CBSE Class 10 Science Magnetic Effects of Electric Current MCQs Set 09

Answer: (c) half rotation

Explanation:
1. The split-ring commutator reverses the current in the coil after every half rotation.
2. This keeps the torque in the same direction so the coil keeps turning.

Teacher's Note:
1. Commutator = current reverser.
2. It reverses the current twice in one full rotation.

Answer: (c) 90°

Explanation:
1. The force is largest when the current is perpendicular to the field.
2. So the angle is 90°.

Teacher's Note:
1. Fleming's left-hand rule uses three mutually perpendicular directions.
2. Force is zero when current is parallel to the field.

Answer: (d) 180°

Explanation:
1. The force is zero when the wire is parallel (or antiparallel) to the field.
2. Among the options, only 180° gives this.

Teacher's Note:
1. Parallel to the field means no force.
2. Perpendicular to the field gives the maximum force.

Answer: (b) electrical energy into mechanical energy

Explanation:
1. A motor takes electrical energy from the supply.
2. The coil rotates, so the energy appears as mechanical energy.

Teacher's Note:
1. Motor: electrical → mechanical.
2. Generator: mechanical → electrical.

Answer: (a) Fleming's left-hand rule

Explanation:
1. Fleming's left-hand rule gives the direction of force on a current-carrying conductor in a magnetic field.
2. Thumb: force; forefinger: field; middle finger: current.

Teacher's Note:
1. Left hand for motors.
2. The three directions are mutually perpendicular.

Answer: (d) zero

Explanation:
1. The force on a moving charge depends on the component of velocity perpendicular to the field.
2. Along the field line, this component is zero, so the force is zero.

Teacher's Note:
1. Maximum force when perpendicular; zero when parallel.
2. Same idea as current parallel to the field.

Answer: (d) south

Explanation:
1. An electron moving east is equivalent to conventional current towards the west.
2. Apply Fleming's left-hand rule: forefinger down (field), middle finger west (current).
3. The thumb then points to the south.

Teacher's Note:
1. The current direction is opposite to the electron's motion.
2. Use the left hand and keep the fingers perpendicular.

Answer: (a) a stationary electric charge

Explanation:
1. A magnetic field acts on moving charges, magnets and magnetic materials such as iron.
2. A stationary charge has no motion, so it feels no magnetic force.

Teacher's Note:
1. No motion means no magnetic force on a charge.
2. Unmagnetised iron is attracted by magnets.

Answer: (c) motion (force)

Explanation:
1. Thumb: motion or force.
2. Forefinger: magnetic field.
3. Middle finger: current.

Teacher's Note:
1. Use FBI: Force (thumb), B-field (forefinger), I-current (middle).
2. Keep the three fingers perpendicular.

Answer: (b) Only (II)

Explanation:
1. The motor works on the force on a current-carrying coil, whose direction is found by Fleming's left-hand rule. So (II) is true.
2. An electromagnet does not need the left-hand rule, and the right-hand rule is for generators. So only (II) is correct.

Teacher's Note:
1. Motor: left hand; generator: right hand.
2. The thumb rule finds the field of a current.

Answer: (d) all of the above

Explanation:
1. The force on a current-carrying conductor is proportional to the current, the field strength and the length of the wire in the field.
2. So increasing any of them increases the force.

Teacher's Note:
1. Force depends on current, field and length.
2. It is largest when the wire is perpendicular to the field.

Answer: (d) show no deflection

Explanation:
1. A neutron has no electric charge, so a beam of neutrons carries no current.
2. A magnetic field exerts force only on moving charges, so the beam is not deflected.

Teacher's Note:
1. No charge means no magnetic force.
2. Alpha particles would be deflected.

Answer: (d) (a), (b) and (c)

Explanation:
1. Split rings reverse the direction of current in the coil every half turn, so (a) is true.
2. They are also called a commutator, so (b) is true.
3. A split ring is a ring broken into two halves, so (c) is true.
4. So all three are correct.

Teacher's Note:
1. Split ring = commutator.
2. It keeps the coil turning in one direction.

Answer: (b) the direction of the current in the coil

Explanation:
1. The turning effect grows with the current, the number of turns and the field strength.
2. The direction of the current only decides the direction of rotation, not its size.

Teacher's Note:
1. Direction affects the way of turning, not the size.
2. Speed can be raised by increasing current or turns.

Answer: (b) electrical energy to mechanical energy

Explanation:
1. An electric fan contains an electric motor.
2. The motor changes electrical energy into mechanical energy of the rotating blades.

Teacher's Note:
1. Fan, mixer and washing machine all use motors.
2. Motor: electrical to mechanical.

Answer: (b) brushes

Explanation:
1. The brushes are fixed carbon (or metal) contacts.
2. They press against the rotating split rings and carry current from the battery to the coil.

