Find operating current or potential difference from electrical power
Rearrange P = IV using non-zero supplied values and check the resulting operating condition.
Section P4.4 · Electrical energy
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Use P = IV.
- Convert watts and kilowatts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many watts equal 1.2 kW?
What you will learn
- Find current using I = P/V.
- Find potential difference using V = P/I.
Choose the unknown
If electrical input power P and non-zero component voltage V are known at the same steady condition, current is I = P/V. A component receiving 24 W at 12 V carries 2 A. If P and non-zero I are supplied instead, use V = P/I; 9 W at 0.5 A requires 18 V across that component.
Use the electrical input power. A motor's useful mechanical output is not enough to infer its electrical current unless efficiency or other input information is supplied. Check units before dividing: 1.2 kW is 1200 W, so at a stated steady 240 V the arithmetic current is 5 A.
Check the operating assumptions
A rated power and voltage can identify the rated current under the stated simple model; a different supplied voltage does not guarantee the device retains its rated power. Real components can change their resistance, temperature or control state. Record the stated condition instead of inventing a universal behaviour for the appliance.
Division by zero is not a valid rearrangement. If I and P are both zero, the relation P = IV does not uniquely establish V. An open interval can have a voltage between two points while sustained current is zero. The equation and the measurement context must be read together.
Sources for this lesson
Pause and explain
At 9 W and 0.5 A, what is the voltage?
Worked example
A steady component receives 24 W at 12 V. Find its current.
Show the worked solution
- Rearrange as I = P/V.
- I = 24/12.
- I = 2 A.
Answer 2 A.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find current for a component receiving 24 W at 12 V.
Give me a hint
Divide electrical input power by the non-zero voltage.
Compare my reasoning
- I = P/V.
- I = 24/12.
- I = 2 A.
2 A.
Look for these in your work
- Use input power.
- State amperes.
Find voltage at 9 W and 0.5 A, then current at 1.2 kW and a stated steady 240 V.
Give me a hint
Solve the two cases separately and convert kilowatts first.
Compare my reasoning
- V = 9/0.5 = 18 V.
- 1.2 kW = 1200 W.
- I = 1200/240 = 5 A.
18 V; 5 A.
Look for these in your work
- Choose the correct rearrangement.
- Convert power units.
A motor delivers 60 W mechanically at 12 V. Is its electrical current necessarily 5 A?
Give me a hint
The useful mechanical output is not the supplied electrical input.
Compare my reasoning
- 60/12 would use output power as input.
- Other transfers may occur in the motor.
- Input power or efficiency is needed to determine the current.
No; electrical input power or efficiency is missing.
Look for these in your work
- Identify the power boundary.
- Do not assume perfect efficiency.
Common mistakes
- Use input power.
- Convert kW to W before dividing by volts.
- Zero input and zero current do not uniquely fix voltage.
What can you explain now?
How do you find V when P and non-zero I are supplied?
Compare with the explanation
V = P/I.
Rearrange the same operating relation without reversing the quotient.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Rearrange the power relation with matching input data and avoid inferring device behaviour or missing input power.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.