Question

A voltmeter of resistance 400 Ω is used to measure the potential difference across the 100 Ω resistor in the circuit shown in the figure (32-E24). (a) What will be the reading of the voltmeter? (b) What was the potential difference across 100 Ω before the voltmeter was connected?
figure 32-E24

Answer 1

But where is the figure ??

Answer 2

Explanation:

(a) The circuit’s effective resistance,

Reff = 100 × 200 + 400500 = 280 Ω

Through the circuit, the current is I = 84280 = 0.3 A.

As 400 Ω and 100 Ω resistor are linked in parallel, across their ends, there will be same potential differences. Let’s consider the current via the resistor of 100 Ω be i1 ; then, via 400 Ω resistor, the current will be i – i1.

100i1 = 400i - i1 ⇒ii = 0.24 A

The voltmeter’s reading = 0.24 $\times$ 100 = 24 V

(b) The two resistors 200 Ω resistor and 100 Ω resistor are in series before the connection of the voltmeter.

The circuit’s effective resistance,

Reff = 100 + 200 = 300 Ω

Through the circuit, the current is,

i = 84300 = 0.28 A

Therefore, across the 100 Ω resistor, the voltage = (100 $\times$ 0.28) = 28 V