Calculate electrical energy using current potential difference and time
Use E = IVt for a stated constant electrical input and distinguish energy amounts from input rates.
Section P4.2.5 · Electrical energy and electrical power
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Calculate power using P = IV.
- Convert minutes to seconds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many seconds are in 2 minutes?
What you will learn
- Calculate electrical energy with seconds, amperes and volts.
- Keep the component's input separate from useful output.
Add the duration to the power
For constant matching I and V over a specified interval, E = IVt. Amperes, volts and seconds give joules. A component at 6 V carrying 0.5 A for 120 s receives 6 × 0.5 × 120 = 360 J. Its power is 3 W; 360 J is the accumulated amount over the interval.
Convert time before substitution: two minutes is 120 s, not 2 s. Convert prefixed current as well: 200 mA is 0.200 A. The equation refers to the measured component voltage and current, rather than an unrelated whole-source voltage.
Interpret the input energy
E = IVt gives electrical input under the stated conditions. It does not by itself tell you how much heats a separate water sample or lifts a load. Some energy can increase the device's own internal store or be transferred elsewhere. Use additional measured outputs or efficiency information for those questions.
If current or voltage changes, treat the interval in stated steady stages or use an appropriately supplied average power. A single initial current reading is not proof that it lasted throughout the interval. These practice conditions specify constant d.c. values so the simple product is justified.
Sources for this lesson
Pause and explain
At constant 6 V, 0.5 A and 120 s, what is E?
Worked example
Find input energy at constant 6 V and 0.5 A over 2 minutes.
Show the worked solution
- Time = 2 × 60 = 120 s.
- P = 6 × 0.5 = 3 W.
- E = 3 × 120 = 360 J.
Answer 360 J.
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 input energy at 6 V and 0.5 A for 120 s.
Give me a hint
Calculate the electrical input rate and multiply by duration.
Compare my reasoning
- P = 6 × 0.5 = 3 W.
- t = 120 s.
- E = 3 × 120 = 360 J.
360 J.
Look for these in your work
- State joules.
- Keep duration explicit.
Find input energy at 12 V carrying 200 mA steadily for 5 minutes.
Give me a hint
Convert both the current prefix and the time unit first.
Compare my reasoning
- I = 0.200 A and t = 300 s.
- P = 12 × 0.200 = 2.4 W.
- E = 2.4 × 300 = 720 J.
720 J.
Look for these in your work
- Convert both units.
- Use the component input.
A heater receives 720 J electrically. Is the energy gained by its water necessarily 720 J?
Give me a hint
Check the heater, container and surroundings as well as the water.
Compare my reasoning
- The electrical input is known.
- Other stores and transfers can take part.
- The water's gain is not separately supplied.
No; the water's gain needs further energy-account information.
Look for these in your work
- Name other destinations.
- Do not assume all input reaches one sample.
Common mistakes
- Use seconds for a joule answer.
- State the constant-condition assumption.
- Input does not equal useful output without evidence.
What can you explain now?
Which quantity is missing if power alone is known but energy is requested?
Compare with the explanation
The transfer duration.
An energy amount needs the power rate and the interval over which it acts.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compute a constant electrical input in joules and explain the duration, units and output boundary.
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.