1.    Describe an experiment to determine the resistance of a resistor.

• Set up the apparatus as show in the diagram above.

• Adjust the rheostat to the maximum so that the smallest possible current flows in the circuit.

• Record the ammeter reading (I) and the voltmeter reading (V).

• Adjust the rheostat to allow a larger current to flow in the circuit. Record the values of I and V.

• Repeat the procedure to obtain five sets of I and V readings.

• Plot a graph of V against I. Draw a best fit line.

• The gradient of the graph gives the resistance of the resistor.

2.    Define Ohm’s law.

• Ohm’s law states that for metallic conductor, the current passing through it is directly proportional to the potential difference provided physical conditions such as the temperature remains constant

3.    Explain why the component with a voltage-current graph as shown below obeys Ohm’s law (is an ohmic conductor).

• For an Ohmic conductor, the current passing through it is directly proportional to the potential difference provided that the temperature remains constant.

• The graph shown is a straight line through the origin which indicates that V is directly proportional to I.

4.    Draw the voltage versus current graph of a filament lamp and explain why Ohm’s law does not apply to the filament lamp. Give a reason for the relationship.

• The graph is a curve which indicates that V is not directly proportional to I. Therefore, it does not obey Ohm’s Law.

• When the current flows through the filament, its temperature increases. This causes its resistance to increase.