Practical 13 Internal resistance of a dry cellTASKTo measure the internal resistance of a fresh and a flat dry-cell battery.

THEORYAny device that generates an EMF (such as a battery, a generator, or a solar cell) offers resistance to the passage of electric charge through itself. This resistance is called the internal resistance of the device. For any device:

V = ξ – IRi

where ξ is the EMF of the battery and Ri is the internal resistance in ohms.

EQUIPMENT• dry-cell batteries: fresh, flat, and different types (alkaline, heavy duty, etc.)• DC voltmeter (0–10 V) or voltage sensor• DC ammeter (0–1 A) or current sensor• rheostat• switch• connecting wires

NOTES ON METHOD

ICTMost current sensors have a 1 A range and are suitable for this experiment. They are generally current-protected, but take care to check the manufacturer’s specifications, and use sensors with care. Exceeding the maximum current rating will permanently damage an ammeter or sensor.

Sample rate: Sample voltage and current manually for each setting of the rheostat. Use a digital display to view and record instantaneous values.

Graph: V against I and use your gradient tool, curve fitting or similar to determine the internal resistance from the gradient of the graph.

1 Connect the circuit as shown in the diagram. Adjust the variable resistor until the current through the ammeter is about 0.1–0.2 A, and read both meters.

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2 Alter the variable resistor through about four or five positions over its full range of values, and obtain a set of corresponding readings of current I and potential difference V between the battery terminals.

3 Draw a graph of V against I and use it to find the internal resistance of the battery.

DATA & ANALYSIS1 Construct a table of corresponding values of V and I.2 Draw a graph of V against I using your values.3 From the graph, determine the value of the internal resistance of the battery and its EMF.

CONCLUSIONBased on your results, how does the EMF and internal resistance of a flat dry cell compare with that of a fresh dry cell?How does the internal resistance of an alkaline battery compare with that of a heavy duty battery?

QUESTIONS1 The batteries in a radio have been used for a long time.

Explain why the volume of the radio is low even though the EMF of the batteries is the same as when they were first bought.

2 Does a power pack have internal resistance? If so, what do you think causes it?

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Teacher’s guidePractical 13 Internal resistance of a dry cellType: EP, SM

Requirements: S, ICTConcept: resistance, cellsLevel: intermediateTime: 30+ minutes

EQUIPMENT• dry-cell batteries; fresh, flat,

and different types (alkaline, heavy duty, etc.)

• DC voltmeter (0–10 V) or voltage sensor

• DC ammeter (0–1 A) or current sensor

• rheostat• switch• connecting wires

ICTIn all electrical circuit activities, voltage or current sensors can be substituted for meters, and electronic measure can be used for recording data and drawing graphs. Excellent simulation software, such as Crocodile Clips, is also available, enabling students to construct and explore virtual circuits.

SAMPLE OR EXPECTED DATAResults will vary depending upon the batteries used. The resistance of a flat battery should be significantly higher than that of a fresh battery of the same type. The internal resistance of an alkaline (high-current) battery is larger than that of a heavy duty battery.

ALTERNATIVE ACTIVITIES• For an introduction to electronic measure, use voltage and

current sensors instead of voltmeters and ammeters.• Determine internal resistance of other sources of EMF, e.g.

solar cells.

NOTESAny device that generates an EMF (such as a battery, generator or solar cell) offers resistance to the passage of electric charge through itself. This resistance is called the internal resistance of the device.

This activity could be replaced with a parallel activity from Chapter 14 ‘Investigations: sustainable energy sources’

QUESTIONS & ANSWERS1 The batteries in a radio have been used for a long time.

Explain why the volume of the radio is low even though the EMF of the batteries is the same as when they were first bought.The internal resistance has increased. As a result, less potential difference is available from the battery to work the radio.

2 Does a power pack have internal resistance? If so, what do you think causes it?Yes, all power supplies do. In the case of a power pack, the internal circuitry has some resistance, so it uses some of the total EMF of the power supply before the EMF is supplied to a circuit from the terminals.

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