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# Investigating Internal Resistance

Extracts from this document...

Introduction

Investigating the internal resistance of a specific lab power pack.

Luke Parker

Planning

Title:

Investigating the internal resistance of a specific lab power pack.

Aim:

To discover the internal resistance of a lab power pack as the EMF and current are altered.

Apparatus:

• Lab power pack
• Wires
• Variable resistor
• Voltmeter
• Ammeter
• Ruler
• Pencil

Diagram (fig.1)

Safety Precautions:

This experiment in general does not have many risks involved however safety precautions need to be taken. The experiment involves electricity; therefore water should be kept well clear. The power should only be supplied for the minimum time possible for results to be taken accurately. This is to prevent the apparatus getting hot and reduce time when the electrics are in use.

Variables:

The possible variables within this experiment are as follows:

• Voltage
• Current
• External resistance

These three will all be varied. The Voltage will be set at a specific setting on the power pack and not be altered until all voltage and current results are recorded for all resistance values. Then the voltage will be changed and once again the voltage and current values taken for each resistance level.

This means the results for each voltage setting will be compared making voltage a fixed variable for a certain set of results. However the voltage setting is changed to repeat the same experiment.

The main variable in this experiment is the external resistance.

Middle

0.07

0.07

R4

1.07

1.07

1.08

0.11

0.11

0.12

R5

1

0.99

1

0.3

0.3

0.31

4V Setting: Emf = 4.7V

 Resistance (Ohms) Voltage (V) Current (A) R1 4.36 4.36 4.35 0.165 0.17 0.16 R2 4.32 4.31 4.32 0.2 0.2 0.2 R3 4.27 4.26 4.26 0.28 0.28 0.28 R4 4.17 4.17 4.18 0.45 0.45 0.45 R5 3.77 3.77 3.76 0.85 0.9 0.9

6V Setting: Emf = 6.66V

 Resistance (Ohms) Voltage (V) Current (A) R1 6.16 6.17 6.16 0.235 0.24 0.24 R2 6.09 6.1 6.1 0.29 0.295 0.29 R3 5.98 5.98 5.99 0.4 0.4 0.4 R4 5.77 5.78 5.77 0.63 0.62 0.63 R5 5.32 5.32 5.31 1.1 1.1 1.1

Tables of averages must also be compiled as this allows voltage and current to be seen as one point and therefore more easily plotted on a graph.

Tables showing the average voltage and current for three voltage settings.

2V Setting:

 Resistance (Ohms) Average Voltage (V) Average Current (A) R1 1.12 0.04 R2 1.11 0.05 R3 1.09 0.07 R4 1.08 0.12 R5 0.99 0.3

4V Setting:

Conclusion

Referring back to my prediction: Based on my background knowledge and the theory I have researched I predict that as the resistance is changed this will affect the voltage, current and internal resistance. As the voltage decreases the current will slightly increase. This makes the internal resistance decrease. Alternatively if the voltage increases the current will decrease and therefore internal resistance will increase.

My prediction also suggests that the results are accurate. The results show that as the voltage decreases when the resistance is changed the current increases. And using the calculations and equations from the theory the internal resistance was also shown to decrease. This is more or less what was stated in the prediction.

Therefore I conclude that the results gathered are sufficient and overall the experiment was successful.

Limitations:

• Unidentified Faulty equipment: give changing or false results
• Environmental conditions: high room temperature may affect resistance and effect results for example.
• Inaccurate method of measuring the external resistance: may give different readings for repeats.

Improvements:

• Test equipment beforehand to ensure everything is working correctly
• Take all the results at the same time to keep experiment in similar environmental conditions throughout.
• Possible computerised way of setting resistance to make sure the value for external resistance is always for repeats.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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