AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Required practical 6: Investigation of the emf and internal resistance of…
Full specification wording
Electromotive force and internal resistance: Required practical 6: Investigation of the emf and internal resistance of electric cells and batteries by measuring the variation of the terminal pd of the cell with current in it.
This practical links the terminal pd V across a cell to the current I drawn from it.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical links the terminal pd V across a cell to the current I drawn from it. A cell has emf ε and internal resistance r, so V = ε − Ir. Vary I using a variable resistor, recording V with a voltmeter across the cell and I with an ammeter in series. Plot V against I; the intercept on the V-axis gives ε and the gradient gives −r, so r is the magnitude of the gradient. Use a high-resistance voltmeter so it draws negligible current, and switch off between readings to limit heating and polarisation. Repeat and average to reduce random error. For a battery, treat it as a single source with combined emf and internal resistance.
What earns the marks
- Connect the cell, ammeter, variable resistor and switch in series, with the voltmeter connected in parallel across the cell terminals to measure terminal pd.
- Record paired values of current I and terminal pd V over a suitable range, adjusting the variable resistor to vary the current.
- Plot a graph of V (y-axis) against I (x-axis) and draw a line of best fit through the points.
- Determine the emf from the intercept on the V-axis and the internal resistance from the magnitude of the gradient, using r = −gradient.
- Use a high-resistance voltmeter and take readings quickly or switch off between readings to reduce heating and polarisation effects.
- Repeat readings and calculate means, or use a graphical method, to reduce the effect of random errors.
Common mistakes and how to put them right
Connecting the voltmeter in series with the cell instead of in parallel across the cell terminals.
Put it right by placing the voltmeter across the cell so it measures terminal pd.
Assuming the gradient of the V–I graph equals +r.
Put it right by recognising the gradient is −r, so r is the magnitude of the gradient.
Leaving the circuit connected continuously so the cell heats and its internal resistance changes.
Put it right by switching off between readings and taking readings quickly.
Exam tips
- State clearly that the intercept is the emf and the magnitude of the gradient is the internal resistance, and show the gradient calculation with the coordinates used.
- Explain why a high-resistance voltmeter is used: it draws negligible current so the measured pd is close to the true terminal pd.
- When describing improvements, link each change to the error it reduces, such as switching off to reduce heating or repeating to reduce random error.
Try the lesson
The start of the Electromotive force and internal resistance: Required practical 6: Investigation of the emf and internal resistance of electric cells and batteries by measuring the variation of the terminal pd of the cell with current in it. lesson, made from checked teaching notes for this specification point.
Have a go first
In Required practical 6, which quantity is plotted on the y-axis when the terminal pd of a cell is measured as the current drawn from it is varied?
Listen
This practical links the terminal pd V across a cell to the current I drawn from it. A cell has emf ε and internal resistance r, so V = ε − Ir. Vary I using a variable resistor, recording V with a voltmeter across the cell and I with an ammeter in series. Plot V against I; the intercept on the V-axis gives ε and the gradient gives −r, so r is the magnitude of the gradient. Use a high-resistance voltmeter so it draws negligible current, and switch off between readings to limit heating and polarisation. Repeat and average to reduce random error. For a battery, treat it as a single source with combined emf and internal resistance. A strong answer earns marks for these points. Connect the cell, ammeter, variable resistor and switch in series, with the voltmeter connected in parallel across the cell terminals to measure terminal pd. Record paired values of current I and terminal pd V over a suitable range, adjusting the variable resistor to vary the current. Plot a graph of V (y-axis) against I (x-axis) and draw a line of best fit through the points. Determine the emf from the intercept on the V-axis and the internal resistance from the magnitude of the gradient, using r = −gradient. A common slip: Connecting the voltmeter in series with the cell instead of in parallel across the cell terminals. Instead, placing the voltmeter across the cell so it measures terminal pd. Exam tip: State clearly that the intercept is the emf and the magnitude of the gradient is the internal resistance, and show the gradient calculation with the coordinates used.
Key words
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Electromotive force and internal resistance
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Terminal potential difference
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Graphical analysis of linear relationships
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Experimental uncertainty and improvement
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See every definition
- Electromotive force and internal resistance
- A cell has emf ε and internal resistance r, so the terminal pd V equals ε minus Ir.
- Terminal potential difference
- The pd V across the cell terminals, measured with a voltmeter connected in parallel across the cell.
- Graphical analysis of linear relationships
- Plotting V against I gives a line of best fit whose V-axis intercept is ε and whose gradient magnitude is r.
- Experimental uncertainty and improvement
- Repeat readings and calculate means, or use a graphical method, to reduce the effect of random errors.
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