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Electrical power can be calculated using P = VI, and for suitable resistive calculations P = I²R. Energy transferred is E = Pt. Joules require watts and seconds; household billing commonly uses kWh, so multiply kW by hours. A kWh is a unit of energy, not power. Read an appliance’s rating and operating time carefully before estimating cost or energy use.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate electrical energy and power
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is a kilowatt-hour?
A unit of energy. It combines power and time.
After watching 2: What is the protective role of an earth wire?
Provide a fault-current path. It helps protective devices disconnect a fault.
What you'll be able to do
Electrical power can be calculated using P = VI, and for suitable resistive calculations P = I²R. Energy transferred is E = Pt. Joules require watts and seconds; household billing commonly uses kWh, so multiply kW by hours. A kWh is a unit of energy, not power. Read an appliance’s rating and operating time carefully before estimating cost or energy use.
The live wire supplies alternating potential difference relative to earth; the neutral provides a return path near earth potential. The earth wire is a protective connection to exposed metal cases. If a fault makes a case live, an appropriate earth path allows a large current that operates a fuse or circuit breaker. The protective device belongs in the live path. A switch being off does not make every internal wire safe, so this lesson is for circuit understanding rather than instructions for working on mains wiring.
The National Grid uses transformers to raise voltage for transmission and lower it for consumers. For the same transmitted power, higher voltage means lower current. Heating losses in cables depend on I²R, so reducing current reduces those losses. Transformers require changing magnetic flux, so ordinary designs operate with alternating supplies. The energy is still conserved: a transformer changes voltage and current rather than creating additional power.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why does raising transmission voltage reduce cable heating for the same power?
Lower current reduces resistive power losses.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.