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Question
In the circuit shown in fig. 6, find the reading of the ammeter A when the switch S is
(a) Opened.
(b) Closed.
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Solution
(a) When the switch S is opened, resistance 60Ω is not connected in the circuit and current flows through the 20Ω resistor.
:. Resistance of the circuit. when the switch is open:
R= 20 + 5 = 25 Ω
Given, voltage = 10 V
:. current through the circuit, I= `"V"/"R" = 10/25` = 0.4 A
The reading of the ammeter is thus 0 .4 A, when the switch is open.
(b) When the switch is closed, resistance 60 Ω gets connected to the circuit.
Thus, in the circuit the parallel combination of 60 Ω and 200 resistors is connected in series with the 5 Ω resistor.
:. Resistance in parallel, `1/"R"_"P" = 1/60 + 1/20 = (1 + 3)/60 = 4/60` Ω
or `"R"_"P" = 15 Ω
Now, total resistance of the circuit= 15 + 5 = 20 Ω
current through the circuit, I= `"V"/"R" = 10/20 = 0.5 "A"`
The reading of the ammeter is thus 0 .5 A, when the switch is closed.
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