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Decay of charge

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Experiment C12 (Physics TAS)16-5-2006

-Decay of charge-

Long Report

Shek Pok Man, Dominic

F.6B (24)


Using an ammeter to investigate the decay of charge in a capactior and plot a decay curve of a capactior by using the data.


  • Battery 9V
  • Resistance substitution box
  • Voltmeter
  • Milliammeter 1 mA
  • Capacitor (eletrolytic) 470 F
  • Clip component holder
  • Stop-watch
  • Switch
  • Connecting leads


In the experiment, the change of the charge on the capactior and the change of the discharge current during discharge are investaged by using an milliammeter.

When a capacacitor C is discharged through a resistor R, the capacitor and the resistor have the same potential difference:


Q/C = IR

...read more.


0 e(-t/CR)and Q =(1/2)Q0

We can get e(-t/CR) =1/2[(t1/2)/CR] = In 2 = -0.69  t1/2 = 0.69 CR


1. The circuit was connected as shown below.


2. The capacitor was removed to find out whether the ammeter reading was affected.

3. The capacitor was replaced and the switched was closed

4. The switch was opened and the stop watch was started at the same time. The time taken was marked.

...read more.



Graph: Current (I/μA) against Time (t/s)


(Results and Discussion)


a) from 80 to 40 μA = 31.5s

b) from 40 to 20 μA = 32.5s

The time is nearly the same.


Half-life = (31.5+32.5)/2 = 32s


Time constant CR = 47s

22. Theoretical t1/2 =0.69 CR = 32.43s


The discrepany is very samall, only in decimal places.

Error analysis

The screw given in the experiment was used to adjust the volumeter and ammeters’ initial reading to zero.

It should be noticed that the capacitance of the capacitor is 470 μF instead of 500 μF written in the book.


After doing the experiment, a (I vs t) graph was plotted and we found that the half-life of the decay is 32s, nearly the same as the theoretical value.

...read more.

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Response to the question

The writer has been able to approach the question to an average standard. They were able to explain the theory of capacitance and discharge, and explain how these equations could be used to show that there is a discharge. The ...

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Response to the question

The writer has been able to approach the question to an average standard. They were able to explain the theory of capacitance and discharge, and explain how these equations could be used to show that there is a discharge. The writer was able to explain the idea of half life and as a result mention this during there analysis. What they could have done is explain why they decided to use particular equipment, for example why they used a milliammeter rather then a regular ammeter. The diagram was clear and made a good visualisation of the set up of the experiment.

Due to the nature of the experiment, it was possible for the reader to repeat this experiment so that they would be able to increase their reliability. This is important when doing an investigation. In terms of their graph, they should have labelled their axes clearly as it wasn't very clear what the curve was trying to show and therefore it looks like the writer isn't sure of what they are trying to show. The conclusion was okay, however they could have explained all found data in more detail. The writer explains about half life, but doesn't refer back to the original question. In addition, the writer should have written an evaluation and explain how the investigation could have been improved.

Level of analysis

The writer clearly has some understanding for the science behind the investigation due to the theory work presented before experimentation. However, what the writer should have done is make sure that this was consistent throughout the report. The writer uses phrases such as "The time is nearly the same." which is a really informal way of saying a point. However, it was good that the writer was able to present the data collected in a nice curve. However, due to the graph not being labelled, the graph lost some validity.

Quality of writing

There were a few spelling and grammar mistakes throughout the report, which the writer could avoid by reading through their work and making sure that it made sense when being read. In addition, some of the analysis was not clearly written down. The writer should make sure that the data being calculated is clearly labelled so that the reader is able to understand what they are reading. They should also make sure that the amount of significant figures/decimals places are consistent throughout the report. The writer should also consider doing page numbers so that it's easy to follow.

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Reviewed by cpdavis 29/02/2012

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