Teacher's Note:
1. Brushes are stationary; the rings rotate.
2. Brushes help in transferring current.

Answer: (a) west

Explanation:
1. A positive charge moving upwards is a current upwards.
2. Fleming's left-hand rule: forefinger north (field), middle finger up (current); the thumb points west.

Teacher's Note:
1. Left hand for force on moving charges.
2. The force is perpendicular to both the field and the motion.

Answer: (c) both (a) and (b)

Explanation:
1. A motor works on the force on a current-carrying coil, whose direction is given by Fleming's left-hand rule.
2. It converts electrical energy into mechanical energy.
3. So both statements are correct.

Teacher's Note:
1. Left hand rule: motors.
2. Energy: electrical to mechanical.

Answer: (a) downward

Explanation:
1. Use Fleming's left-hand rule: forefinger north (field), middle finger west (current).
2. The thumb points downward, which is the direction of the force.

Teacher's Note:
1. Force is perpendicular to both field and current.
2. Reverse the current and the force becomes upward.

Answer: (d) zero if v is parallel to B

Explanation:
1. A magnetic force acts on a moving charge only when it has a component of velocity across the field.
2. If v is parallel to B, there is no such component, so the force is zero.

Teacher's Note:
1. Parallel motion: zero force. Perpendicular motion: maximum force.
2. The same idea holds for a current-carrying wire.

Answer: (d) Fleming's right-hand rule

Explanation:
1. The rule for the direction of induced current is Fleming's right-hand rule.
2. Thumb: motion; forefinger: field; middle finger: induced current.

Teacher's Note:
1. Right-hand rule for generators.
2. Keep the three fingers mutually perpendicular.

Answer: (a) Electric motor: Fleming's right-hand rule

Explanation:
1. An electric motor works on the force on a current-carrying conductor in a magnetic field, and its rule is Fleming's left-hand rule.
2. So the pairing in (a) is not correct.

Teacher's Note:
1. Motor: left hand. Generator: right hand.
2. A compass needle is a small pivoted magnet.

Answer: (c) geyser

Explanation:
1. A ceiling fan, a blender and a washing machine all contain electric motors, which work on the magnetic effect of current.
2. A geyser works on the heating effect of current, so it is the answer.

Teacher's Note:
1. Motors use the force on a current-carrying coil in a magnetic field.
2. Heaters use the heating effect.

Answer: (a) A-(ii), B-(iv), C-(iii), D-(i)

Explanation:
1. Force on a current-carrying conductor: Fleming's left-hand rule, so A-(ii).
2. Induced current: Fleming's right-hand rule, so B-(iv).
3. Field of a straight conductor: right-hand thumb rule, so C-(iii).
4. Field direction at a point: direction of force on a north pole, so D-(i).

Teacher's Note:
1. Left hand: force. Right hand: induced current.
2. The thumb rule gives the field of a current.

Answer: (d) I-(1)-(ii), II-(2)-(i)

Explanation:
1. I: the field near a current is found by the right-hand thumb rule, and its discoverer is Oersted. So I-(1)-(ii).
2. II: current in a moving conductor is found by Fleming's right-hand rule, and induction was discovered by Faraday. So II-(2)-(i).

Teacher's Note:
1. Oersted: current gives magnetism.
2. Faraday: magnetism gives current.

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

Explanation:
1. A current-carrying conductor in a magnetic field experiences a force.
2. This force deflects the rod, so R explains A.

Teacher's Note:
1. Direction of force follows Fleming's left-hand rule.
2. The force is largest when current is perpendicular to the field.

Answer: (c) A is true, but R is false.

Explanation:
1. A motor converts electrical energy into mechanical energy, so A is true.
2. Its working follows Fleming's left-hand rule, so R is false.

Teacher's Note:
1. Motor: left hand rule.
2. Generator: right hand rule.

Answer: (c) A is true, but R is false.

Explanation:
1. A motor takes electrical energy and gives mechanical energy, which runs the pump. A is true.
2. R states the opposite conversion, so R is false.

Teacher's Note:
1. Motor: electrical to mechanical.
2. Generator: mechanical to electrical.

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

Explanation:
1. A conductor carrying current in a magnetic field experiences a force, so A is true.
2. The current produces its own field, which interacts with the external field, and this interaction causes the force. So R is true and explains A.

Teacher's Note:
1. Force is largest when the conductor is perpendicular to the field.
2. Parallel conductor feels no force.

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

Explanation:
1. The forces on opposite sides of the coil are equal in size and opposite in direction.
2. So they cancel and the net force on the coil is zero; R explains A.
3. They may still form a couple that turns the coil.

Teacher's Note:
1. Equal and opposite forces on different lines give turning, not a net push.
2. This is how a motor coil rotates.