Say you were to conduct an experiment with a parallel plate capacitor. You measu
ID: 2091483 • Letter: S
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.