Physics Current Electricity DUHS 2022
PMDC Verified Question 68 of 112
A battery of emf 'E' has an internal resistance 'r'. If current 'I' is drawn then terminal potential \(V_t\) is given by:
A
\( V_t = E + Ir \)
B
\( V_t = E - Ir \)
C
\( V_t = \frac{E}{1-r} \)
D
\( V_t = Er \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \( V_t = E - Ir \)
Concept:

When a battery is discharging (supplying current to an external circuit), its terminal voltage is lower than its EMF due to internal losses.

Formula:

$$ V_t = E - Ir $$

Solution:

  • The total EMF (\(E\)) must overcome the internal resistance of the battery itself.


  • The voltage dropped inside the battery is \( Ir \).


  • Therefore, the usable voltage at the terminals is \( E \) minus the internal drop: \( V_t = E - Ir \).


Why other options are incorrect:

  • Opt A: This is the formula for a battery that is being charged by an external source.


  • Opt C, D, E: These are mathematically invalid formulas for battery potential.

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