CBSE Class 10 Science Magnetic Effects of Electric Current Important Questions Set 01

Answer: (c) If magnetic field lines are parallel and equidistant, they represent zero field strength. Parallel and equidistant lines represent a uniform field of constant strength, not zero field.

Teacher's Note:
1. Closeness of field lines shows the strength of the field.
2. Uniform means the same strength and direction everywhere.

Answer: Face A is the north pole. By the clock rule, the face at which the current appears anticlockwise is a north pole, and the face at which it appears clockwise is a south pole.

Teacher's Note:
1. The clock rule gives the polarity of a current-carrying loop.
2. Field lines come out of the north face of the loop.

Answer: (i) Yes, because alpha particles are positively charged, and moving charges constitute a current which produces a magnetic field.
(ii) No, because neutrons carry no charge, so their motion is not a current.

Teacher's Note:
1. A magnetic field is produced only by moving electric charges.
2. Neutrons are electrically neutral.

Answer: End A will be a south pole. Clockwise current at an end makes that end a south pole (clock rule).

Teacher's Note:
1. Check the sense of the current as seen from the end.
2. Clockwise means south and anticlockwise means north.

Answer: (a) End X is a south pole (clockwise current).
(b) End Y is a north pole (anticlockwise current).
(c) Clock rule: looking at a face of the coil, if the current is anticlockwise the face is a north pole, and if it is clockwise the face is a south pole. (The right-hand thumb rule for a solenoid gives the same result.)

Teacher's Note:
1. The two ends of a solenoid always have opposite polarity.
2. Use the rule to decide the polarity from the sense of the current.

Answer: 1. Rotate the coil faster, which increases the rate of change of magnetic flux.
2. Use a stronger magnet, more turns of wire in the coil, or a coil of larger area.

Teacher's Note:
1. Induced current depends on how quickly the flux linked with the coil changes.
2. More turns and stronger fields also increase it.

Answer: (a) Direct current (DC) flows in one direction only. Alternating current (AC) reverses its direction after equal intervals of time.
(b) DC sources: dry cell, battery, DC generator. AC sources: AC generator (alternator) and the mains supply.
(c) AC can be transmitted over long distances with much less loss of energy.
(d) 50 Hz, that is the current reverses direction 100 times per second.

Teacher's Note:
1. The direction of the current is the main difference.
2. Indian AC reverses every 1/100 s, which gives a frequency of 50 Hz.

Answer: The only difference is in how the ends of the coil are connected to the external circuit. A DC generator has a split-ring commutator of two half rings, while an AC generator has two full slip rings, each in contact with a brush.

Teacher's Note:
1. A commutator reverses the connection every half turn, and slip rings do not.
2. This is why DC generator output is unidirectional.

Answer: (a) The induced current increases.
(b) The direction of the induced current is reversed.
(c) The induced current increases.
(d) No current is induced.
(e) No current is induced, because no field lines are cut.

Teacher's Note:
1. Current is induced only when the wire cuts magnetic field lines.
2. The size depends on the rate of cutting and the field strength.

Answer: Zero volt. The neutral wire is at earth potential, whereas the live wire is at 220 V relative to it.

Teacher's Note:
1. The neutral wire completes the circuit with the live wire.
2. The potential difference of 220 V is between the live and neutral wires.

Answer: Current drawn I = P/V = 180/240 = 0.75 A. The fuse should be rated just above this, so the 1 A fuse is the most suitable.

Teacher's Note:
1. Find the normal working current first.
2. A fuse below 0.75 A would blow in normal use and a very large one would not protect the amplifier.

Answer: Maximum power = V × I = 220 × 5 = 1100 W. Number of bulbs = 1100/60 = 18.3, so at most 18 bulbs can be used.

Teacher's Note:
1. Total power must not exceed the fuse limit.
2. The answer is rounded down because a fraction of a bulb is not possible.

Answer: Total power = 60 + 500 + 1200 = 1760 W. Current I = P/V = 1760/220 = 8 A, which is less than 10 A, so the fuse will not blow.

Teacher's Note:
1. In parallel the powers add.
2. Compare the total current with the fuse limit.

Answer: (a) The live wire is at 220 V, whereas the neutral wire is at about zero potential. Touching the live wire lets current flow through the body to the earth.
(b) A bird's body is not connected to the earth, and its feet are at the same potential, so no current flows through it.

Teacher's Note:
1. Current flows only when there is a potential difference across the body.
2. A bird on a single wire has no path to the earth.

Answer: Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. If the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of the force on the conductor. It is used to find the direction of the force on a current-carrying conductor in a magnetic field, as in an electric motor.

Teacher's Note:
1. Forefinger is field, middle finger is current and thumb is force.
2. The force is largest when the current is perpendicular to the field.

Answer: Electromagnetic induction is the production of an induced current in a coil when the magnetic field linked with it changes. The direction of the induced current is given by Fleming's right-hand rule: with the thumb, forefinger and middle finger perpendicular, the thumb shows the motion of the conductor, the forefinger the field, and the middle finger the induced current.

Teacher's Note:
1. A changing magnetic field induces a current in a closed coil.
2. The principle is used in generators.

Answer: An electric motor works on the principle that a current-carrying coil placed in a magnetic field experiences a force and rotates. The split-ring commutator reverses the direction of the current in the coil after every half rotation, so the coil keeps turning in the same direction.

Teacher's Note:
1. A motor converts electrical energy into mechanical energy.
2. Without the commutator the coil would only swing to and fro.

Answer: Inside a solenoid the field lines are parallel straight lines, so the field is uniform and strong. A soft iron core placed in it forms an electromagnet, which is used in electric bells, cranes and loudspeakers.

Teacher's Note:
1. A solenoid behaves like a bar magnet.
2. Its field is stronger when it has more turns or a larger current.