A cell of emf 12 V and internal resistance 1 ohm supplies a current of 2 A. The terminal potential difference is
Correct answer: C. 10 V
- A. 12 V
- B. 14 V
- C. 10 V
- D. 2 V
Explanation
The lost volts inside the cell are I times r, that is 2 multiplied by 1, giving 2 V, so the terminal voltage is 12 minus 2, which is 10 V. This is why a car's headlights dim when the starter motor draws a very large current. Adding rather than subtracting gives the impossible 14 V.
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About Internal Resistance of Sources
Internal resistance is the opposition to current inside a cell or other source, causing terminal voltage to fall below its emf when current flows. Questions use E = V + Ir, current in complete circuits, lost volts, source combinations, and the difference between internal resistance and the external load resistance.
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