Electrical energy: E = VIt

What you will learn

Calculate energy and rearrange for voltage, current or duration.

Specification mapping: C2.1.4

Prerequisites: C2.1.1, C2.1.3

Physical principles

Combining P = VI with E = Pt gives E = VIt for constant voltage and current, or suitable average power. The energy transferred depends on all three factors. With volts, amperes and seconds the result is joules, because V A = W and W s = J.

Rearrange to t = E/(VI), I = E/(Vt) or V = E/(It). Include brackets in calculator input so the full denominator is used. Convert mA to A, minutes to seconds and kJ to J. A 12 V supply does not imply that every individual component in series has 12 V across it.

In calorimetry, VIt is the electrical energy supplied to the heater. Not all necessarily reaches the sample: the heater, container and surroundings may absorb some. For varying voltage or current, measure power over time and sum or integrate appropriately rather than multiplying unrelated averages.

Scientific diagram and interpretation

Equations and relationships

E = VIt

Worked examples and applications

Common misconceptions

Electrical input energy and thermal energy gained by a sample differ when losses or container heating are significant.

Examination technique

State whether a calculated value is input energy or useful transferred energy.

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.