Electric circuits: Ohm’s law, resistors and the cost of electricity
Answers are for parents and teachers.
1
A 24 V source drives a current of 8 A through a resistor. Find R.
R = 24 V ÷ 8 A = 3 Ω.
2
Find the voltage across a 12 Ω resistor that carries 0.25 A.
V = 0.25 A × 12 Ω = 3 V.
3
Five identical 6 Ω resistors: find Req in series and in parallel.
Series: 5 × 6 Ω = 30 Ω. Parallel: 6 Ω ÷ 5 = 1.2 Ω.
4
A 1500 W appliance runs for half an hour. Find the energy in kW·h, and the cost at an assumed 2 pounds per kW·h.
1.5 kW × 0.5 h = 0.75 kW·h; cost = 0.75 × 2 = 1.50 pounds.
5
Explain why four lamps in a series string all go out when one is removed.
The loop is broken, so there is no closed path for the current.