Voltmeter measures potential difference across the resistor, not the cell

Steps:

• Identify circuit components: cell with emf and internal resistance, switch, resistor, voltmeter across resistor.
• When switch closed, current flows, energy transfers mechanically/thermally via resistor.
• Electromotive force (emf) equals open-circuit voltage; terminal pd drops due to internal resistance.
• When switch open, no current, so voltmeter across resistor reads 0 V, not emf.

Why D is correct:

• Voltmeter connected across resistor measures its pd, which is 0 V (no current) when open, not cell's emf (definition: emf is open-circuit pd across cell terminals).

Why the others are wrong:

• A: Matches emf definition as work per unit charge by cell.
• B: Chemical energy converts to electrical, then heat/light in circuit when closed.
• C: Internal resistance causes voltage drop, so emf > terminal pd (V = emf - Ir).

Final answer: D