Internal resistance
EMF & Internal Resistance — AQA A-Level Physics
Key Definition
Internal resistance — The resistance to current flow inside the power supply itself, caused by collisions between charge carriers and atoms within the source.
$$\varepsilon = IR + Ir$$
- Where:
- $\varepsilon$ = EMF of the source (V)
- $I$ = current in the circuit (A)
- $R$ = external resistance (ohm)
- $r$ = internal resistance (ohm)
- Every real cell has internal resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω).The resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). within the source of EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). itself, which causes energyThe capacity to do work. Measured in joules (J). to be dissipated inside the source..
- Charge carriers collide with atoms inside the cell as they flow.
- These collisions transfer energyThe capacity to do work. Measured in joules (J). to the cell's internal structure.
- This is why batteries get hot during use.
- EnergyThe capacity to do work. Measured in joules (J). 'lost' inside the cell cannot reach the external circuit.
Key Definition
Lost volts — The potential difference dropped across the internal resistance of the source. Also called 'lost p.d.'.
$$v = Ir$$
- Where:
- $v$ = lost volts (V)
- $I$ = current (A)
- $r$ = internal resistance (ohm)
Key Definition
Terminal potential difference — The p.d. measured across the terminals of the cell when current is flowing. Always less than the EMF when current flows.
$$V = \varepsilon - Ir$$
- Where:
- $V$ = terminal potential difference (V)
- $\varepsilon$ = EMF (V)
- $I$ = current (A)
- $r$ = internal resistance (ohm)
e-diag1
A real cell modelled as a perfect source in series with its internal resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).The resistance within the source of EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). itself, which causes energy to be dissipated inside the source..
Circuit diagram showing a cell modelled as a perfect EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). source (epsilon) in series with internal resistanceThe resistance within the source of EMF itself, which causes energy to be dissipated inside the source. r, connected to an external resistance R. VoltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. across the terminals reading V (terminal p.d.).
Examiner Tips and Tricks
- Think of a real cell as two things in series: a perfect EMF source and a small resistor r.
- The voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. across the terminals reads V, not epsilon.