Measuring capacitance
Lab 8 – Measuring capacitance
Capacitors are useful circuit elements because they can store charge. For a capacitor, there is a fundamental relationship between the applied voltage and the amount of charge stored on the capacitor. In this experiment you will measure the charge on a capacitor versus the applied voltage for several values of the applied voltage. A plot of charge vs. applied voltage should form a straight line with a slope equal to the capacitance.
(1)
You will use this relation to measure the capacitance of capacitors with different areas and distances between plates. Then you will verify the relation below using these different values.
(2)
objectives
· Determine the mathematical relationship between charge, applied voltage, and capacitance.
· Determine the capacitance of a capacitor.
· Verify the theoretical formula
MATERIALS
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Computer w/ internet access |
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Logger Pro or Excel |
PRELIMINARY SEtup (done in lab)
Do the following in your groups, then return to class to discuss to make sure you know how to use site:
1.Go to the following website:
2. Open the capacitor only screen.
3. Play with the controls until you are familiar with how they work. Make sure you can use the voltmeter to measure the potential difference between the plates.
PROCEDURE
You will measure the capacitance for each capacitor using the following protocol. Note that the theoretical value is given to you at the top of the screen—-your fit of your data should agree with this value.
1.Set the distance and area to the required values.
2.Position voltmeter to measure potential difference.
3.Repeat the following until you have at least 7 points, starting at 0 and going up in steps of 0.2 V.
a) Set the battery voltage to required value.
b) record voltage and charge in first data table.
4.When complete, use Excel or LoggerPro to plot a graph between Q (Y axis) and V (X axis) and do linear curve fitting and determine slope. The slope is C (Cs). Remember to use SI units when plotting! Include all plots with fits in lab report.
5. You will repeat the above for three values of A and d. These values will be given to you by your professor. When you complete this task, fill out the second data table.
6. You will also calculate value of C (Cc) from equation 2.
7. Find percent error between three values of C as shown in the table 2.
DATA TABLEs
Add rows and columns as needed:
Data Table 1
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Voltage (V) |
Q1 (C) |
Q2 (C) |
Q3 (C) |
Same here:
Data Table 2
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capacitor |
A (mm2) |
d (mm) |
Theoretical Cth (pF) |
Calculated Cc (pF) |
Slope – Cs (pF) |
Percent Error 1 for Cth and Cc |
Percent Error 1 for Cth and Cs |
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1 |
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2 |
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3 |
ANALYSIS
1.You have already plotted the Q vs. V and determined the slopes. This is the first part of the analysis. Put these slopes in column 4 of data table 2.
2.For the second part of the analysis, use Eq. (2) and the data in columns 2 and 3 in data table 2 to calculate the capacitance for each capacitor geometry you measured. Put these values in column 5 of data table 2.
3. You will compare the results of C in columns 4 and 5, 4 and 6 in your lab report.
© 2021 Vernier, PHET and modified by Dr Dipti Sharma
