For each question: list what you know, decide which equation to use, then use FIFA. The solution starts by showing how to pick the equation.
Q1 (F) A current of 3.0 A flows for 60 s. How much charge flows?
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Which equation? Know current and time, want charge → Q = It
I: Q = 3.0 × 60
A: 180 C
Q2 (F) A current of 2.0 A flows through a 6.0 Ω resistor. Calculate the potential difference across it.
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Which equation? Know current and resistance, want p.d. → V = IR
I: V = 2.0 × 6.0
A: 12 V
Q3 (F) A 12 V motor draws a current of 2.5 A. Calculate its power.
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Which equation? Know p.d. and current, want power → P = VI
I: P = 12 × 2.5
A: 30 W
Q4 (F) A 60 W lamp is on for 300 s. How much energy does it transfer?
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Which equation? Know power and time, want energy → E = Pt
I: E = 60 × 300
A: 18 000 J
Q5 (F) 5.0 C of charge passes through a 9.0 V battery. How much energy is transferred?
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Which equation? Know charge and p.d., want energy → E = QV
I: E = 5.0 × 9.0
A: 45 J
Q6 (F) A current of 0.50 A flows through a 20 Ω resistor. Calculate the power.
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Which equation? Know current and resistance, no p.d. given, want power → P = I²R
I: P = 0.50² × 20
A: 5.0 W
Q7 (F/H) A 2300 W kettle is plugged into the 230 V mains. Calculate the current.
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Which equation? Know power and p.d., want current → P = VI
I: 2300 = 230 × I
F: I = 2300 ÷ 230
A: 10 A
Q8 (F/H) A component has 6.0 V across it and 0.25 A through it. Calculate its resistance.
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Which equation? Know p.d. and current, want resistance → V = IR
I: 6.0 = 0.25 × R
F: R = 6.0 ÷ 0.25
A: 24 Ω
Q9 (F/H) A 40 Ω heating element carries a current of 1.5 A. Calculate its power.
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Which equation? Current and resistance again → P = I²R
I: P = 1.5² × 40
A: 90 W
Q10 (H) A phone battery stores 3600 C of charge. How many minutes does it take to charge with a current of 1.2 A?
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Which equation? Know charge and current, want time → Q = It
I: 3600 = 1.2 × t
F: t = 3600 ÷ 1.2 = 3000 s
A: 3000 ÷ 60 = 50 minutes
Q11 (H) A 2.0 kW heater is on for 30 minutes. How much energy does it transfer, in MJ?
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Which equation? Know power and time → E = Pt
Convert: 2.0 kW = 2000 W; 30 minutes = 1800 s
I: E = 2000 × 1800 = 3 600 000 J
A: 3.6 MJ
Q12 (H) A 1.2 kW microwave runs on the 230 V mains. Calculate the current, to 2 significant figures.
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Which equation? Know power and p.d. → P = VI
Convert: 1.2 kW = 1200 W
I: 1200 = 230 × I
F: I = 1200 ÷ 230 = 5.217…
A: 5.2 A (2 s.f.)
