4.2.3: Resistance

All components in a circuit have their own resistance. Resistance is the ratio between the pd across a component and the current in a component. The unit of resistance is the ohm. The ohm is the resistance of a component when a pd of 1 V is produced per ampere of current. In other words, 1 Ω is 1VA⁻¹.

Resistance can be calculated using the equation V=IR.

Ohms law states that or a metallic conductor kept at constant temperature, the current in the wire is directly proportional to the p.d. across its ends.

It is important to remember that temperature affects resistance (which is why Ohms law states at a constant temperature). In the example graph below, the p.d of the wire remains constant (1.5V) but the current deceases with time. This is because resistance increases with time (by looking at V=IR we can see that if V is constant then I is inversely proportional to R). Over time, the temperature of the wire increases and as the wire gets hotter its resistance increases. This is because the positive ions inside the wire have more internal energy therefore they vibrate with greater amplitude about their mean positions. The frequency of collisions between charge carriers and positive ions increases and this means that charge carriers do more work (transfer more energy as they travel throughout the wire).

Photo credit: kerboodle OCR A physics textbook p.147

As well as temperature, the material of the wire, the length of the wire, and the cross-sectional area of the wire all affect resistance.