The potentiometer is the device that works on the principle of Kirchoff’s law. The word potentiometer is the combination of potential difference and metering. It is an electrical device that is used to regulate EMF and the internal resistance of the given cell.

## Kirchoff’s Law of Electrical Network

**Kirchoff’s Current Law: **The algebraic sum of currents entering and leaving the junction of the electrical network will be equal to zero.

**Kirchoff’s Voltage Law: **Kirchoff’s voltage law works on the principle of the law of conservation of charge. The current entering the node is taken as positive, and the current leaving the node is taken as negative. The algebraic sum of the current entering the node is equal to the sum of the currents leaving the node.

## Potentiometer

The potentiometer is a long piece of uniform wire across which a standard cell is connected. In the actual instrument, the wire is cut into many pieces, and it is connected at the ends with a thick strip. These wires are stretched parallel to each other on a wooden board. There is a meter scale attached parallel to the wires. The position of the sliding contact is varied across the uniform resistance to find the value of the balancing length. The potentiometer is used to measure the internal resistance and the EMF of the two cells. In a potentiometer, when the current is passed through the wire of a uniform cross-sectional area, the potential difference between the two points is directly proportional to the length of the wire between the two points.

The formula to calculate the internal resistance of the cell is given by

r = R (l_{1}/l_{2} – 1)

R is the resistance unplugged in the resistance box

l_{1} is the balancing length before introducing the resistance R

l_{2} is the balancing length after a resistance R is introduced

## Application of Potentiometer

1) The potentiometer is used to compare the EMF of two primary cells

2) To find the internal resistance of the primary cell

3) To find the value of the high resistance

4) To determine the EMF of the cell

## RC Circuit

In an RC circuit, the passive components like the resistor and the capacitor are connected in series across an AC voltage source. The capacitor in the circuit will store the energy, and the resistor in the circuit will control the rate of charging and discharging. The charging and discharging is a time-consuming process. In an** **RC circuit, a fully discharged capacitor is connected to the resistor, and the switch is left open. When the switch is closed, the current starts flowing through the capacitor and the resistor. The capacitor will charge slowly through the resistor, till the voltage across the resistor and the supply voltage are equal.

## Time Constant

The time that is needed to increase the charge on the capacitor by 37% of its final charge is the time constant during the charging of the capacitor. It is the measure of how fast the capacitor can be charged.