These eight questions use original numbers. Write the formula, the substitution and the unit before you open each answer.
The set belongs to SPM Physics electricity. The timed original practice session builder can turn it into a timed attempt.
Questions
Q1. A current of 0.50 A flows for 2.0 minutes. Find the charge that passes.
Answer
Time = 2.0 × 60 = 120 s. Q = I × t = 0.50 × 120 = 60 C.
Q2. A resistor has a potential difference of 6.0 V across it and a current of 0.25 A. Find its resistance.
Answer
R = V ÷ I = 6.0 ÷ 0.25 = 24 Ω.
Q3. The V–I graph of a filament lamp curves away from a straight line. Explain why.
Answer
As the current rises, the filament gets hotter, and its resistance increases. The ratio V ÷ I grows, so the graph bends towards the voltage axis. The lamp is not an ohmic conductor.
Q4. Resistors of 3 Ω, 5 Ω and 4 Ω are in series with a 6.0 V battery. Find the current and the voltage across the 5 Ω resistor.
Answer
Total R = 3 + 5 + 4 = 12 Ω. I = 6.0 ÷ 12 = 0.50 A.
Voltage across 5 Ω = 0.50 × 5 = 2.5 V.
Q5. Resistors of 6 Ω and 12 Ω are connected in parallel. Find the total resistance.
Answer
1/R = 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4, so R = 4 Ω. The answer is smaller than 6 Ω, as it must be.
Q6. A kettle rated 2.0 kW runs on 240 V for 3.0 minutes. Find the current and the energy used.
Answer
I = P ÷ V = 2000 ÷ 240 = 8.3 A (2 s.f.).
Energy = P × t = 2000 × 180 = 360 000 J, which is 0.36 MJ.
Q7. A cell of e.m.f. 3.0 V and internal resistance 1.0 Ω drives a 5.0 Ω resistor. Find the terminal voltage.
Answer
I = 3.0 ÷ (5.0 + 1.0) = 0.50 A. V = IR = 0.50 × 5.0 = 2.5 V.
Check: lost volts = 0.50 × 1.0 = 0.50 V, and 2.5 + 0.50 = 3.0 V.
Q8. An appliance rated 720 W works on 240 V. Fuses of 3 A, 5 A and 13 A are available. Which should be used?
Answer
I = 720 ÷ 240 = 3.0 A. A 3 A fuse would be at its limit and could blow in normal use. The 13 A fuse is too high to protect the wire.
Choose the 5 A fuse, the smallest rating above the normal current.
If you got these wrong
- Q1: relating current, charge and time.
- Q2 and Q3: applying potential difference, resistance and Ohm’s law.
- Q4 and Q5: solving series and parallel circuits.
- Q6 and Q8: calculating electrical energy and power.
- Q7: electromotive force and internal resistance.
Use the simple circuit calculation tutor for more circuit practice and log slips in the mistake log. For a teacher to go through new questions with you, see online one-to-one Physics tuition or the one-hour trial class (from RM50).