Physics

Physics

Problem

When will the terminal voltage of a real cell be equal to its emf? a) When the external resistance equals the internal resistance, R=rR = r b) When the external circuit is open, R→∞R \rightarrow \infty c) When the external resistance is much larger than the internal resistance, R≫rR \gg r d) When the external resistance is zero (short-circuit), R=0R = 0
Intuitive explanation

The key insight is recognizing that terminal voltage Vt=ε−IrV_t = \varepsilon - Ir, where the voltage drop across internal resistance rr disappears only under one specific condition. Terminal voltage equals emf when there's no current flowing through the cell—think of it as the "ideal" moment before any load demands energy. This happens in an open circuit, where the cell is disconnected from any external circuit, so I=0I = 0 and the internal resistance has no effect on what you'd measure across the terminals.

Intuitive explanation

The key insight is recognizing that terminal voltage Vt=ε−IrV_t = \varepsilon - Ir, where the voltage drop across internal resistance rr disappears only under one specific condition. Terminal voltage equals emf when there's no current flowing through the cell—think of it as the "ideal" moment before any load demands energy. This happens in an open circuit, where the cell is disconnected from any external circuit, so I=0I = 0 and the internal resistance has no effect on what you'd measure across the terminals.

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