A circuit is set up as shown. The variable resistor is adjusted so that the ammeter reading decreases.

Question 28

A circuit is set up as shown.

The variable resistor is adjusted so that the ammeter reading decreases.

How do the readings of the voltmeters change?

reading on voltmeter P           reading on voltmeter Q

A                      decreases                               decreases

B                      decreases                               increases

C                     increases                                 decreases

D                     increases                                 increases

Reference: Past Exam Paper – June 2017 Paper 12 Q37

Solution:

Answer: B.

Ohm’s law: V = IR

Current I = V / R

The ammeter gives the value of current flowing in the whole circuit since the components are connected in series.

For the current in the circuit (ammeter reading) to decrease, the total resistance in the circuit should increase.

The resistance of the fixed resistor on the left cannot be increased. It can be concluded that it is the resistance of the resistor that has increased.

Voltmeter Q gives the potential difference across the variable resistor.

V = IR

Since the resistance of the variable resistor has increased, the p.d. across it (reading on voltmeter Q) also increases. [A and C incorrect]

The sum of p.d. in the circuit should be equal to the e.m.f. in the circuit. That is, the sum of readings on voltmeter P and Q should be equal to the e.m.f.

 

Since the reading on voltmeter Q has increased, the reading on voltmeter P should decrease. [B is correct]