name____________________ period _______ lab partners___________________________________
The Capacitor--Its History and Function
Introduction
Capacitors are devices that store electrical energy by maintaining charge separation across two conductive
metal regions called plates: one positively charged, the other negatively charged. To keep these positive and
negative charges from coming together, an insulating material (plastic, cardboard, etc.) is placed between the
plates. One of the first capacitors--the Leyden Jar (Figure #1a) developed in the middle of the 18th century-used glass as the insulator. The ability to store charge (or more accurately, to keep charges separated) is
called capacitance--with symbol C used to denote it.
<==Figure #1a Figure #1c==>
Figure #1b
Since work must be done to separate charges, a source of electrical energy (such as a battery)is needed to
charge a capacitor. In Part A this will be supplied by a power supply (see Figure #2 in Procedure). There
you'll see the charging and discharging of a capacitor. As it is charged, the amount of charge on each plate of
the capacitor--+ q on one, - q on the other--grows, and so does the potential difference V or voltage between
the plates, measured in volts. The charge q and voltage V increase proportionally: q = C V (equation #1).
Capacitance C is measured in farads (F). A one farad capacitor can store one coulomb of charge for every
volt maintained across its plates. An electric field E also exists between the positively and negatively
charged plates. Between the plates of a parallel plate capacitor (pictured in Figure #1b) the electric field
ideally is uniform--something you'll investigate in Part B of the lab. You'll do this by looking for points in
between the plates that have the same voltage--connecting those gives equipotential lines. If the electrical
field is uniform, equipotential lines should be parallel (and perpendicular to electric field lines). In making an
analogy between lifting mass m in the uniform gravitational field (of strength g) at the Earth's surface and
lifting charge q, and concluding that work done W= q V, we assumed a uniform electric field.
If its plates are separated by distance d, the voltage V and electric field E are related by V=E d or E=V / d
(equation #2). The presence of insulating material between the plates of most capacitors allows greater
voltages--and correspondingly larger electric fields--to be maintained across them. Without an insulator, air
subject to electric fields greater than 3.00 x 106 V / m will electrically break down--meaning previously
separate positive and negative charges come together. (This happens in nature when lightning strikes.)
Capacitors have diverse uses. Their history is also interesting since it parallels the story of harnessing
electrical energy and putting it to work. With the aid of the tutorials and simulations on an excellent website
maintained by National High Magnetic Field Laboratory, this lab ends (in Part C) with your exploration of
how a capacitor works, its history and practical application. (Figure #1c shows a capacitor you can build.)
Objectives
1) to appreciate a) that charging a capacitor involves electrical energy doing work to separate charges, and
that this energy is available to do useful work as the capacitor discharges and the charges come back together.
2) to verify / check that between the plates of a parallel plate capacitor is a region of uniform electric field
3) to appreciate the place of the capacitor in the harnessing of electricity and putting it to work.
4) to provide practical experience with electric circuits (including circuit diagrams) and measuring voltage.
Materials
DC power supply (for charging capacitor in part A)
capacitor, resistor, switches, wires (part A)
computer interface, voltage sensor (part A)
power supply (for maintaining voltage, part B)
Leyden Jars (exhibits, associated with part C)
black conductive paper (part B)
digital multimeter (for reading voltage, part B)
connecting wires, electrical leads (part B)
computer, web access, web browser (part C)
miscellaneous capacitors (exhibits, associated with part C)
Procedure
A. Instructor Demo: Charging and Discharging of a Capacitor
Watch and listen as your
instructor uses the circuit at
the right: Figure #2 =====>
Note: C is the capacitor.
Record data, answer the
questions, etc. in Data / Data
Handling / Questions for
part A.
B. The Electric Field Between the Plates of a Capacitor--CAUTION: observe electrical safety precautions!
Use the silver conductive ink / black conductive
paper representation of the capacitor Figure #3 ==>
The DC power supply will be used to put 10.00 volts
across the upper capacitor plate at y = 10 and the
lower capacitor plate at y = 5. Measure voltage to
nearest 0.01 volt using digital multimeter at 24
points in the central region between the plates.
Record data, answer the questions, etc. in Data / Data
Handling / Questions for part B.
C. Exhibits / Exploring the National High Magnetic Field Lab Website
As you do the following, record data, answer the questions, etc. in Data / Data Handling / Questions part C.
1) Look over the exhibits provided at your lab station: Leyden Jar, miscellaneous capacitors.
2) Go to the Molecular Expressions: Electricity & Magnetism Introduction--Interactive Java Tutorials
http://micro.magnet.fsu.edu/electromag/java/index.html
Visit the following:
a) Charging and Discharging a Capacitor
b) Factors Affecting Capacitance
c) Lightning--An Example of A Natural Capacitor:
3) Go to the Mag Lab U: Interactive Java Tutorials
http://www.magnet.fsu.edu/education/tutorials/java/index.html
Visit the following:
a) Electrostatic Generator:
b) Leyden Jar
c) Voltaic Pile
d) Simple Electrical Cell
e) The Van De Graf Generator
4) Explore elsewhere on this website (18th century timelines?) and in your text re: capacitors as time permits
Data / Data Handling / Questions
Part A (Note: Study of Figure #2 will help answer some of what follows.)
