A student investigated the variation of the current I flowing through a filament bulb for a range of different values of potential difference V - Leaving Cert Physics - Question 4 - 2005

The student varied the potential difference by using a variable resistor (rheostat) or potentiometer in the circuit. By adjusting this component, the voltage supplied to the filament bulb was modified, which allowed for measuring the corresponding current at different voltages.

The relationship between the current and the potential difference is not proportional because the graph shows a curve rather than a straight line. This implies that as the voltage increases, the current does not increase linearly. This is characteristic of non-ohmic conductors, such as filament bulbs, where resistance changes with temperature.

To calculate the resistance at different voltages, we can use Ohm's law, which states:

$R = \frac{V}{I}$

1. At 1 V:

   • Given that the current I is 0.028 A, we find: $R = \frac{1}{0.028} \approx 35.7 \Omega$

2. At 5 V:

   • Given that the current I is 0.091 A, we find: $R = \frac{5}{0.091} \approx 54.9 \Omega$

3. Change in resistance: $\text{Change in resistance} = 54.9 \Omega - 35.7 \Omega = 19.2 \Omega$

The resistance of the filament bulb changes due to the temperature increase as the current flows through it. As the temperature of the filament rises, metal atoms vibrate more vigorously, which makes it harder for electrons to move through the filament. Therefore, the resistance increases with temperature.