## What is the heating effect of electric current ?

Ans : When an electric current is passed through a conducting wire, the electrical energy is converted into heat energy. This effect of electric current is called heating effect of electric current. It is an irreversible process.   Heat produced does not depend upon the direction of current.

## What is the Internal Resistance of the cell ?

Ans:  The opposition offered by the electrolyte of the cell to the flow of electric current through it is called internal resistance of the cell. It is denoted by ‘ r ‘

Internal Resistance depends on

(1)  Distance between electrodes (r ∝ d)

(2)  Area of electrodes ( dipped in electrolyte) (r ∝ 1/A)

(3)  Temperature of electrolyte.

## Why does the mass of a body change when it emit or absorb photons ?

Question : Why does the mass of a body change when it emit or absorb photons ?
Ans. As photon has energy hν and according to theory of relativity, energy E is equivalent to a mass (E/c²), so photon has a mass (hν/c²). So theoretically the mass of a body will decrease if it emits photons and will increase if it absorbs them. However due to small value of h =(6.6 x 10-34 Js) and large value of c( = 3 × 108 m./s) as the mass of an optical photon is too small, the change in mass of a body due to emission or absorption of photons is practically negligible as compared to the mass of the body.

## Why the name Transistor ?

Ans: By forward biasing input junction (emitter-base junction) and reverse biasing output junction (collector-base junction), a low resistance of forward biased junction can be transformed into a high resistance of reverse biased junction.

Also by interchanging biases, a high resistance can be transformed into a low resistance. Hence the junction triode is called TRANsformer of reSISTOR, which in short becomes TRANSISTOR.

## When current in a coil changes with time , how is the back e.m.f induced in the coil related to it ?

Ans. A/c to Faraday’s law of E.M.I , Back e.m.f induced in the coil , e = – L dI/dt  . Where L is the coefficient of self – inductance of the coil and dI/dt is the rate of change of current through the coil .

## A lamp in a circuit consisting of a coil of large number of turns and a battery does not light up to full brilliance instantly on switching on the circuit . Why ?

Question: A lamp in a circuit consisting of a coil of large number of turns and a battery does not light up to full brilliance instantly on switching on the circuit . Why ?

Ans. When circuit is switched  on , current increases through the lamp and also through the coil . An induced e.m.f developes in the coil which opposes the growth of current . As current takes time to grow to maximum value , the lamp does not light instantly  upto full brilliance .

## A bar magnet falls through a metal ring . Will its acceleration be equal to ‘ g ‘ ?

Question: A bar magnet falls through a metal ring . Will its acceleration be equal to ‘ g ‘ ?

Ans. No , acceleration of magnet will not be equal to g . It will be less than ‘g ‘ . This is because as magnet falls amount of magnetic flux linked with the ring changes  . An induced current is developed in the ring which opposes the downward motion of the magnet .After the magnet has crossed the metal ring , amount of magnetic flux linked with the ring goes on decreasing . An induced current is developed in the ring and opposes the fall of magnet . Therefore downward acceleration of magnet continues to be less than ‘ g ’ .

## Why does moving part of a moving coil galvanometer come to rest almost instantaneously ?

Question : Why does moving part  of a moving coil galvanometer come to rest almost instantaneously ?

Ans. Due to electromagnetic damping , that is production of induced current in the frame and the coil which strongly opposes motion according to Lenz’s Law .

## If the polar ice caps melt & spread uniformly , how will the length of the day be affected ?

Question: If the polar ice caps melt & spread uniformly , how will the length of the day be affected ?

Ans. If the polar ice caps were to melt , the water formed would spread on the surface of the earth and so its moment of inertia would increase . By the law of conservation of angular momentum , the speed of rotation would be reduced , that is , the length of the day would increase .

## Why does Microwave oven heats up the food items containing water molecules most efficiently ?

Question: Why does Microwave oven heats up the food items containing water molecules most efficiently ?

Ans. Microwave oven heats up the food items containing water molecules most efficiently because the frequency of microwaves matches the resonant frequency of water molecules .