Science (086) — AI-generated practice question

A rheostat is a variable resistor. It works by changing the effective length of the resistance wire in the circuit — a longer length increases resistance, a shorter length decreases it. Since resistance depends directly on the length of the conductor, sliding the contact alters the resistance without touching the source.

By Ohm's law: $I = \dfrac{V}{R}$

The source voltage $V$ remains constant. When the rheostat increases resistance $R$, current $I$ decreases; when it decreases $R$, current increases. Thus, the rheostat controls current by varying the resistance of the circuit.

Source: Chapter 11 (Electricity), Sections 11.3 and 11.4

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• Examiners expect three clear points: (1) what a rheostat does physically (changes effective length → changes resistance), (2) the relevant property (resistance ∝ length of conductor), and (3) Ohm's law to show how changed resistance controls current at fixed voltage.
• Writing the formula $I = V/R$ is essential for full marks — it directly links resistance to current.
• Don't confuse "voltage of the source" with "potential difference across the rheostat"; the source EMF is unchanged, which is what the question asks about.