BRAINCAKE

Exam Style Question - 2

Edexcel AS-level PhysicsElectrical CircuitsLesson 20 of 22

Question

Two heating elements, of resistance 24 Ω and 12 Ω, are connected in parallel to a 12 V supply of negligible internal resistance. The heater is switched on for 5.0 minutes.

(a) Calculate the current in each element and the current drawn from the supply.
(b) Calculate the power of each element.
(c) Calculate the total charge that flows from the supply and the total energy transferred.

(a) The elements are in parallel, so each has the full 12 V across it.

I1 = 12/24 = 0.50 A and I2 = 12/12 = 1.0 A

By the current rule, the supply current is 0.50 + 1.0 = 1.5 A.

(b)

P1 = VI1 = 12 × 0.50 = 6.0 W and P2 = 12 × 1.0 = 12 W

The smaller resistance dissipates the greater power: with the same p.d., P = V2/R, so halving R doubles P. The total power is 18 W, which matches VI = 12 × 1.5.

(c) First convert the time: t = 5.0 × 60 = 300 s.

Q = It = 1.5 × 300 = 450 C
W = QV = 450 × 12 = 5400 J

Check: Pt = 18 W × 300 s = 5400 J.

Exam tip: always convert minutes to seconds before using Q = It or W = Pt. Using t = 5.0 gives an answer 60 times too small.