The diagram in the figure shows a cell of e.m.f. $ε = 2$ volt and internal resistance r = 1 ohm connected to an external resistance R = 4 ohm. The ammeter A measures the current in the circuit and the voltmeter V measures the terminal voltage across the cell. What will be the readings of the ammeter and voltmeter when (i) the key K is open, and (ii) the key K is closed?

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Solution

(i) Ammeter reading = 0 because its an open circuit.
Voltmeter reading = e.m.f. of the cell = 2 V (As no current is being drawn from the cell)

(ii) Ammeter reading
$I equals fraction numerator epsilon over denominator left parenthesis R plus r right parenthesis end fraction equals fraction numerator 2 over denominator 4 plus 1 end fraction equals 0.4 space a m p V o l t a g e space r e a d i n g space V o l t a g e space comma space V equals epsilon minus I r V equals 2 minus 0.4 cross times 1 equals 2 minus 0.4 equals 1.6 V$

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