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Resistance is the opposition offered by a conductor to the flow of current.

A good conductor has low resistance.

An insulator, or poor conductor, has high resistance.

Electric wires are made of metal, which have electrical resistance.

The atoms in a solid metal are arranged in a regular lattice structure.

The outer electrons from each atom are free to move through the structure.

When these flow in the same direction a current flows.

However, they may collide with ions of the metal or meet defects in the lattice.

Hence, there is resistance to the current flow, which in turn, reduces the size of the current.

Higher tier

• Describe and carry out an experiment to obtain the voltage–current characteristic graph (V-I graph) for a filament lamp with voltage on the y-axis and current on the x-axis.
• Show that the resistance of a filament lamp increases as the current through the filament increases by taking the ratio of the voltage to the current at different values of the current.

Investigation of the voltage-current (V-I) characteristics of a filament lamp

The voltage across and the current through a component (eg a filament lamp) can be measured and the results plotted on a graph to show the I-V characteristic of the component.

In this practical activity, it is important to measure and observe the voltage and current accurately.

Method

1. Connect the circuit as shown in the diagram above.
2. Adjust the variable resistor until the voltmeter reads 1 V.
3. Record the readings on the ammeter and on the voltmeter in a suitable table.
4. Adjust the variable resistor to increase the voltmeter reading to 2 V.
5. Record the new readings on the ammeter and the voltmeter in the table.
6. Repeat steps 4 and 5, increasing the voltage in steps 1 V, until the voltmeter reads 12 V.

Results