The diagram shows a circuit used to investigate the variation of current with potential difference for a filament lamp - Leaving Cert Physics - Question 4 - 2011

The component V is a rheostat (or variable resistor). It adjusts the resistance in the circuit, thereby controlling the current flowing through the circuit and the filament lamp.

To plot the graph:

1. Label the x-axis as 'Potential Difference (V)' and the y-axis as 'Current (A)'.
2. Plot the points from the table:
   • (2, 0.9)
   • (4, 1.6)
   • (6, 2.1)
   • (7, 2.5)
   • (8, 2.8)
   • (9, 3.0)
3. Draw a smooth curve through the points to represent the relationship. Make sure to correctly label the axes and ensure the points are reasonably spaced.

The graph indicates that the current increases with potential difference, but not in a linear fashion. This suggests that the filament lamp does not follow Ohm's Law at higher voltages, as the resistance increases with temperature. This is characteristic of non-ohmic materials.

To calculate the resistance (R) using Ohm's Law:
$R = \frac{V}{I}$
From the graph, at V = 5.5 V, estimate the current (I) to be approximately 2.1 A. Then, substituting the values gives: $R = \frac{5.5}{2.1} \approx 2.62 \: \Omega$
Thus, the resistance of the lamp at 5.5 V is approximately 2.62 ohms.