Say you were to conduct an experiment with a parallel plate capacitor. You measu

Question

Say you were to conduct an experiment with a parallel plate capacitor. You measure the capacitance for varying plate distance ranging from 2 mm to 12 mm. The measured capacitance should range from about 150 pF to 30 pF. To take the first measurement, set the plate separation to 2 mm and attached leads to the insulated posts of the capacitor. From this you are to linearize the capacitance equation and find the permitivity of free space from the slope. When conducting the experiment, what possible sources of error could result? When doing the experiment, the capacitance reading on the meter fluctuated, but I don't really know why it did so.

Explanation / Answer

source of error :

1. if we do notzero out the capacitance meter.

This step is necessary because the lead wires of the meter are conductors, and will have

a capacitance of their own when they are connected to the parallel-plate apparatus. This

capacitance should not be large, but it must be taken into account.

2. ignoring value of lead capacotance at large vaue of d.

Consulting a reference book on

electronics, one can find that the capacitance of a coaxial cable, such as you will use between

your electrometer and the plates, is typically 1 pF for each cm of cable length. If the coax cable

is 30 cm long, it will have a capacitance of approximately CL ? 30 pF. This ignores the additional

capacitance of the two unshielded leads at the end of the cable. In the pre-lab questions, you will

compute the capacitance of your parallel plates, when they are separated by a typical distance of

10 cm, and you will find a value that is comparable what we just estimated for the cable.

3. the capacitor plates are not parallel.

If the plates are not parallel, ask your TA for help. The plates can be adjusted by turning the mounting

screws, which press against a spring

4.one possible problem

is that you may have used the wrong cable, or connected it incorrectly, or made a loose

connection. If you moved the cables about during the measurement ,that might also have corrupted

your results

5.Another possible problem is electrostatic charge on your clothes

6.As you increase the distance between the plates, you should nd that the relative

error between the theoretical prediction of the capacitance and the measured value of

capacitance will increase. This error is due to fringing eects.

reason for fluctuation of the capacitance reading on the meter :

Observe the digital number on your capacitance meter.
if the number fluctuates during the capacitance reading, there may be a leak in the voltage of the capacitor.

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