Two identical cells are connected to a resistor R and an ammeter in two ways as shown in the diagrams. The current flowing through the ammeter in both circuits are equal. What is the internal resistance of each cell?

AR/4

BR/2

D2R

Explanation: Series (single cell shown as EMF with internal r, or two cells in series): I₁ = 2E/(R+2r) or single cell E/(R+r). Parallel: I₂ = E/(R+r/2). Setting the currents equal for the two configurations gives r = R. Answer C.

ZIMSEC Physics Paper 1, November 2002, Q15

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Two identical cells are connected to a resistor R and an ammeter in two ways as shown in the diagrams. The current flowing through the ammeter in both circuits are equal. What is the internal resistance of each cell?

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