Analysis

For each pair of results calculate the resistance, R using R = \frac{V}{I}. Record your results in the third column of the table.

Plot a graph of voltage, V on the y-axis v current, I, on the x-axis.

For a filament bulb, voltage and current are not directly proportional as the graph is not a straight line through the origin. Doubling the voltage does not double the current.

Evaluation

For a filament bulb, voltage and current are not directly proportional as the graph is not a straight line through the origin.

Doubling the voltage does not double the current.

It is clear from the resistance column in the results table that, as current increases the resistance of the lamp increases.

As the current through the lamp increases, the filament gets hotter and has a higher resistance.

Conclusion

The resistance of a filament lamp increase as the current through the filament increases and hence as the temperature increases.

Risk assessment

HazardRiskControl measures
Hot lamps can burnBurns to the skin from hot lampsDo not touch the lamp whilst the circuit is connected; allow time for the lamp to cool
WaterElectric shockDo not set up the experiment near taps, sinks etc.

The heating effect of an electric current and resistance

The free electrons collide with ions of the metal as they pass through the conductor.

As a result, the free electrons lose electrical energy and the metal ions gain energy.

This causes them to vibrate faster and with bigger amplitude – which means a higher temperature.

The heating effect of an electric current is useful in devices such as electric heaters, toasters, grills, hair dryers and straighteners.

The heating effect of an electric current is not useful in devices such as televisions, filament lamps, electric drills and battery chargers.