Straight-line comparison
EMF & Internal Resistance — AQA A-Level Physics
- Vary the external 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). R using a variable resistor.
- Record pairs of terminal p.d. (V) and 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). (I) for each setting.
- Plot V on the y-axis and I on the x-axis.
- The equation $V = \varepsilon - Ir has$ the form $y = mx + c$.
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- The y-intercept gives the EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). (epsilon).
- The gradientThe rate of change of one variable with respect to another; the slope of a graph at a point. gives the negative of the internal resistanceThe opposition to current flow. The ratio of potential difference to current. 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 energyThe capacity to do work. Measured in joules (J). to be dissipated inside the source. (-r).
- Use a line of best fit — do not force the line through any single data point.
e-diag2
V-I graph for determining EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). and internal resistanceThe resistance within the source of EMF itself, which causes energyThe capacity to do work. Measured in joules (J). to be dissipated inside the source..
V vs I graph showing a straight line with negative gradient. Y-intercept labelled as epsilon. Gradient labelled as -r. Data points scattered around the line of best fit.
Examiner Tips and Tricks
- When reading the gradient, choose two points on the LINE OF BEST FIT — not data points.
- Use a large triangle for accuracyHow close a measurement is to the true or accepted value. High accuracy means low systematic error..
- Convert mA to A if the x-axis is in mA.
Common Mistake
MEDIUM
Students often: Forgetting to convert units when the x-axis is in mA.
Instead: If $gradient = -0.0023 V/mA$, then $r = 0.0023 V/mA = 2.3 V/A = 2.3 ohm. Always convert mA$ to A.
Instead: If $gradient = -0.0023 V/mA$, then $r = 0.0023 V/mA = 2.3 V/A = 2.3 ohm. Always convert mA$ to A.
- Connect the circuit for as short a time as possible when taking readings.
- This prevents the cell from heating up and changing its internal resistanceThe resistance within the source of EMF itself, which causes energyThe capacity to do work. Measured in joules (J). to be dissipated inside the source..
- Use a high-resistance voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. so negligible current flows through it.
- Take at least 6 data points across a wide range of currents.