Why no current flows through galvanometer at the null point of potentiometer?

The fig. Shows a network of resistance in which the point A is earthed

 

When a p.d of 2v is applied across a resistor, 10j of energy are covered. What charge flows through the resistor?

 

Please provide it's solution. In video form 

Find the equivalent resistance of the given figure

The resistivity of material is 10 ohm m at 30 C when the temperature drops to 28 then the resistivity  drops by 2. The temperature coefficient of resistivity of material is 

Three cells of 2.0v each and internal resistance of 1.0 each are connected in series and in parallel calculate the new emf and new internal resistance in each connection