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Question
The diagram in Figure shows a cell of e.m.f. ε = 4 volt and internal resistance r = 2 ohm connected to an external resistance R = 8 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
- the key K is open, and
- the key K is closed
Solution
ε = 4 volt
r = 2 ohm
R = 8 Ω
V = ?
I = ?
- When key is open: Since there will be no current flowing through the circuit, the voltmeter reads zero (A = 0) and the electromotive force (e.m.f.) of the cell is two volts.
- When key is closed:
`I = ε/("R" + r)`
= `(4 " volt")/(8 + 2)`
= `4/10`
= 0.4 A
Ammeter reading = 0.4 A
`"R" = "V"/"I"` or V = IR
V = 0.4 × 8
= 3.2 volt
voltmeter reading = 3.2 volt
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