1) The value of the capacitor used in this demo is _________ micro Farads. It is located between terminals #
___ and # ___. The switch should connect what two terminal #s for the capacitor to charge? ____ What is
the maximum voltage the capacitor charged to? Give value in volts ___ How long did it take the capacitor to
charge? time =_____seconds The switch should connect what two terminal #s for the capacitor to discharge?
_____ The value of the resistor used in this demo is ______ohms. It is located between terminals # ___ and #
___. How long did it take the capacitor to discharge to 37% of its fully charged voltage? time =____seconds
2) Sketch the voltage vs. time graph for capacitor charging and discharging.
3) It turns out that the bigger the product of resistance R x capacitance C in the circuit we're using, the longer
the capacitor requires to charge and discharge. Compute the product RC = ______ (units are seconds)
Theory predicts this is the amount of time the capacitor needs to discharge to 37% of its fully charged
voltage. Compare this theoretical value with the experimental one and compute % difference.
4) Using equation #1, compute the charge on each plate of our fully charged capacitor.
Part B (Note: Refer to Figure #3 for (x, y) co-ordinate system used in the table below)
1) Measured Voltages in Central Region Between Capacitor Plates--all values in volts
x,y
x=10
x=11
x=12
x=13
x=14
y=9
y=8
y=7
y=6
x=15
2) Using your data and calculator (TI 83, TI 84, etc.) enter (use STAT, then EDIT) y=9 voltages in List 1,
y=8 voltages in List 2, y=7 voltages in List 3, and y=6 voltages in List 4. Tell the calculator to compute the
average or mean value ( x bar) of these voltages, and also compute a measure of the variation in the values
known as the sample standard deviation (Sx). It will do this if you make a 1-Vars Stats request (look on
STAT, CALC menu), choosing 1-Vars Stats, then typing in the name of the list L1, L2, etc. then hitting
Enter. (Alternatively, you can use Setup on STAT menu.) Record data in the table below.
mean voltage, Vave std. dev, volts = ΔV (ΔV / Vave) x 100%
y=9
y=8
y=7
y=6
3) Look at the mean voltages in the chart above. Ideally, we might expect them to be 8.00 volts, 6.00 volts,
4.00 volts, and 2.00 volts (assuming there is exactly 10.00 volts between the upper capacitor plate and the
lower plate--assumed to be at ground potential = 0.00 volts). How do your measured mean voltages compare
with the idealized values--compare % differences.
4) Look at the (ΔV / Vave) % values in the chart above. How uniform is the electric field in this region? Is it
as uniform as the gravitational field (measured by g) at Earth's surface? Discuss.
5) Using equation #2, measuring d from Figure #3 which is at the same scale as the actual capacitor
representation you used, and assuming V = 10.00 volts, compute the expected electric field strength E.
Part C --Fill in the blanks, answer the questions, etc.
1) The largest of the capacitors exhibited is called the ________________. The numbers on the smallest of
the capacitors exhibited are ___________. Explain what at least one of them means____________________
2)
The insulator between the plates of a capacitor is also
called a ______________. Three factors affecting the
capacitance of a parallel plate capacitor are ___________,
_____________, and _____________. Explain how each
affects capacitance: the capacitance increases when the
____________ increases, increases when ___________
increases, and decreases when the _____________
increases. Natural Capacitor: water droplets in clouds
collide with _______, ionizing radiation, and other water
droplets leading to charge separation. This can lead to a
huge natural capacitor: the bottom of the cloud serving as
the negative plate, the ground serving as the positive
plate, and air as the dielectric. A _____________ results
when the insulating capability of the air breaks down.
3) Which came first: capacitors or electrostatic generators? (circle one) In the early electrostatic generator
of the simulation, the two materials that rub together to separate charges are __________ and ____________.
What turns this generator? ____________________. The charge that builds up on the brass ball is positive
or negative (circle one). From the brass ball it jumps to _________________. In the Leyden Jar, electrons
move from the brass rod to the chain and eventually to the inner or outer (circle one) tin foil. Discharging
the Leyden Jar is accomplished by using a _____________________, which is accompanied by a _______
as the voltage between the plates falls to zero. The significance of the voltaic pile is that it represented
__________________________________________. In it, voltage builds as alternating disks of _______ and
___________ separated by ______________ are piled or stacked. Each cell added to the stack increases the
voltage by roughly ____ volt. Unlike the voltaic pile, the simple electrical cell is wet / dry (circle one).
Name one chemical species shown migrating in the simulation of its operation:______________________.
Approximately when (give date or time frame) was the simple electrical cell invented ? ________________.
The van de Graf generator was originally developed to __________________________________________.
By the early 1930s van de Graf generators could produce a potential difference of __________ volts.
4) miscellaneous historical trivia: The first unit of capacitance was not the farad but rather a unit called the
___________. (One of these was about equal to one nanofarad.) In his famous 1752 kite experiment, Ben
Franklin collected charge from a thunderstorm cloud in a _________________. The important instrument
for making precise measurements known as ________________(hint: it has already been pictured on a past
handout) was invented in 1785 and most notably used by ____________________.
5) Pages in your text which refer to capacitors are in chapter 17 pp. ___________ and in chapter 19 pp.
_____________. From exploration of both the website and your text list several applications of capacitors:
6) Describe something that you find especially interesting that you learned regarding capacitors or electricity
in general from your explorations of the National High Magnetic Field Lab website.