The potential difference between the terminals of a cell is 20 V when a current of 2 A flows through the circuit. When the direction of current in the circuit is reversed, the potential difference between the terminals of the cell is 30 V . The internal resistance of the cell is

A straight uniform wire of resistance $36 \Omega$ is bent in the form of a semi-circular loop. The effective resistance between the ends of the diameter of the semi-circular loop is

The ratios of the voltage sensitivities, resistances and areas of the coils of two moving coil galvanometers $A$ and $B$ are $4: 3,3: 4$ and $1: 2$ respectively. If the number of turns of the coil of galvanometer $A$ is 200 , then the number of turns of the coil of galvanometer $B$ is (All other quantities remain same in both the cases)

The potential difference between points $C$ and $D$ of the electrical circuit shown in the figure is