Investigating Capacitors
Physics 4B – Chabot College – Scott Hildreth
Work in your new lab teams on this exercise; record your observations, answers to questions, data
tables, and results in general in a separate report that you will submit, with the names of all active
participants. This lab does not require a formal write up. You should turn it in within one week of
the completion of the lab.
Part 1: Exploring Capacitors with PhET Simulations http://phet.colorado.edu/en/simulation/capacitor-lab
1) Start with the “INTRODUCTION” tab selected. Using the default separation and plate area,
change the battery voltage from -1.5 V to 1.5 V. Record your observations. Which way do
electrons move? Towards what end of the battery?
2) Now connect the meter marked “Plate Charge”, and the Voltmeter. You’ll need to
connect the voltmeter leads to the plates. (Which side goes where?) Change the battery
voltage, and make three measurements of Charge Q, and the corresponding voltage V.
From these, calculate the expected Capacitances, based on the expected relationship of
C = Q/V.
Q (coulombs)
V (volts)
Calculated
Capacitance (farads)
Displayed
Capacitance (farads)
Then, for each value of the battery voltage, verify the calculated capacitances match the
meter values by turning on the Capacitance meter as well, and recording the displayed
capacitances in the table.
3) Now you will investigate what happens
when we change the dimensions of our
capacitor. First, disconnect all meters
and the battery.
3a) Vary the area of the plates. How
does the Capacitance vary with area A?
Illustrate with a calculation using actual
values.
3b) Vary the separation of the plates. How does the Capacitance vary with the
separation distance d? Illustrate with a calculation using actual values.
3c) Using the expected expression for the capacitance of a parallel capacitor, calculate
the constant of proportionality, ε0, from your data. Use the proper number of
significant figures based on your measurements. Compare this with the known value of
the permittivity of free space, to the same number of significant figures.
4) Reconnect the battery and adjust to 1.5 V. Check the View “Electric Field Lines” box, and
the Charge meter box. With the battery connected, watch what happens when the
separation of the plates is increased.
a) What happens to charge when the plate separation is increased, and why? (Please take
note of any charge flow)
b) What happens to the strength of the field when the separation is increased, and why?
5) With the separation set to 5 mm, disconnect the battery.
a) What happens to charge when the plate separation is increased, and why?
b) What happens to the voltage when the plate separation is increased, and why?
c) What happens to the strength of the field when plate separation is increased? (You can
use the field meter to check this.) Why?
6) Repeat step 5, but now add the stored energy meter to the display.
a) What happens to the stored energy when the plate separation is increased? Why?
b) Where does this energy come from, or go to?
7) Now repeat step 6, but with the battery connected.
a) How are the results different than those from Step 6? Why do you think the change
takes place?
b) Where does this energy come from, or go to?
c) Did mechanical work have to be done to increase the separation of the plates? If so, how
do you reconcile this with the fact that the energy stored DECREASED when you increased
separation.
8) Now click on the DIELECTRIC tab at the top of the simulation, and explore what happens as
you insert a dielectric material between the plates. Click on the small radio button “Show
all charges”, and compare what you see to when you just “Show excess charges.” Vary the
type of dielectric as well.
Look at the values in each meter (Capacitance, Plate Charge, Stored Energy, Voltmeter, and
E Field detector.) Make a simple table that shows what happens to each values of these
variables as you move a dielectric material between the plates. Do they increase or
decrease? Or does the quantity stay the same? You don’t need values here – just relative
differences.
9) Develop your own experiment that demonstrates numerically how capacitance changes
with a dielectric, using values from the simulation. Use a standard process of science:
a. What is the hypothesis you want to explore?
b. How will you use the simulation to explore this hypothesis? (What is your
procedure?)
c. What data did you collect as you ran the simulation?
d. Analyzing that data, was your hypothesis supported or rendered invalid?
e. What uncertainties do you have in your data and results?
For example, you might say that you want to explore how the insertion of a dielectric
between the plates changes the E field. Your hypothesis might be that the E field would
decrease, and that C increases. You set up the experiment to show the E field, and record
the plate separation d and Voltage differences of the top and bottom plate (so you can
calculate the E field numerically from V = Ed). You run the experiment with a variety of
voltages, record the results, and verify that the relationship of C = A/d holds where


10) Now click on the MULTIPLE CAPACITORS tab at the top of the simulation, and explore
how capacitance, plate charge, voltage, and stored energy vary with different
configurations of capacitors. Set the battery voltage to some constant value like 1 V
throughout your exploration.
a. For 2 capacitors in series, use the voltmeter to measure the voltage across the
battery, and across each capacitor. Change the capacitance of one of the capacitors
using the slider bars. Verify for Capacitors in Series:
i)
Voltage adds to equal the voltage of the battery
ii)
Charge is the same on the plates (you’ll have to calculate how
much charge is on each independently using Q = CV)
iii)
C equivalent = (1/C1 + 1/C2)-1
b. For 2 capacitors in parallel, use the voltmeter to measure the voltage across the
battery, and across each capacitor. Change the capacitance of one of the capacitors
using the slider bars. Verify for Capacitors in Parallel:
i)
Voltage across each capacitor stays the same
ii)
Charge on each plate adds to the total charge
iii)
C equivalent = C1 + C2
11) Choose either 2 Capacitors in Series + 1 in Parallel, or 2 in Parallel + 1 in Series, and
develop an experiment that demonstrates the expected value for equivalent capacitance.
Make a sketch of your circuit, and record all values. Run the experiment with at least three
trials, with different values of capacitances for each of the components. Verify that charge
is the same on components in series, and voltage is the same across components in parallel.
Adapted from Baird, D. (2011) The Two-Plate Special. Physics Education Tutorials. University of Colorado.
http://phet.colorado.edu/en/contributions/view/3369