IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Verification of Ohm's Law

## Introduction

Ohm's law states that potential difference $$V$$ across a conductor is proportional to the current $$i$$ passing through it. The relation is given as $$V=iR$$, where $$R$$ is the resistance of the conductor.

## Apparatus

Voltmeter, Ammeter, Power supply, resistors, and connecting wires.

## Procedure

Take four or five dry cells, a thin wire (AB), a voltmeter, an ammeter, a plug key and some thick connecting wires. Connect the circuit as shown in figure, using one cell. The plug key allows you to switch off the current when not required. The wire becomes quite hot when current passes through it for some time. This drains the cell as well. Therefore, insert the key into the plug to switch on the current only when taking measurements.

The ammeter measures the current $$i$$ through the circuit, and the voltmeter measures the potential difference $$V$$ between the ends A and B of the wire. Note these values. Now, connect two cells in series in the circuit. You will find that the reading of the voltmeter increases, indicating the fact that a larger potential difference has been applied across the wire AB. You will also find that the reading of the ammeter increases as well. Note down the new values of $$V$$ and $$i$$. Repeat the experiment by connecting in series three cells, four cells, and so on. In each case measure the potential difference and the current. If you calculate $$V/i$$ for each case, you will find that it is almost the same. So, $$V/i=R$$ is a constant, which is another way of stating Ohm's law. Here, $$R$$ is resistance of the wire AB. If you plot a graph of the current of the current $$i$$ against the potential difference $$V$$, it will be a straight. This shows that the current is proportional to the potential difference.