An electric device, which heats water by immersing a resistance wire in the water, generates 50 cal of heat per second when an electric potential difference of 12 v is placed across its leads. what is the resistance of the heater wire?

0.686 Ohm

Explanation:

Heat energy H = 50 cal = 50 x 4.2 J = 210 J, time t = 1 second, V = 12 V

Let R be the resistance.

Heat energy = V^2 x t / R

210 = 12 x 12 x 1 / R

210 = 144 / R

R = 144 / 210 = 0.686 Ohm

the duration of a photographic flash is related to an rc time constant, which is 0.100 μs for a certain camera.