LU5_Capacitors
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Name ______________________ Class Section ________ Date _____________
PHY 242 – Laboratory
LABORATORY 5: CAPACITORS
Objectives:
Determine the relationship between charge and voltage for a capacitor.
Explore the effect of plate separation and plate area on the capacitance
of a parallel plate capacitor in a circuit.
Determine the relationship between the energy stored in a capacitor
and the charge on the plates and the voltage for capacitor.
Materials Required:
Computer with Excel and access to simulation
Capacitor Lab Basics
: https://phet.colorado.edu/en/simulation/capacitor-lab-basics
Software Requirements
: Windows
Macintosh
Chromebook
Linux
iPad
Mobile Phone
Chrome, Edge
Chrome, Safari
Chrome
Not recommended
Safari
Not recommended
Introduction:
A capacitor
is an electronic component that stores electrical energy in an electric field. Most capacitors contain two
electrical conductors separated by a nonconducting dielectric medium (glass, ceramic, plastic film, paper, mica, and oxide layers) acting to increase the capacitor's charge capacity. When two conductors experience a potential difference, an electric field develops across the dielectric, causing a net positive charge to collect on one plate and net negative charge to collect on the other plate. Capacitance is defined as the ratio of the electric charge on each conductor to the potential difference between them C
=
Q
V
. The unit of capacitance in the SI is the farad (
F
=
1
C
/
V
). A parallel plate capacitor, the simplest model capacitor, consists of two conducting plates, each of area A
, separated by a gap of thickness d
containing a dielectric. It is assumed the gap is much smaller than the dimensions of the plates. The capacitance of a capacitor is proportional to the surface area of the plates (conductors) and inversely related to the gap between them C
=
ε
o
A
d
. To increase the charge and voltage on a capacitor,
work must be done by an external power source to move charge from the negative to the
positive plate against the opposing force of the electric field. The work required to move the
1
charge is stored as energy in the increased electric field between the plates. The total stored energy U
can be calculated as: U
=
1
2
C V
2
=
1
2
QV
=
1
2
Q
2
C
.
In a simple parallel-plate capacitor, a voltage applied between the two conductive plates creates a uniform electric
field between those plates. The electric field strength in a capacitor is directly proportional to the voltage applied
and inversely proportional to the distance between the plates: E
=
V
d
. Since the electric field strength must not
exceed the breakdown field strength of the capacitor, there is a maximum rated voltage for any capacitor. If the
breakdown voltage is exceeded, an electrical arc is generated between the plates, and can destroy the capacitor
instantly. Activity 1: Relationship between the Charge on the Plates and Voltage
1.
Start the Capacitor Lab Basics
PhET simulation and explore it. Choose the Capacitance
tab. Make sure that
the Plate Charges
, Bar Graph
(
Capacitance, Top Plate Charge, and Stored Energy), and Electric Field
boxes are checked. 2.
Select a separation distance d
and a plate area A
. Once you made such a selection, the capacitor has a fixed capacitance C
. List the corresponding values below:
3.
Connect the battery to the plates of the capacitor. Use the battery slider to increase the battery potential difference (voltage) V
battery
to the maximum value (
1.5
V
)
4.
Connect one of the voltmeter’s leads to the top plate, and the other lead to the bottom plate, to measure the difference in potential V
between the plates. Use the Top Plate Charge
on the Bar Graph
meter to read the charge Q
on the top plate of the capacitor. Record them in Table 1 below.
5.
Without changing the separation distance d
and the plate area A
, use the battery slider to change the
potential difference between the plates and the charge on the top plate of the capacitor. Record the values in
Table 1. Repeat the procedure to have at least five sets of values.
Table 1
Battery Voltage, V
battery
(
V
)
Capacitor Voltage, V
(
V
)
Top Plate Charge, Q
(
10
−
12
C
)
2
A
=
¿
C
=
¿
6.
Use Excel
to plot a graph of Q vs .V
(see LU0_Excel file for help). Make sure that your graph is a scatter
plot
. Customize the graph - graph title and label the axes (using the Chart Tools
menu). Add the best-fit
line passing through your data points (use the Trendline
menu) and check the Display Equation on Chart
option near the bottom. Insert a copy (screenshot) of your graph in the space below.
7.
Does a linear fit describe the dependence of the charge on the top plate Q
on the difference in potential V
between the plates? Record the slope of the line.
8.
Compare the slope of the graph to the capacitance recorded above, by calculating the % diff.
