Link the potential differences and numbers of turns of a transformer to…

The idea

This Higher tier, separate-science statement asks a learner to connect three ideas. First, the turns ratio of a transformer sets the ratio of potential differences: the ratio of secondary to primary turns equals the ratio of secondary to primary potential difference. Second, an ideal transformer transfers the same power, so if potential difference rises the current falls by the same factor, because power equals potential difference multiplied by current. Third, transmitting at high voltage therefore means a low current, and since heating in cables depends on current, less energy is wasted. A correct multiple-choice response selects the option that keeps power constant while linking turns, potential differences and current, and identifies high-voltage transmission as reducing wasted heating.

What earns the marks

• Selects the relationship that the ratio of secondary to primary turns equals the ratio of secondary to primary potential difference.
• Recognises that an ideal transformer transfers the same power, so an increase in potential difference is accompanied by a proportional decrease in current.
• Applies the power relationship to show that high-voltage transmission carries a lower current for the same power.
• Identifies that a lower transmission current reduces thermal energy wasted in the cables, which is the advantage of high-voltage transmission.
• Rejects options that claim a transformer increases power or that high voltage alone causes greater cable heating.

Common mistakes and how to put them right

• Chooses an option stating that a step-up transformer increases power.

  Put it right by noting that an ideal transformer keeps power the same while changing potential difference and current.

• Inverts the turns ratio.

  Put it right: State that more secondary turns than primary turns gives a higher secondary potential difference.

• Selects an option saying high voltage increases heating in cables.

  Put it right: Explain that the accompanying lower current reduces heating losses.

Exam tips

• Write down the turns-ratio relationship and the power relationship before reading the options, then match each option against them.
• Check whether an option keeps power constant; discard any that implies a transformer creates power.
• Link the chosen option explicitly to reduced current and reduced wasted heating in the cables.