Test of the reed switch capacitors in series and in parallel

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Title

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

Objective

  • 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

Apparatus

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

Theory

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.

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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. And the capacitance of the capacitor can be estimated by the formula C = I/ Vf.

Capacitors in parallel

If capacitors C1, C2, …, CN are connected in parallel, the charges stored in each capacitor are shown as below:

Q1 = C1V, Q2 =C2V, …, QN =CNV, where V is the common p.d. across the capacitors.

The total charge Q stored in the combination

= Q1 + Q2 + … + QN = C1V + C2V ...

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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.

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.

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.