OBJECTIVES
·
To define capacitance
·
To determine how the
capacitance of conducting parallel plates is related to the
separation distance between the plates and the surface area of the
plates
·
To determine the
permitivvity constant
INTRODUCTION
Capacitors are widely used in a variety of
electric circuits to provide extra energy or help keep energy
levels at a constant value.
As an example, a home cooling (AC) unit will have a
capacitor that stores charge (energy). When the system is started, the capacitor
can release the stored energy to assist the unit in starting the
compressor necessary to cool the home. Electronics with flashing lights use a
capacitor in a timing or RC circuit.
The classical design of a capacitor, which you
will use in this lab, is two parallel conducting plates, separated
by an insulator as shown below.
Charges of opposite sign are stored on the two
plates, establishing an electric field between the plates. The capacitance can be
defined as a ratio of charge to voltage, or
where C is the capacitance, in Farads, Q the
charge on one plate in Coulombs (each plate has equal magnitude
charge), and V is the separation Voltage.
However, it is a misnomer to think that the
capacitance of a capacitor is defined by the amount of charge and
voltage. Capacitance
is defined by the geometry of the capacitor design, or
particularly on the cross sectional area of the plates and the
separation distance of the plates (and also the material, if any,
placed between the plates).
In this lab, you will use a pHet simulation to
derive the relationship for capacitance, as a function of both
separation distance and the area of the plates.
PROCEDURE
Use Capacitors
Phet for this lab.
PART 1
In this part of the lab, you will determine the
relationship between capacitance and plate area. Using the simulation,
fix the voltage at 1.5 V (the default), the plate Area at 100 mm2
(default), and the separation distance at 5.0 mm. Select the Capacitance
meter, and measure the capacitance.
Repeat for the five values of plate area shown below.
Plate Area (mm2) |
Capacitance (F) |
100 |
|
150 |
|
200 |
|
300 |
|
400 |
|
Graph your data in EXCEL, Google Sheets, or on
a graphing calculator.
1.
What is the mathematical
function suggested by the graph (linear, quadratic/parabola,
inverse/rational)? If
you are unclear as to the shapes of these graphs, visit Types
of Graphs
2.
Write a corresponding
equation based on your answer, using C as the symbol for
Capacitance, A as the symbol for plate Area, and k as some
arbitrary constant.
For example, if you felt the relationship was linear, you would
write C = kA
. If you felt the relationship was quadratic,
you would write C = kA2
. If you felt this is an inverse relationship,
you would write C=k/A
.
PART 2
Now the relationship
between Capacitance and plate separation will be investigated. Keep the plate area at
400 mm2 and the battery voltage at 1.5 V. Complete the table
below for the indicated plate separations
Plate separation (mm) |
Capacitance (F) |
5 |
|
6 |
|
7 |
|
8 |
|
9 |
|
10 |
|
Graph your data in EXCEL, Google Sheets, or on
a graphing calculator.
1.
What is the mathematical
function suggested by the graph (linear, quadratic/parabola,
inverse/rational)?
2.
Write a corresponding equation based on
your answer, using C as the symbol for Capacitance, d as the
symbol for plate separation, and k as some arbitrary constant. For example, if you
felt the relationship was linear, you would write C = kd
and so forth.
CONCLUSIONS