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Objects that store charge are called capacitors; the size of their capacitance is usually measured in micro-farads (mf). A common capacitor is made from two long strips of metal foil separated by a strip of insulator (waxed paper or plastic).

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Introduction

Scientific Knowledge

Objects that store charge are called capacitors; the size of their capacitance is usually measured in micro-farads (mf). A common capacitor is made from two long strips of metal foil separated by a strip of insulator (waxed paper or plastic).

If one strip is charged negatively and the other positively charged they store electrical charge. It is storing energy rather like a small rechargeable battery. Also the amount of charge stored is measured in coulombs.

The symbol for a capacitor in a circuit is

The time taken for a capacitor to reach two thirds of the supply voltage is called the time constant and is given by

T=RC

  • T= time in seconds
  • R= ohms
  • C= farads

There are two elements which may affect how long it takes for a capacitor to charge up the resistance (given in ohms) or the size of the capacitor (given in farads or micro-farads). The greater the resistance the longer the capacitor will take to charge up and the bigger the capacitance the longer it will take for the capacitor to reach two thirds of the supply voltage.

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Middle

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Enormous amounts of energy are stored in the banks of capacitors that power giant lasers in national laboratories.

Key variables

  • Size of capacitor
  • Size of resistance
  • Size of supply voltage

Chosen variable

I have chosen to vary only the resistance as we have a good range of resistors in school, which will give a large spread of results. Also it would be difficult to alter the supply voltage as a whole class of students are carrying out this experiment at the same time in the same room, all sharing the same supply voltage. This would prove to be impossible to coordinate.

Aims of the experiment  

I aim to find out how the size of the resistor will affect the rate at which a capacitor charges. Hopefully, I will find a strong correlation between the formula and my results but also I am hoping for slight differences, which I can explain and improve this experiment to eradicate them from further investigations.

Prediction

As I vary the size of the resistor I expect the rate at which the capacitor charges to vary too. According to scientific theory, the relationship between the size of the resistor and the rate at which the capacitor charges should be that of when I raise the size of the resistor the rate at which the capacitor charges should decrease.

Using the formula (T=RC) I can predict the following: the resistance (    ) multiplied by the capacitance (   ) will equal the time taken for the capacitor to reach two thirds of the supply voltage.

Predicted results using the formula

Resistor/ohms

Time taken to reach two thirds of the supply voltage using the formula t=rc (seconds)

1k

1

4.7k

4.7

5.6k

5.6

10k

10

100k

100

150k

150

680k

680

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Conclusion

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The time taken in seconds for the capacitor to reach two thirds of the supply voltage

Averagetime in seconds

1

2

3

1k

1.46

1.90

1.45

1.60

4.7k

4.69

5.01

4.86

4.85

5.6k

6.56

6.11

6.30

6.32

10k

11.15

10.87

10.67

10.90

100k

109.64

107.67

108.80

108.70

150k

168.16

166.51

170.26

168.31

680k

698.34

695.56

701.42

698.44

This is just a small sample of results to help me plan the main practical. Hopefully it will help me to improve and enable me to plan my experiment more efficiently.

Fair test

To keep this investigation fair I must keep every factor the same except for varying the size of the resistor, which I must keep constant throughout my experiment. To do this, I must always keep the same capacitor as even though there maybe others with the same capacitance they still may vary a little. I must always use the same supply voltage which is 9 volts and the same connection in the class room as the voltage does vary very slightly as you move around the room. The same person must do the timing as reaction times vary and so may the accuracy.

(George Barrett)

...read more.

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