CBSE Class 10 Science Magnetic Effects of Electric Current MCQs Set 12

Answer: (b) Oersted. Hans Christian Oersted observed that a compass needle is deflected by a nearby current-carrying wire.

Teacher's Note:
1. Faraday is linked to electromagnetic induction instead.
2. Keep the two discoveries separate.

Answer: (b) from north to south. Field lines emerge from the north pole and enter the south pole outside the magnet, and continue inside from south to north.

Teacher's Note:
1. Field lines form closed loops.
2. The direction of the field is the direction in which a north pole would move.

Answer: (b) the field at the point of intersection would have two directions. At a crossing point a compass needle would have to point in two directions, which is impossible.

Teacher's Note:
1. Say that the field has only one direction at any point.
2. The crowding of lines shows a stronger field.

Answer: (b) closest together. The relative closeness of lines represents the strength of the field.

Teacher's Note:
1. Near the poles the lines are crowded, so the field is stronger there.
2. Far from the magnet the lines are spread out.

Answer: (b) the right-hand thumb rule. If the thumb of the right hand points along the current, the curled fingers show the direction of the field lines.

Teacher's Note:
1. The field lines are concentric circles around the wire.
2. Reversing the current reverses the direction of the field.

Answer: (b) decreases as the distance from the wire increases. The field becomes weaker with distance and stronger with a larger current.

Teacher's Note:
1. The concentric circles drawn around the wire become farther apart as we move away.
2. This shows the weaker field.

Answer: (c) uniform and strong, along parallel straight lines. Inside a solenoid the field lines are parallel straight lines, showing a uniform field.

Teacher's Note:
1. Outside, the pattern resembles that of a bar magnet.
2. This property is used in making electromagnets.

Answer: (a) bar magnet. One end of the solenoid acts as a north pole and the other as a south pole, like a bar magnet.

Teacher's Note:
1. If suspended freely, it would align itself in the north-south direction.
2. The poles are reversed by reversing the current.

Answer: (b) soft iron. Soft iron gets magnetised strongly when the current flows and loses its magnetism when the current is switched off.

Teacher's Note:
1. Steel keeps its magnetism, so it is used for permanent magnets.
2. This is a typical reasoning question.

Answer: (b) increasing the current and the number of turns. The field grows with the current and with the number of turns per unit length, and a soft-iron core strengthens it further.

Teacher's Note:
1. Write at least two factors in your answer.
2. A core of soft iron is an additional factor.

Answer: (b) perpendicular to the field. The force is largest when the conductor is at right angles to the field, and zero when it is parallel.

Teacher's Note:
1. The direction of the force is perpendicular to both the current and the field.
2. Use Fleming's left-hand rule for the direction.

Answer: (a) force, field, current. The thumb shows the direction of the force, the forefinger the magnetic field and the middle finger the current.

Teacher's Note:
1. Keep the three fingers mutually perpendicular.
2. The phrase 'FBI' (Force, B-field, I-current) may help.

Answer: (c) south. By Fleming's left-hand rule, with the field upward and the current eastward, the force is directed towards the south.

Teacher's Note:
1. The force is perpendicular to both the velocity and the field.
2. Check that your three directions are at right angles.

Answer: (b) electrical energy into mechanical energy. A current-carrying coil in a magnetic field experiences a torque and rotates, producing mechanical energy.

Teacher's Note:
1. It works on the force on a conductor in a magnetic field.
2. A generator does the opposite job.

Answer: (b) reverse the direction of current in the coil every half rotation. By reversing the current every half turn, it keeps the coil rotating in the same direction.

Teacher's Note:
1. Without it the coil would just oscillate.
2. The brushes maintain contact with the rings.

Answer: (b) detect the presence of a current in a circuit. A galvanometer shows the presence and direction of a small current through the deflection of its pointer.

Teacher's Note:
1. It has zero at the centre of its scale.
2. A sensitive one can detect very small currents.

Answer: (b) Faraday. Michael Faraday found that a changing magnetic field around a coil induces a current in it.

Teacher's Note:
1. Oersted discovered the opposite link, in which a current produces a field.
2. Link each discovery to its scientist.

Answer: (b) a magnet is moved relative to the coil. A current is induced only while the magnetic field linked with the coil changes, as when a magnet moves in or out.

Teacher's Note:
1. The induced current stops as soon as the motion stops.
2. Faster motion gives a larger induced effect.

Answer: (b) mechanical energy into electrical energy. A coil rotated in a magnetic field has a current induced in it, which is mechanical energy turned into electrical energy.

Teacher's Note:
1. It works on electromagnetic induction.
2. The direction of the induced current is found by Fleming's right-hand rule.

Answer: (b) 50 Hz. The AC supply in India has a frequency of 50 Hz, so the direction reverses once every 1/100 s.

Teacher's Note:
1. The current completes 50 cycles in one second.
2. The usual supply voltage is 220 V.

Answer: (b) can be transmitted over long distances with less energy loss. AC can be sent over long distances at high voltage with little loss of power and then stepped down for use.

Teacher's Note:
1. DC from a cell flows in one direction only.
2. Remember that long-distance transmission is the key advantage.

Answer: (c) green. By the common convention, the live wire is red, the neutral wire black, and the earth wire green.

Teacher's Note:
1. Older wiring may use different colours.
2. The newer standard uses brown, blue and green-yellow.

Answer: (b) prevent electric shock by providing a low-resistance path to the ground. If a live wire touches the body, the current flows to the earth and the user is saved from a severe shock.

Teacher's Note:
1. The earth wire is especially needed for appliances like geysers, refrigerators and irons.
2. It is connected to a metal plate deep in the ground.

Answer: (a) the live and neutral wires touch each other. When the insulation fails and the live and neutral wires come into contact, the resistance falls to a very small value and a very large current flows.

Teacher's Note:
1. Overloading is a different cause of excess current.
2. A fuse protects against both.

Answer: (b) series with the live wire. A fuse in the live wire breaks the circuit when excess current flows, protecting the appliance and the wiring.

Teacher's Note:
1. Placing it in the live wire ensures that the appliance is completely disconnected from the live supply.
2. Fuses carry a fixed current rating.

Answer: (a) Both Assertion (A) and Reason (R) are true and R is the correct explanation of A. Both are true and the reason correctly explains why lines cannot cross.

Teacher's Note:
1. Such questions are answered by testing whether the second statement gives the cause.
2. Here it does.

Answer: (a) Both Assertion (A) and Reason (R) are true and R is the correct explanation of A. Both are true, and the reversing action explains why a split ring is used.

Teacher's Note:
1. A full ring would not reverse the current.
2. The reversal keeps the torque in the same sense.

Answer: (a) Fleming's right-hand rule. Fleming's right-hand rule gives the direction of the induced current when the motion and field directions are known.

Teacher's Note:
1. The left-hand rule applies to motors and the right-hand rule to generators.
2. Remember this as 'motor-left, generator-right'.