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Test of the reed switch capacitors in series and in parallel

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Class Number:




11th May, 2008



Test of the reed switch ─ capacitors in series and in parallel


  • To use a reed switch to measure the capacitance of some real capacitors, including those of series and parallel combinations
  • To investigate how the reed switch current varies with the frequency


Reed switch                                x1                Signal generator                        x1

Resistance substitution box        x1                Battery box with 4 cells                x1

Milliammeter                                x1                Voltmeter                                        x1

Capacitors C1 and C2                                Connecting wires


Reed switch current

In the experiment, the reed switch allows the capacitor to be charged up and discharged rapidly. If a capacitor with capacitance C is charged up at a voltage V, the charge Q stored in it will be equal to CV. If the frequency f is operated by the reed switch, the charging up and discharging process will be repeated f times per second, the charge Q in the capacitor will be delivered to the milliammeter at the same rate.

Assuming the capacitor is fully charged and discharged every time, the total charge

Q total passing through the milliammeter per second is equal CVf, which is the theoretical current I.

...read more.


1 + V2 + … + VN = Q/ C1 + Q/ C2 + … + Q/ CN = Q (1/ C1 + 1/ C2 + … + 1/ CN)

Therefore, the equivalent capacitance C of the combination

= Q/V

 1/C = V/Q = 1/ C1 + 1/ C2 + … + 1/ CN


Measuring capacitance

  1. The reed switch was connected in a circuit as shown as below: image00.png
  2. With the signal generator powered off, the frequency and output was set to 100Hz and 0V respectively.
  3. The signal generator was switch on and the output was increased slowly until some ‘Zee’ sound was heard from the reed switch, which meant the reed switch was operating.
  4. The frequency was then increased slowly to the maximum value of 400Hz while the output was changed slowly such that the sound from the reed switch was clear and loud. The frequency finally used was recorded.
  5. A suitable p.d. across the capacitor was applied. The variable resistor was adjusted until a maximum reading was shown on the milliammeter. The reading of the milliammeter was taken.
  6. For the frequency and p.d. used, the theoretical current was estimated by using the formula C = I/ Vf.

Capacitors in series

  1. The set-up was same as the first experiment except connecting 2 capacitors, C1 and C2, in series. The frequency and p.
...read more.


There are edge effects in the capacitors.There is the stray capacitance around the capacitors.The connecting wires contain resistance.The milliammeter and voltmeter are not ideal.There are reading errors in measuring the p.d. across the capacitor and reed switch current.


  1. Digital multimeters should be used for measuring the p.d. across the capacitor and reed switch current.
  2. Other unnecessary things should be removed to reduce stray capacitance.
  3. Connecting wires with smaller resistance like copper wires should be used.


In the experiments, the current measured increases when the frequency of the signal generator increases form 100Hz to 400Hz, holding the p.d. across the capacitor and resistance of the variable resistor constant.

At a fixed frequency value, there is a limiting resistance Rl for delivering the maximum current Imax, holding the p.d. across the capacitor constant. The current increases to Imax when the resistance increases to Rl. The current decreases from Imax when the resistance exceeds Rl.

When the p.d. across the capacitor and the capacitance of the capacitor remains constant, the frequency of the signal generator is directly proportional to the current.

For further investigation, we can study how the limiting resistance varies with different capacitors of different capacitance connected at a fixed frequency value.

...read more.

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Response to the question

Response to question - To what extent has the student answered the set question? How explicit is their response?

The writer has done a good job of answering the question set and as a result produced a good report. ...

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Response to the question

Response to question - To what extent has the student answered the set question? How explicit is their response?

The writer has done a good job of answering the question set and as a result produced a good report. They started off with a preliminary investigation which involved going through the theory of the experiment. The evaluation could have gone into more detail, which would include all the good parts of their experiment and what could have been thoroughly improved. Although the writer had an improvement section, this in general could have gone into more detail and relate these evaluations to the original experiment. It's worth noting that when drawing graphs, by you doing it in a computer program such as Microsoft Excel compared to doing it by hand, you loose a lot of precision and therefore could have a less accurate quantity. I would say using a program such as QTIPlot where you are able to calculate the PMCC and add error bars would be more suitable. The layout is okay, however it could be fixed so that it flows better on the page. In terms of procedure and apparatus, although the writer did write these clearly they should also discuss as to why each piece of equipment is suitable for taking certain measurements. The diagram was very good. They could mention all potential errors within the experiment before actually carrying it out, and mention this within their evaluation to see if these predictions were correct. They could have in addition mentioned any predictions in general before carrying out their experiment when the writer was talking about the theory behind the experiment.

Level of analysis

Level of analysis - To what extent does the writer show appropriate analytical skills for this level of qualification? Have they made evaluative judgements using suitable evidence? Have these examples been developed throughout the response and has an appropriate conclusion been reached?

Physics content is generally correct. Should use SI Units to show understanding of measurements. The writer could discuss precision and accuracy in more detail. As mentioned before, the graph being done by hand would have made results more valid.

Quality of writing

Quality of writing - Is the writing accurate in terms of spelling, grammar and punctuation? Has the writer used technical terms expected at this level of qualification? To what extent does the writer follow conventions and expectations for written work at this level?

Spelling and grammar relatively good. Layout however didn't flow very well. This was noticed when reading the apparatus and when the data was not all in tables. If you put all the data collected into tables and the apparatus in a bullet pointed listed, it will make the layout flow better.

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