Question

Solve

1. To carry how much charge between two points having potential difference equal to 220 V, 1760 J of work is done? Ans. 8c

2. The EMF of a cell falls from 3 volts to 2.8 volts when its terminals are joined to an electrical load of 4 Ohms. Calculate the internal resistance of the cell.

Answer 1

a)

$$W=q\Delta V,$$ $$q=\dfrac{W}{\Delta V}=\dfrac{1760\ J}{220\ V}=8\ C.$$

b) Let's first find the load current in the circuit from the Ohm's law:

$$I=\dfrac{V}{R}=\dfrac{2.8\ V}{4\ \Omega}=0.7\ A.$$

The EMF of the cell can be found as follows:

$$\Epsilon=V+Ir.$$

From this formula we can find the internal resistance of the cell, $r$:

$$r=\dfrac{\Epsilon-V}{I}=\dfrac{3\ V-2.8\ V}{0.7 A}=0.286\ \Omega.$$