EMF equation (expanded form)
EMF & Internal Resistance — AQA A-Level Physics
- The source supplies 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). (epsilon). This energyThe capacity to do work. Measured in joules (J). splits two ways.
- Some energyThe capacity to do work. Measured in joules (J). drives 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). through the external resistor R (useful).
- Some energy is wasted inside the cell across r (lost voltsThe potential difference across the internal resistance of a cell. Equal to Ir.).
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- In words: $EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). = terminal p.d. + lost voltsThe potential difference across the internal resistance of a cell. Equal to Ir.$.
- $V = IR is the p.d. across the external resistor (what the voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. reads).$
- $v = Ir is the lost voltsThe potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). across the internal resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω).The resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). within the source of EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). itself, which causes energy to be dissipated inside the source. of a cell. Equal to Ir. dropped inside the cell.$
- When I = 0 (open circuit), $V = \varepsilon. The voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. reads$ the full EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V)..
- When I increases, lost voltsThe potential difference across the internal resistance of a cell. Equal to Ir. increase, so terminal p.d. falls.
Common Mistake
MEDIUM
Students often: Using $V = IR$ for the whole circuit, forgetting the internal resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).The resistance within the source of EMF itself, which causes energy to be dissipated inside the source..
Instead: $V = IR$ only applies to the external resistor. For the whole circuit, use $\varepsilon = I(R + r)$.
Instead: $V = IR$ only applies to the external resistor. For the whole circuit, use $\varepsilon = I(R + r)$.
Common Mistake
MEDIUM
Students often: Confusing V (terminal p.d.) with epsilon (EMF). Writing $V = \varepsilon$ when current flows.
Instead: $V = \varepsilon ONLY$ when I = 0. When current flows, V < epsilon because of lost volts.
Instead: $V = \varepsilon ONLY$ when I = 0. When current flows, V < epsilon because of lost volts.