Question

In the circuit shown in fig. 6, find the reading of the ammeter A when the switch S is
(a) Opened.
(b) Closed.

Answer 1

(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.