Question

In which of the following cells, the potential difference between the terminals of a cell exceeds its emf

• A. a
• B. b
• C. c
• D. d

Answer 1

Answer: (B)

Electromotive force is directly related to the source of potential difference, such as the particular combination of chemicals in a battery. However, emf differs from the voltage output of the device when current flows. The voltage across the terminals of a battery, for example, is less than the emf when the battery supplies current, and it declines further as the battery is depleted or loaded down. However, if the device's output voltage can be measured without drawing current, then output voltage will equal emf (even for a very depleted battery).
Terminal voltage is given by the equation V=emf-Ir,
where r is the internal resistance and I is the current flowing at the time of the measurement.
I is positive if current flows away from the positive terminal. The larger the current, the smaller the terminal voltage. Likewise, it is true that the larger the internal resistance, the smaller the terminal voltage.
Hence, the potential difference between the terminals of a cell exceeds its emf when current flows away from the cell.