%
diff
=
|
slope
−
recorded capacitance
recorded capacitance
|
×
100
Activity 2: Dependence of the Capacitance of a Parallel Plate Capacitor on the Plate Separation
9.
Within the Capacitor Lab Basics
PhET simulation, choose the Capacitance
tab. Make sure that the Plate Charges
, Bar Graph
(
Capacitance C
, Top Plate Charge Q
, and Stored Energy U
), and Electric Field
E
boxes are checked. 10.
Connect the battery and set the plates to the maximum area (400.0 mm
2
), maximum separation (10.0 mm) and maximum positive battery voltage (1.5 V) to begin.
11.
Using the provided meter and the Bar Graph
readings in the simulation fill-in Table 2 below.
Table 2:
Trial
V
(V)
d
(mm)
A
(mm
2
)
C
(F)
U
(J)
Q
(C)
E
(V/m)
1
1.5
10.0
400
2
1.5
8.0
400
3
1.5
6.0
400
4
1.5
4.0
400
5
1.5
2.0
400
12.
Which of the calculated variables (
C
, U
, Q
, E
) increase and which decrease as the plates are moved further
apart (i.e. d
increases)?
3
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13. Use Excel
to plot a graph of C vs.
1
d
. Add the best-fit line passing through your data and display its
equation on graph. Insert a copy (screenshot) of your graph in the space below.
14. Does a linear fit describe the dependence of the magnitude of the capacitance C
of the capacitor on the
inverse of the distance between the plates 1
d
?
15.
The density of the electric field lines is proportional with the strength of the electric field in a certain area of
space. Visually, does the change in the density of electric field lines as d
change, confirm the change in the calculated value of E
listed in Table 2? Activity 3: Dependence of the Capacitance of a Parallel Plate Capacitor on the Plate Area
16.
Within the Capacitor Lab Basics
PhET simulation, choose the Capacitance
tab. Make sure that the Plate Charges
, Bar Graph
(
Capacitance, Top Plate Charge, and Stored Energy), and Electric Field
boxes are checked. 17.
Connect the battery and set the plates to the maximum area (400.0 mm
2
), maximum separation (10.0 mm) and maximum positive battery voltage (1.5 V) to begin.
18.
Using the provided meter and the Bar Graph
readings in the simulation fill-in Table 3 below.
Table 3:
Trial
V
(V)
d
(mm)
A
(mm
2
)
C
(F)
U
(J)
Q
(C)
E
(V/m)
1
1.5
10.0
400
2
1.5
10.0
350
3
1.5
10.0
300
4
1.5
10.0
250
5
1.5
10.0
200
19.
Which of the calculated variables (
C
, U
, Q
, E
) increase and which decrease as the area of the plates A
decreases?
20. Use Excel
to plot a graph of C vs. A
. Add the best-fit line passing through your data and display the its
equation on graph. Insert a copy (screenshot) of your graph in the space below.
4
21.
Does a linear fit describe the dependence of the magnitude of the capacitance C
of the capacitor on the area
of the plates A
?
22.
The density of the electric field lines is proportional with the strength of the electric field in a certain area of
space. Visually, does the change in the density of electric field lines as A
change, confirm the change in the calculated value of E
listed in Table 3? 23.
Do your graphs show that the capacitance of a capacitor is proportional to the surface area of the plates (conductors) and inversely related to the gap between them (
C
=
ε
o
A
d
)?
Activity 4: Energy Stored in a Parallel Plate Capacitor
24.
Select a separation distance and a plate area. Once you made such a selection, the capacitor has a fixed capacitance. List the corresponding values below:
A
=
¿
C
=
¿
25. Without changing the separation distance d
and the plate area A
, use the battery slider to change the
potential difference between the plates and the charge on the top plate of the capacitor. Record the values in
Table 4. Repeat the procedure to have at least seven sets of values.
Table 4
Capacitor
Voltage, V
(
V
)
Top Plate Charge,
Q
(
10
−
12
C
)
Stored
Energy,
U
(J)
1
2
C V
2
(J)
1
2
QV
(J)
1
2
Q
2
C
(J)
Do your Table 4 values confirm that the total stored energy U
can be calculated as U
=
1
2
C V
2
=
1
2
QV
=
1
2
Q
2
C
?
References: CC-BY license, PhET Interactive Simulations, University of Colorado Boulder, http://phet.colorado.edu 5
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