Question

Current is passed from a battery having negligible internal resistance through two resistors 12 kilo ohm and 24 kilo ohm in series. An accurate voltmeter of resistance 12 kiloohm reads 4.8 volt when joined across the 24 kiloohm resistor What is the effect of battery and what would be the voltmeter read if joined across the 12 kilo ohm resistor?

Answer 1

Voltmeter reading if it is joined across 12 kilo ohm resistor is 2.4 V

Given-

• Resistor in series are 12 kilo ohm and 24 kilo ohm
• Resistance of voltmeter = 12 kilo ohm
• Reading of voltmeter = 4.8 V

12 and 24 kilo ohm resistors are connected in parallel. So effective resistance is 8 kilo ohm.

Net resistance = 20 kilo ohm

Let E be the emf of the cell

I = E/20 kilo ohm

Now it is given potential drop across voltmeter is 4.8 volt

I × 8 = 4.8 V

E/20 × 8 = 4.8 V

E = 12 volt

Now this 12 kilo ohm are placed parallel to another 12 kilo ohm resistor.

So effective resistance is = 6 kilo ohm

So net resistance = 30 kilo ohm

I = 12/30 kilo ohm

Potential drop = I × 6 = 12 × 6/30 = 2.4 volt