Explain ways of reducing unwanted energy transfer through low resistance…

The idea

Wires in a circuit always have some resistance, so some energy is transferred to the thermal store of the surroundings instead of to the component you want to power. The heating of a wire depends on its resistance and on the current: a larger current transfers more energy per second to the surroundings. To reduce this unwanted transfer, use thick wires, because a thicker wire has a larger cross-sectional area and therefore lower resistance. Use short wires, because a shorter wire has lower resistance. Choose a material of low resistivity, such as copper, for connecting leads. Keep the current as small as the task allows. In each case the aim is to lower resistance so that less energy is dissipated as heat in the wires themselves.

What earns the marks

• Identifies that a thicker wire has a larger cross-sectional area and therefore lower resistance, reducing unwanted heating.
• States that a shorter wire has lower resistance and so transfers less energy to the surroundings.
• Selects a low-resistivity material such as copper for connecting wires to reduce resistance.
• Explains that reducing the current in the wire reduces the energy transferred as heat per second.

Common mistakes and how to put them right

• Claims that a thicker wire has higher resistance.

  Put it right by stating that a larger cross-sectional area gives lower resistance.

• Assumes that resistance of a wire is fixed regardless of length.

  Put it right by stating that a longer wire has greater resistance.

• Confuses reducing unwanted transfer with increasing the current.

  Put it right by stating that a smaller current reduces heating in the wires.

Exam tips

• Link each method to the idea of lowering resistance, then to less energy dissipated as heat.
• Name the property of the wire that changes (length, thickness, material) rather than giving a vague answer.
• Check the option you choose actually reduces resistance, not just changes the wire.