A cell maintains a fixed potential difference across its terminals even when no current is drawn from it. (a) When the cell is connected to a resistor, explain the role this potential difference plays in driving current through the circuit. (b) What energy transformation occurs inside the cell to sustain this current, and why does the potential difference across the external resistor fall if the internal resistance of the cell is taken into account?
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(a) The chemical action within the cell generates a potential difference across its terminals. When connected to a resistor, this potential difference sets the charges in motion through the conductor, producing an electric current. It acts like an "electric pressure" that drives electrons around the circuit.
(b) Inside the cell, chemical energy is converted into electrical energy to sustain the current. When internal resistance is considered, some potential difference is used up in driving current through the cell itself (voltage drop = $I \times r$). Thus, the terminal voltage available across the external resistor becomes $V = \varepsilon - Ir$, which is less than the cell's EMF.
Source: Chapter 11, Sections 11.2 and 11.7
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Explanation
- For part (a), the key phrase from the textbook is: "the potential difference sets the charges in motion in the conductor." Use the water-pressure analogy if needed but keep it brief.
- For part (b), examiners expect two things: the energy transformation (chemical → electrical) AND the reason for voltage drop (internal resistance consumes part of the EMF). Writing $V = \varepsilon - Ir$ earns the formula mark even if not explicitly asked.
- Avoid writing long paragraphs; concise, pointed sentences score better in board exams.