Ten identical cells connected in series are needed to heat a wire of length one meter and radius 'r' by 10^oC in time 't'. How many cells will be required to heat the wire of length two meter of the same radius by the same temperature in time 't' ?

The resistances of the four arms P, Q, R and S in a Wheatstone's bridge are 10 ohm, 30 ohm, 30 ohm and 90 ohm, respectively. The e.m.f. and internal resistance of the cell are 7 volt and 5 ohm respectively. If the galvanometer resistance is 50 ohm, the current drawn from the celll will be

The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10 $$\Omega $$ is

A wire of resistance 4 $$\Omega $$ is stretched to twice its original length. The resistance of stretched wire would be