Key Equations
Current Electricity — AQA A-Level Physics
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Resistance
$$R = \frac{V}{I}$$
- Where:
- $R$ = resistance (Ω)
- $V$ = potential difference (V)
- $I$ = current (A)
Definition of resistance. Applies to ALL components, not just ohmic ones.
Current
$$I = \frac{\Delta Q}{\Delta t}$$
- Where:
- $I$ = current (A)
- $\Delta Q$ = charge flow (C)
- $\Delta t$ = time interval (s)
Rearranges to Q = It.
Potential difference
$$V = \frac{W}{Q}$$
- Where:
- $V$ = potential difference (V)
- $W$ = work done / energy transferred (J)
- $Q$ = charge (C)
1 V = 1 J C⁻¹.
Resistivity
$$\rho = \frac{RA}{L}$$
- Where:
- $ρ$ = resistivity (Ω m)
- $R$ = resistance (Ω)
- $A$ = cross-sectional area (m²)
- $L$ = length (m)
Material property. Unit is Ω m, not Ω m⁻¹.
Electrical power
$$P = IV$$
- Where:
- $P$ = power (W)
- $I$ = current (A)
- $V$ = potential difference (V)
Primary power equation. Combine with R = V/I for the derived forms below.
Power (current form)
$$P = I^{2}R$$
- Where:
- $P$ = power (W)
- $I$ = current (A)
- $R$ = resistance (Ω)
Derived from P = IV and V = IR. Useful when V is unknown.
Power (voltage form)
$$P = \frac{V^{2}}{R}$$
- Where:
- $P$ = power (W)
- $V$ = potential difference (V)
- $R$ = resistance (Ω)
Derived from P = IV and I = V/R. Useful when I is unknown.