There is a current from P to R in the resistor network shown. The potential difference (p.d.) between P and Q is 3 V.

Question 32

There is a current from P to R in the resistor network shown.

The potential difference (p.d.) between P and Q is 3 V.

The p.d. between Q and R is 6 V.

The p.d. between P and S is 5 V.

Which row in the table is correct?

Reference: Past Exam Paper – June 2016 Paper 11 Q35

Solution:

Answer: A.

Current flows from P to R.

Recall that current cannot flow in opposite directions in the same wire.

There are different paths that the current could possibly flow.

Path 1: P to Q to R

Path 2: P to Q to S to R

Path 3: P to S to R

Path 4: P to S to Q to R

The p.d. between P and R is constant. This means that the sum of p.d. in any of the paths should be the same.

Let the p.d. between Q and S be QS and the p.d. between S and R be SR.

Considering the different paths,

Path 1: p.d between P and R = 3 + 6 = 9 V

Path 2: p.d. between P and R = 3 + QS + SR

The sum of p.d. in path 2 should also be 9 V as the p.d. between P and R is constant.

3 + QS + SR = 9

QS + SR = 9 – 3 = 6

QS + SR = 6               eq (1)

Path 3: p.d. between P and R = 5 + SR = 9

SR = 9 – 5 = 4 V

Substituting SR = 4 V in eq (1),

QS + 4 = 6

QS = 6 – 4 = 2 V

Path 4 may not be a possible path as the sum of p.d. is different.