Electrical power and energy
Distinguish a rate of energy transfer from the total energy transferred.
What you will learn
- Use P=VI and E=Pt.
- Derive alternative power formulas for resistive components.
Power is a rate
Potential difference is energy transferred per unit charge and current is charge per unit time. Multiplying them gives energy per unit time, which is power. The watt is a joule per second.
For a constant power, multiply by elapsed time to calculate energy. Use seconds with watts if the result is needed in joules. Energy in kWh uses kilowatts and hours instead.
Use the quantities you know
Combining P=VI with V=IR gives P=I²R or P=V²/R for a resistive component. Do not use these substitutions blindly for complex alternating-current components; the simple starter calculations concern direct-current circuit quantities.
Worked example
A component has 10 V across it and carries 0.6 A for 2 minutes. Find power and energy transferred.
Show the worked solution
- Power is 10×0.6=6 W.
- Convert two minutes to 120 seconds.
- Energy is 6×120.
Answer 6 W and 720 J
Common mistakes
- A watt is not a unit of energy.
- Remember to square the current in I²R.
What can you explain now?
A 5 Ω resistor carries 2 A. Find its power.
Compare with the explanation
20 W
P=I²R=4×5=20.
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These lessons teach the topics represented in our current practice sets. They are not a complete course for every paper or option in the qualification.
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