Lab 2 - Capacitance

# Discuss the effects of fringing of the fields why is

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Discuss the effects of “fringing” of the fields. Why is Eq. 3 an approximation to the capacitance of a parallel plate capacitor? Something to think about: Dielectrics are insulating materials that are capable of being polarized by applied electric fields; a semiconductor, on the other hand hand may have some non-zero conductance. What do you think might happen if one placed a semiconductor between the plates of the capacitor instead of a dielectric? 3

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6 Variable Capacitor In this section you will measure the change in capacitance of a parallel plate capacitor as plate separation is changed. With this data you will verify that the capacitance is proportional to the inverse of plate separation. Additionally, by measuring the the area of the plate you will also approximate the dielectric constant of the enclosed medium. You will again be using the mounted parallel plates, from the previous section, to construct your capacitor as well as the LCR meter to measure it’s capacitance. Get one of your textbooks (preferably one with close to 1000 pages). Determine the thickness of one single page by measuring the thickness of the majority of the pages with the calipers then divide this thickness by the number of pages enclosed. The sheets of paper will act as the dielectric medium in your capacitor. Starting with 50 pages between them, press the capacitor plates flush together and, with the LCR meter, measure and record the capacitance. Repeat this process with 100, 200, 300, 400 and 500 number of pages. By doing this you’re varying distance between the plates but maintaining the same dielectric material in between. Determine the plate separation corresponding to each of these numbers of pages. Plot the capacitance versus the inverse of plate separation (1 / d). Fit a line to the data and determine its slope (See Section 8.2 for more details on fitting). How do you expect the slope of this line relates to physical characteristics of the capacitor? In the previous section you measured the dimensions of your parallel plates. Using this and the slope of the plot you’ve made, calculate a value for the permittivity of the paper. 7 RC Circuit (qualitative) First, to get a rough idea of the behavior of a charging and discharging capacitor you will use a light bulb in series with a capacitor to monitor the current in the circuit. Unlike the previous sections you will be using capacitors more commonly found in electronics (Figure 1). Recall the brightness of the bulb is proportional to the square of the current passing through it. As current flows through the circuit the capacitor will charge (or discharge) until the voltage of the capacitor matches that of the power supply at which point the current will have dropped to zero. Therefore the brightness of the bulb can be used as a rough measure of how long it takes for the capacitor to charge (or discharge).
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