Current, potential difference, energy and power

What you will learn

Define electrical quantities and units: A, V, W, J and Ω.

Specification mapping: C1.2, C1.2.1

Prerequisites: C1.1

Physical principles

Electric current is the rate of flow of charge: I = ΔQ/Δt. One ampere is one coulomb per second. In metals the mobile charge carriers are electrons; conventional current points opposite to their drift. Charge is conserved in a circuit, so a component does not “use up” current.

Potential difference is energy transferred per unit charge between two points: V = E/Q. One volt is one joule per coulomb. A supply transfers energy to charges; circuit components transfer energy from the electrical system to other forms. Potential difference is measured between points, not “through” a component.

Energy is the capacity for transfer by processes such as electrical work, measured in joules. Power is the rate of energy transfer, measured in watts, where 1 W = 1 J s⁻¹. Resistance relates potential difference to current at an operating point, R = V/I, in ohms (Ω = V A⁻¹). A constant resistance requires suitable conditions; not every component is ohmic.

The charge symbol Q in I = Q/t is not the thermal energy ΔQ in later calorimetry equations. Meaning is determined by the equation and unit: coulombs for charge, joules for thermal transfer. Use explicit labels in mixed problems.

Scientific diagram and interpretation

A cell drives a resistor with an ammeter in series. A voltmeter connects across the resistor through a separate parallel branch. The cell’s longer plate is positive.

Equations and relationships

I = Q/t; V = E/Q (supporting definitions)

Worked examples and applications

Energy per charge

A component transfers 24 J when 4.0 C passes. V = E/Q = 24/4.0 = 6.0 V.

Charge flow

At 0.50 A for 20 s, Q = It = 0.50 × 20 = 10 C. The current is the rate, not the accumulated charge.

Common misconceptions

Current is charge per second; voltage is energy per charge. Neither is a quantity of energy stored in a wire.

Examination technique

Pair each definition with its unit equivalence.

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.