WAEC Physics 2021 Theory — Question 24
Question 24 of 35 from the West African Examinations Council (WAEC) Physics 2021 Theory paper, with the correct answer and a full explanation.
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11(b). An electrical device rated 120 V, 60 W is operated on a 240 V, 50 Hz mains supply. The circuit has a capacitor connected in series with the electrical device and the supply. Calculate the capacitance of the capacitor. [π=3.142]
Model answer
Resistance of device R=V²/P=120²/60=240 Ω. Using V²=VR²+VC²: 240²=120²+VC² → VC=207.85 V. Current I=VR/R=120/240=0.5 A. Reactance Xc=VC/I=415.7 Ω. Since Xc=1/(2πfC): C=1/(2π×50×415.7) ≈ 7.66 µF.
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