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# To determine the internal resistance of a dry cell using an ammeter and a voltmeter.

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Introduction

Date :27-05-06                                    Mark:

Title :Internal Resistance of a Cell

Objective :To determine the internal resistance of a dry cell using an ammeter and a voltmeter.

Apparatus :        Dry cell        2

Ammeter (with 1A and 5A shunts)        1

Voltmeter (with 1 V and 5V multipliers)        1

Rheostat         1

Switch         1

Connecting wires        8

Theory :The e.m.f.,ξ, of a cell indicates how much chemical energy would be transferred to electrical energy when unit charge passes through it. However ,a cell is a complex chemical device, and charge cannot move through it without encountering internal resistance . As a result ,a cell behave in a circus like a pure e.m.f. source in series with a resistor .This internal resistance ,r ,and the equivalent circuit element for a cell are shown below: Notice that when no current is being drawn from the cell ,there is no potential drop across the internal resistance r .

Middle

The switch S was closed and the rheostat was adjusted so that the cell supplied a small current through the circuit.
1. The rheostat was adjusted and different pairs of ammeter reading and voltmeter reading were recorder(note :over the widest possible range).
1. The experiment was repeated by replacing the cell X by cell Y.

Results :

(i) Experiment with the cell X

Measured e.m.f. = 1.40 V

 Ammeter reading I/A 0.05 0.1 0.15 0.2 0.25 Voltmeter readingV/V 1.32 1.18 1.08 0.977 0.855

The graph of the potential difference across the cell V against the current I : (ii)Experiment with the cell Y

Conclusion

The higher is the resistance ,the more accurate is the e.m.f. of the cell .

1. State the main precautions in this experiment .
1. Take the readings only when the values shown on the voltmeter and ammeter are steady.
1. Don’t touch the electric wires which are not wrapped by plastic in order to avoid electric shock.
1. The equipments should be connected correctly.
1. Discuss the sources of error in this experiment .
1. The internal resistance is seldom constant .However ,sometimes we assume it to be constant for simplicity.
1. The resistance of equipments such as wires ,ammeter and voltmeter are assume to be negligible .In fact ,some of the electric energy is dissipated by Joule heating inside the battery.
1. The readings may not reach the final value.

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### Response to the question

Response to question - This was a generally sound report with obvious understanding of how to determine a cell's internal resistance, however it needs work to improve its evaluation of the experiment. The initial discussion of physical principles behind internal ...

### Response to the question

Response to question - This was a generally sound report with obvious understanding of how to determine a cell's internal resistance, however it needs work to improve its evaluation of the experiment. The initial discussion of physical principles behind internal resistance was good and the procedure fine. The results section was OK but units should always be labelled on a graph's axes and the line of best fit should fill a significant portion of the graph, in this case a false origin should have been used so the line was comparatively larger with respect to the axes. It is highly recommended that uncertainties are stated with the final answer, these can often be ascertained from the graph. The questions within the discussion section are not answered to a suitable level. 1) is fine but would benefit from an explanation of the significance of the internal resistance. 2) Voltage can not be drawn. Using the ideas of potential dividers, the writer should show that the voltmeter can not measure the potential difference across itself and this would be unaccounted for if the resistance was not very large compared to the other circuit. 3) The precautions could be more specific I believe these only relate to safety though so iii) is irrelevant. The sources of error would be better if they had been quantified to an extent and if it was explained what the effect would be on the final outcome. Discussion on how these errors could be reduced would also be useful.

### Level of analysis

Level of analysis - Generally analysis was good with relevant physics being handled well. As stated earlier, precision of words is key in science and this could be improved in places. The lack of diagram was poor and errors within the graph let down this piece a bit.

### Quality of writing

Quality of writing - Reports have to concisely communicate the points and this is, for the most part, achieved. The tone is correct and appropriate scientific terms are used throughout.

Reviewed by k9markiii 17/02/2012

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