Determining the e.m.f. of a test cell

Apparatus

• chemical cell
• digital d.c. supply
• 100Ω resistor
• digital ammeter
• connecting leads

Introduction

The e.m.f. of a cell may be determined by connecting a high-resistance voltmeter across its terminals. An alternative method makes use of Kirchhoff ’s second law.

The arrangement shown here may be used to determine the e.m.f. of a cell by applying Kirchhoff ’s second law. The direction of the current in the circuit depends on the relative magnitudes of the e.m.f.s. A digital ammeter will show the direction of the current.

According to Kirchhoff ’s second law:

sum of e.m.f.s in a closed loop = sum of p.d.s in that loop

Therefore:

e.m.f. of supply – e.m.f. of cell = IR

Es – Ecell = IR

When the e.m.f. of the supply is equal to the e.m.f. of the cell, the current I in the circuit is equal to zero.

Procedure

1 Set up the circuit as shown in the diagram above.

2 Set the e.m.f. Es of the supply to zero.

3 Measure the current I in the circuit – make sure that you also take note of the ‘sign’.

4 Measure the circuit current I as Es is increased in steps of 0.5 V.

5 Suitably record your results and plot a graph of I against Es.

6 Draw straight a line of best fit through the points (see sketch).

7 Use the graph to determine the e.m.f. of the test cell.

What is the uncertainty in your value for the e.m.f.?

Safety

Always take sensible precautions when using mains-operated supplies. Teachers and technicians should follow their school and departmental safety policies and should ensure that the employer’s risk assessment has been carried out before undertaking any practical work.