# Verification of Ohm’s Law experiment

We have already learned the Ohm’s Law of current electricity. Now in this article, we are going to perform an experiment for the verification of ohm’s law. This practical verification is very important for the students of class 10 and 12. This is a lab based experiment to verify ohm’s law or the Ohm’s law practical.

## Aim of the Experiment

Aims of the ohm’s law experiment are as followings –

1. Verification of Ohm’s Law by showing that the Voltage to Current ratio is constant.
2. To determine the resistance of a wire by plotting a graph for potential difference (V) versus current (I) using Ohm’s Law.
3. To find the resistivity of a wire by plotting a graph for potential difference versus current.

## Theory of the experiment

From Ohm’s law we know that the relation between current and voltage or potential difference is

V=IR or, R=$\frac{V}{I}$ ………….. (1)

where I is current, V is potential difference and R is the resistance.

Again, the formula for the resistance of a wire is, $R=\frac{\rho L}{A}$

or, resistivity, $\rho = \frac{RA}{L}$ ………. (2)

where, A is Cross-section area of the wire. A= πr2 where r is the radius of the wire. L is the length of the wire.

In this experiment we will find the current and the potential difference across the sample wire by using Ammeter and Voltmeter respectively. Then the resistance of the wire can be found by using equation (1).

Again, We have to take at least five set of data for different voltage and current. Then a graph is needed to plot with current along positive Y-axis and the potential difference along positive X-axis.

• Ohm’s Law can be verified by finding the Voltage to current ratio. If the ration remains constant [equation- (1)] for all set of data, then we can say that the voltage across the resistance is proportional to current through it which is nothing but Ohm’s Law.
• One can easily find the value of resistance of the wire from the slope of the graph. R = $\frac{V}{I}$
• One can find the resistivity of the wire from the equation (2) by using the value of R from the graph. Usually, examiner supplies the radius (r) or diameter (2r) and length (L). If radius and Length are not given then we have to find those by using screw gauge and meter scale respectively.

## ApparatusUsed

Apparatus used for this experiment are

1. A power supply (Voltage source or Battery): The used battery can supply the voltage from 0 to 12 volts.
2. An Ammeter (A) to measure current. This Ammeter can measure the current from 0 to 3 ampere.
3. A Voltmeter (V) to measure Voltage. The used Voltmeter can measure the voltage from 0 to 3 volts.
4. A rheostat to control and adjust the current through the circuit.

## Circuit Diagram

Fig. (1) gives the circuit diagram of the experiment for verification of Ohm’s Law.

Here, R is the resistance of the wire, A is the ammeter, V is the Voltmeter, Rh is the rheostat and K is a key. Arrow sign indicates the direction of current flow in the circuit.

## Formula used

The used formulae are

R=$\frac{V}{I}$ ………….. (1)

and

$\rho = \frac{RA}{L}$ ………. (2) where, A= πr2

## Experimental data

The least count of Ammeter = Smallest division of Ammeter = 0.05 ampere

The least count of Voltmeter = Smallest division of voltmeter = 0.05 Volt

So, we can see that in each observation the voltage to current ratio is almost same. Thus, the voltage across the wire is proportional to the current through the wire. Hence Ohm’s law is verified.

Now we got the calculated value of the resistance of the wire is R= 1.02 ohm.

We also need to plot I-V graph to confirm the experimental value of R.

## Current versus Voltage graph (Ohm’s Law graph)

If we plot the Current as a function of voltage with the help of above data then we will get a straight line passing through origin.

## Calculations

#### Calculation of resistance from graph

The inverse of the I-V graph gives the resistance of the wire.

Now, from the graph, change in current, ∆I = AB = 0.5 amp

corresponding change in voltage, ∆V = BC = 0.5 volt

Thus, the Resistance from the graph, R= ∆V/∆I = 0.5/0.5 = 1.00 ohm

#### Calculation of resistivity of the wire

Given that

Length of the wire is, L = 50 cm = 0.5 m

Radius of the wire. r=0.25 mm = 0.25 × 10-3 m

So, the cross-section area of the wire, A=πr2 = 3.14 × (0.25×10-3)2 = 0.196 × 10-6 m2

Thus from the equation-2 we get the resistivity of the material of the wire is,

$\rho$ = (1 × 0.196 ×10-6 )/0.5

or, $\rho$ = 0.392 × 10-6 = 3.92 ×10-7 ohm.m

Thus the resistivity of the material of the wire is 3.92 ×10-7 ohm.m

## Final result

The resistance of the wire from the Current-Voltage graph is, R=1.00 ohm

The calculated value of the resistance of the wire is, R=1.02 ohm.

Resistivity of the material of the wire is 3.92 ×10-7 ohm.m

## Discussions

1. We observed that when voltage V=0 the reading of ammeter is zero. That means current through the wire is zero. Now, one cannot calculate the resistance for this data because one cannot measure the opposition faced by the current until the current flows.
2. While taking data we can see that for last two data current have not increased as much as first three observations. This is because of increase in resistance of the wire due to heating. Here current flow through the wire causes joule’s heating.
3. The calculated value of resistance almost matches the resistance calculated from the graph.
4. If the radius (r) and length (L) of the wire are not supplied, then we have to determine those parameters by screw gauge and the meter scale respectively.
5. In this experiment 1) verification of ohm’s law is done 2) Unknown resistance of the wire and 3) Resistivity of the material of the wire are determined.

This is all from this post on an experimental verification of ohm’s law. If you have any queries on this experiment you can ask me in the comment section.

Thank you!

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3. Drift velocity of electrons
4. Mobility of electrons and holes
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