Key Equations

EMF & Internal Resistance — AQA A-Level Physics

On Data Sheet
Not on Data Sheet
EMF equation (expanded)
$$\varepsilon = IR + Ir$$
  • Where:
    • $\varepsilon$ = EMF of the source (V)
    • $I$ = current in the circuit (A)
    • $R$ = external resistance (ohm)
    • $r$ = internal resistance (ohm)
IR = terminal p.d. (useful), Ir = lost volts (wasted inside the cell). On the AQA data sheet as ε = I(R + r).
Terminal p.d.
$$V = \varepsilon - Ir$$
  • Where:
    • $V$ = terminal potential difference (V)
    • $\varepsilon$ = EMF (V)
    • $I$ = current (A)
    • $r$ = internal resistance (ohm)
V decreases as I increases. When I = 0, V = epsilon.
EMF definition
$$\varepsilon = \frac{E}{Q}$$
  • Where:
    • $\varepsilon$ = electromotive force (V)
    • $E$ = energy transferred by the source (J)
    • $Q$ = charge (C)
E is the total electrical energy the source gives to charge Q.
Lost volts
$$v = Ir$$
  • Where:
    • $v$ = lost volts (V)
    • $I$ = current (A)
    • $r$ = internal resistance (ohm)
Energy per coulomb wasted inside the cell.
Back to EMF & Internal Resistance Overview