Electrical power: P = IV

What you will learn

Power, voltage and current; rearrangements, units and operating conditions.

Specification mapping: C2.1, C2.1.1

Prerequisites: C1.2, C1.3

Physical principles

Electrical power transferred by a component is P = IV, using the potential difference across that component and the current through it. With I in amperes and V in volts, P is in watts. The equation follows from energy per charge multiplied by charge per second.

Useful rearrangements are I = P/V and V = P/I. Values must describe the same component and operating state. Do not multiply total source voltage by one unrelated branch current to find every component’s power. For multiple components, total input power equals the sum of transferred powers in a steady circuit after accounting for all relevant parts.

P = IV is directly suitable for DC or appropriate instantaneous quantities. For mains appliances in this course, use the supplied operating voltage and rated power for the intended calculation. Real AC systems with phase differences can require power factor; do not generalise a simple rated-appliance calculation to every AC load.

Power is a rate, not energy. A high-power device can transfer less total energy than a low-power device if operated for a much shorter time. The rated power also may differ from actual average power when a thermostat or control cycles the load.

Scientific diagram and interpretation

Equations and relationships

P = IV

Worked examples and applications

Common misconceptions

Watts measure energy per second, not stored energy. Use matching current and voltage readings.

Examination technique

Write P = IV before substitution and check whether a milliamp reading needs conversion.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.