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# Find The Internal Resistance Of A Power Supply

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Introduction

## Coursework To Find The Internal Resistance Of A Power Supply

The electrical supply on satellites needs to be kept at a constant voltage and the lost volts need to be taken into account when drawing different currents.

## Aim

The aim of this investigation is to find the internal resistance of a power supply and to see if this value changes at five different voltage settings. Using a variable resistor to vary the load resistance at different voltage settings and obtaining both ampere and voltage readings and then plotting graphs of terminal potential difference against current to find the different values for the internal resistance of the power supply.

## Theory

We can investigate the internal resistance of a power supply by setting up a circuit such as the one shown in figure 1. The voltmeter is connected in parallel to measure the potential difference across the power supply and the ammeter is connected in series to measure the flow of current.

We would connect the voltmeter across the terminals of the power supply. The variable resistor is used to vary the load resistance to gain a series of values for current and corresponding voltmeter readings. Given that the power supply supports ohms law, potential difference is proportional to current under constant physical conditions. Due to this relationship, a graph of current (amps) against terminal potential difference (volts) can be drawn which can be used to work out the internal resistance of the cell. This has been shown in figure 2.

In figure 2, he power supply in the circuit is supplying a current (amps) to the external circuit. The internal resistance (r) of the power supply is constant provided that temperature of the equipment is constant.

Middle

Voltage3

(volts)

Average Voltage

(volts)

Current1

(amps)

Current2

(amps)

Current3

(amps)

Average Current

(amps)

1

3.95

3.96

3.95

3.95

0.22

0.22

0.22

0.22

2

3.91

3.92

3.92

3.92

0.39

0.38

0.39

0.39

3

3.89

3.90

3.89

3.89

0.53

0.53

0.52

0.53

4

3.86

3.86

3.85

3.86

0.69

0.69

0.67

0.68

5

3.78

3.78

3.77

3.78

0.89

0.89

0.89

0.89

6

3.70

3.69

3.69

3.69

1.13

1.14

1.14

1.14

7

3.58

3.62

3.57

3.59

1.46

1.47

1.46

1.46

8

3.43

3.47

3.41

3.44

1.90

1.71

1.83

1.81

 Setting 6V Voltage1(volts) Voltage2(volts) Voltage3(volts) Average Voltage(volts) Current1 (amps) Current2(amps) Current3(amps) Average Current(amps) 1 5.73 5.75 5.73 5.74 0.28 0.27 0.28 0.28 2 5.72 5.70 5.66 5.69 0.48 0.48 0.47 0.48 3 5.67 5.67 5.60 5.65 0.69 0.69 0.69 0.69 4 5.63 5.62 5.56 5.60 0.88 0.88 0.88 0.88 5 5.53 5.53 5.48 5.51 1.12 1.13 1.11 1.12 6 5.44 5.40 5.38 5.41 1.54 1.51 1.53 1.53 7 5.17 5.17 5.23 5.19 1.98 2.01 2.03 2.01 8 4.97 4.92 4.94 4.94 2.73 2.81 2.76 2.77
 Setting 9V Voltage1(volts) Voltage2(volts) Voltage3 (volts) Average Voltage(volts) Current1 (amps) Current2(amps) Current3(amps) Average Current(amps) 1 8.60 8.67 8.64 8.64 0.36 0.36 0.36 0.36 2 8.45 8.50 8.48 8.48 0.61 0.61 0.62 0.61 3 8.40 8.43 8.40 8.41 0.89 0.89 0.89 0.89 4 8.30 8.33 8.28 8.30 1.23 1.23 1.21 1.22 5 8.16 8.19 8.18 8.18 1.75 1.75 1.74 1.75 6 7.97 8.01 8.03 8.00 2.41 2.38 2.34 2.38 7 7.72 7.72 7.84 7.76 3.15 3.03 3.01 3.06 8 7.38 7.33 7.38 7.36 4.15 3.93 3.97 4.02
 Setting 12V Voltage1(volts) Voltage2 (volts)

Conclusion

The temperature of the power supply was increasing as I changed from a lower voltage setting to a higher one. To avoid this scenario, I could do this experiment in colder temperatures such as in a freezer. This will help ensure that the power pack stays cool and does not heat up, so helping to keep the internal resistance constant. The freezing environment will also lower the temperature in the wires and therefore the resistance in the wires will be more negligible. Collisions between electrons flowing through the circuit and positive ions in the metal are reduced because fewer vibrations occur due to the less heat energy provided by the surroundings.

Anomalous results:

In the tables on pages 5 and 6, I have highlighted two cells which I have counted as anomalous results. I have omitted these results when calculating the average voltage and current because they skew the average higher. Such a high current reading of 1.60A compared with the other two results at the same resistance setting could have occurred because the power supply was delivering peak current at that point in time. The other highlighted cell showing a low voltage reading of 9.00V could have been because of a poor connection in the circuit and therefore a lower reading was obtained. I have not decided to repeat these results because only one result from three repeats was anomalous and therefore it is reasonable to say that the other two results are reliable as they are very close to each other.

Bibliography

Books:

Title: Salters Horners Advanced Physics (AS level)

Publisher: Heinemann

First published 2000

Page reference: pg81, internal resistance and layout for equations.

Title: A Level Physics

Authors: Jim Breithaupt and Ken Dunn

Publisher: Letts

First published: 1983

Edition: 5th revised edition

Page reference: pg208 – pg209, steps for working out lost volts and understanding of electromotive force.

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