Careful Measurement of Emf and or Internal Resistance of a Cell

1664 Words Jan 19th, 2013 7 Pages
Careful measurement of emf and or internal resistance of a cell
Aim
To measure voltage and current in the circuit and from that figure work out emf and internal resistance of the cell, to identify errors involved and deal with them in the most effective way and maintain a safe working environment at all times.
Background
EMF (electromotive force) is a measure of work done per unit of charge and is measured in volts. Internal resistance is resistance in ohms of the cell. I will be using a 1.5V battery in the experiment. I will measure the voltage and current using multimeters.
Calculation method
I intend to rearrange the equation ‘ε=Ir+IR’ to form ‘V= -Ir +ε’ and then draw the y=mx+c graph equation to find EMF and internal resistanc`e. The
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The circuit diagram is the same as before with resistance on a side experiment measured.
The results were initially good but then tailed off and the correlation even went in the wrong direction. Two attempts returned very similar results. I thought this may have been as a result of the battery overheating as it had been on for a long time, as I initially struggled to get a reading as the resistance reading was very low. When the resistance on the rheostat is

very low anomalies are produced. The battery produces a large fluctuating current, this is because if there is low resistance a high current is produced. The chemical energy in the battery isn’t quick enough to regenerate the power.
I also used the same safety precautions as before as there was no new dangers associated with my slightly altered experiment

Current (A) | Uncertainty (A) | Voltage (V) | Uncertainty (V) | 0.57 | +/- 0.01 | 1.063 | +/- 0.01 | 0.43 | +/- 0.01 | 1.180 | +/- 0.01 | 0.34 | +/- 0.01 | 1.261 | +/- 0.01 | 0.27 | +/- 0.01 | 1.308 | +/- 0.01 | 0.23 | +/- 0.01 | 1.341 | +/- 0.01 | 0.20 | +/- 0.01 | 1.348 | +/- 0.01 | 0.17 | +/- 0.01 | 1.344 | +/- 0.01 |

Third experiment
I changed my experimental method considerably for my third experiment. I worked out that the rheostat may have been overheating on its own or at the same time as the battery so I stopped taking