OCR.4.1.2
electromotive force (EMF)
Energy, Power & Resistance - OCR A-Level Physics
Key Definition
electromotive force (EMF)
The work done per unit charge, transferring energy into electrical energy. Symbol: epsilon. Sources include cells, generators, and solar cells.
The work done per unit charge, transferring energy into electrical energy. Symbol: epsilon. Sources include cells, generators, and solar cells.
Key Definition
potential difference (PD)
The work done per unit charge, transferring energy from electrical energy into other energy stores (e.g. light, thermal).
The work done per unit charge, transferring energy from electrical energy into other energy stores (e.g. light, thermal).
$$\varepsilon = \frac{W}{Q}$$
$$V = \frac{W}{Q}$$
- EMFElectromotive force. The energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). by a source in driving chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). around a complete circuit. Measured in volts (V). pushes energyThe capacity to do work. Measured in joules (J). INTO the circuit from the source.
- PDPotential difference — the energy transferred per unit charge between two points. Measured in volts (V). takes energyThe capacity to do work. Measured in joules (J). OUT of the circuit at a component.
- Both are measured in volts (V). 1 V = 1 J $C^{-1}$.
- A voltmeterAn instrument that measures potential differenceThe energy transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V).. Connected in parallel across the component. Has very high resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω).. measures PD. Connect it in parallel.
- An ideal voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. has infinite resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). so it draws no current.
Examiner Tips and Tricks
- EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is energy in, PD is energy out.
- Examiners want you to say 'work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). per unit charge' - not just 'energy per charge'.
Common Mistake
MEDIUM
Saying EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). and PD are the same thing. EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is at the source; PD is across a component. They transfer energy in opposite directions.
Related:Current Electricity
Circuits Kirchhoff