Power input and output, in watts, W Students should be able to: explain…

The idea

A transformer works because an alternating current in the primary coil produces a continuously changing magnetic field. The iron core carries this changing magnetic field through the secondary coil, which induces an alternating potential difference across it, driving a current if the circuit is complete. Two equations govern this. The turns rule states Vₚ ÷ Vₛ = nₚ ÷ nₛ, so more secondary turns step the potential difference up. The power rule assumes 100% efficiency, meaning power in equals power out (Vₚ × Iₚ = Vₛ × Iₛ). The National Grid uses step-up transformers to increase potential difference and decrease current. Since energy wasted heating cables depends on the square of the current, a smaller current significantly reduces energy transferred to the surroundings.

What earns the marks

• An alternating current in the primary coil creates a changing magnetic field.
• The iron core directs this changing magnetic field through the secondary coil, inducing a potential difference.
• A correct substitution into the turns ratio equation, keeping primary values on the same side.
• Using the principle that power input equals power output to calculate an unknown current.
• Transmitting at a high potential difference reduces the current, which decreases energy wasted heating the cables.

Common mistakes and how to put them right

• Forgetting to mention that the magnetic field must be changing to induce a potential difference.

• Inverting the turns ratio, resulting in a step-up transformer calculating a smaller output potential difference.

• Stating that a step-up transformer increases both potential difference and current, violating the conservation of energy.

Exam tips

• When explaining how a transformer works, explicitly state that the alternating current creates a changing magnetic field.
• Write out the power equation (Vₚ × Iₚ = Vₛ × Iₛ) before substituting values to avoid calculation errors.