. Multiply by the number of turns.