Edexcel IGCSEPhysicsGeneral Applications12 Marks

Figure 15 shows a circuit with a 12 V power supply and three resistors.

(a)Calculate the total resistance of the circuit. Show your working.[4]
(b)Calculate the current from the power supply. Show your working.[2]
(c)Calculate the potential difference across the 60 Ω resistor. Show your working.[3]
(d)The 30 Ω resistor is removed from the circuit. Explain what happens to the total resistance of the circuit.[3]

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Mark scheme

Marking points from the official mark scheme. The mark or mark type is shown in brackets.

15 (a) M1 for 1/R_parallel = 1/20 + 1/30
M1 for 1/R_parallel = 3/60 + 2/60 OR 5/60
M1 for R_parallel = 12 (Ω)
A1 for R_total = 12 + 60 = 72 (Ω)
15 (b) M1 for I = V/R OR 12 / 72
A1 for 0.17 (A)
15 (c) M1 for V = I × R OR 0.17 × 60
M1 for using I from (b) and R=60Ω
A1 for 10 (V) OR 10.2 (V)
15 (d) B1 for (total resistance) increases
B1 for (the parallel combination of 20Ω and 30Ω gives) 12Ω
B1 for removing 30Ω means resistance of parallel combination is now 20Ω (as only 20Ω is left) / (effectively 20Ω and 60Ω in series)

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