# Electromagnetic Induction – Short Questions Collection With Answer | Physics Class 12

## Very Important Questions Collection HSEB | Science Faculty Electromagnetic Induction – Physics Grade XII Short Questions with Answer (Solution)

1. Show that Lenz’s law is an example of conservation of energy.
Ans. According to Lenz’s law, the direction of induced current in a coil is such that it always opposes the cause which produces it. Energy can neither be created nor be destroyed but one form of energy can be changed into another. This is conservation of energy. When a magnet is moved towards or away from a coil, an induced emf is set up in the coil at the expense of mechanical energy spent by the external agent. Hence. the mechanical energy is converted to electrical energy. That’s how ; the Lenz’s law is in accordance with the law of conservation of energy.

2. What is the difference power looses in a transformer? What measures do you take to minimize these losses?
Ans. A practical transformer has efficiency less than one due to following types of power looses:

1. Copper loss : It can be minimized by talking wires of suitable thickness.
2. Hysteresis loss : This can be minimized by using core materials having narrow hysteresis loop.
3. Iron (or eddy current) : It can be minimized by using laminated iron core.
4. Humming loss : It can be reduced by using proper material with low vibration.
5. Flux loss : It is minimized by designing the core for maximum linkage between the primary and secondary coils.

3. Why are the coils of a resistance box made of doubled insulted wire?
Ans. Double insulted wire is used to make the coil of a resistance box. In such condition, there are equal and opposite current in such section of the coil so that induced e.m.f produced by them becomes zero. Such coils are called non-inductive coil and have the following advantage :

1. The current attains the final value quickly at that time of join or break.
2. The alternating current through such coils is frequency independent.

4. Why can’t transformer be used to step up or down the dc voltage?
Ans. Transformer work on the principle of mutual induction. But, when primary coil of the transformer is connected to a dc current which produces constant magnetic flux, the fluxed connected with the secondary coil of the transformer is not charged. As there is no change in magnetic flux, e.m.f is not induced in the secondary coil. That’s why; a transformer can’t be used to step up or down the dc voltage.

5. An artificial satellite with a metal surface has an orbit over the equator. Will the earth’s magnetism induce current in it ? Explain.
Ans. The satellite will cut vertical component of earth’s magnetic field and vertical component in the equatorial plane is zero. Consequently , no current will be induced on the aircraft satellite with a metal surface has an orbit over the equator.

6. Why is the cylindrical core of soft iron used in moving coil galvanometer?
Ans. An instrument which is used to measure small amount of electric current flowing through the circuit is called moving coil galvanometer. When a cylindrical coil of soft iron used in it, a radial magnetic field is produced so that the deflection becomes directly proportional to the current flowing through the coil. As a result, the scale showing the current value is uniform one i.e. equal division along the calibrated scale represents equal steps in current. That’s why, the cylindrical core of soft is iron used in moving coil galvanometer.

7. How is magnetic field made radical in a moving coil galvanometer?
Ans. In a moving coil galvanometer, magnetic field is made radial by using cylindrically cut pole pieces and putting a soft iron core within its coil. The advantage of radial magnetic field is that the deflection becomes directly proportional to the current flowing through the coil. As result, the scale showing the current values is a uniform one.

8. Spark is produced in switch when the light is put off. Explain why ?
Ans. When the light is put off, a large e.m.f is produced momentarily due to sudden interruption in circuit current which opposes the decay of current in the circuit. Consequently the air between the contact of switch is ionized which causes sparking in the switch.

9. What is eddy current ? Write down its used?
Ans. When a block of metal moves through a magnetic field, induced current circulate through the volume of the metal block. These currents are called eddy currents. The eddy currents exist in the core of the transformer when alternating current flows through it. These currents are disadvantageous because they dissipate energy in the form of heat and they themselves set up a magnetic flux which opposes the flux change in the input circuit. Moreover , eddy currents are used in damping galvanometer when current is switched off and hence the coil in the galvanometer comes to rest sooner.

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### One Comment

1. Arpan Tiwari
Posted May 25, 2016 at 10:42 pm