# Measuring the e.m.f. And Internal Resistance of a Cell

Measuring the e.m.f. And Internal Resistance of a Cell

Planning

I wish to find out the electromotive force (e.m.f.) and Internal Resistance of a direct current battery cell.

Circuit Diagram

. Set up apparatus as shown.

2. Use ammeter to adjust variable resistor to vary of currents.

3. Read voltmeter value and record.

4. Disconnect circuit and take e.m.f reading from voltmeter across cell.

5. Repeat for another 9 resistor settings. Use variable resistor and ammeter to adjust the set up to get a range of 0.1 to 1.0A.

6. Repeat entire experiment to get more accurate average.

Safety Aspects

. Do not have circuit connected up for too long, this will run the battery down and may cause it to heat up and maybe explode.

2. Wear safety goggles and lab-coats during experiment.

3. Follow standard laboratory safety procedures.

Apparatus

0 - 1.0A analogue ammeter

2 digital voltmeters

1 variable resistor

1 cell

8 connecting wires.

Safety goggles and lab-coat

Paper and pen

Arranging Evidence

Results

Current (Amps)

Potential Difference (V)

2

3

Average

0.1

.46

.47

.46

.46

0.2

.42

.42

.41

.42

0.3

.36

.39

.36

.37

0.4

.33

.33

.33

.33

0.5

.27

.29

.28

.28

0.6

.24

.23

.23

.23

0.7

.18

.19

.20

.19

0.8

.13

.15

.14

.14

0.9

.10

.09

.11

.10

.0

.04

.03

.06

.04

Using these results I will be able to draw a graph to find the e.m.f. and the internal resistance of the cell.

Calculating Internal Resistance

The current is the same in any part of the circuit and the electromotive force = the sum of the potential differences.

? = I R + I r

Since voltage = Current x Resistance and can be substituted into the above equation. ? ? = V + I r Rearrange to get: V = -r I + ?

This equation is now in the form y = m x + C

This means that ; y is the terminal voltage

m is the negative internal resistance

x is the current

C is the e.m.f.

Since the gradient is the change in y divided by the change in x, this is the terminal ...