Power equations must use the component’s operating point
Power can be calculated from the voltage and current at a component’s operating point. A filament lamp’s resistance changes as it heats up, so one fixed resistance cannot describe its whole graph.
Follow the reasoning
- Read voltage and current for the same operating point.
- Calculate power from their product.
- If using a resistance formula, calculate R = V/I at that point.
Try it in a past paper 6
Open the question, then use its mark scheme to check your work.
Question 3
Calculate resistance in a series-and-parallel network.
Build the supporting skills. Practise Resistance before exploring this connection. This question has not been checked as a combined example of the connection.
Question 19
Recognise electrical power as energy transferred per unit time.
Build the supporting skills. Practise Power in circuits before exploring this connection. This question has not been checked as a combined example of the connection.
Question 23(a)–(b)
Calculate microwave-oven efficiency and evaluate the time needed to heat soup.
Build the supporting skills. Practise Power in circuits before exploring this connection. This question has not been checked as a combined example of the connection.
Question 3(b)
Calculate and evaluate the output of a thermistor potential divider.
Build the supporting skills. Practise Resistance before exploring this connection. This question has not been checked as a combined example of the connection.
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Question 5
Find the resistance of a series–parallel network.
Build the supporting skills. Practise Resistance before exploring this connection. This question has not been checked as a combined example of the connection.
Question 19
Combine measurement uncertainties for electrical energy.
Build the supporting skills. Practise Power in circuits before exploring this connection. This question has not been checked as a combined example of the connection.
A curated collection checked against the question papers and mark schemes; not a complete past-paper catalogue. Links open official OCR PDFs in a new tab.
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Specification outcomes (3)
- 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
- 4.2.3(a) Define resistance using R = V / I and state the unit ohm.
- 4.2.3(c)(i) Describe and sketch I–V characteristics for resistor, filament lamp, thermistor, diode and LED.