Purpose of this Minilab. Learn about charging and discharging of a capacitor through a resistor. Learn how to build a simple resistor and capacitor from “everyday materials”. What is a “Capacitor”?. Capacitor = an object that can store electric charge Q. Example: Two metal plates. +. .
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“everyday materials”.
Capacitor = an object that can store electric charge Q.
Example: Two metal plates
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+

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+
+

+
Relationship between charge and voltage
Total charge on one plate: Q
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+

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+
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Capacitance
of the capacitor
Voltage V
Current (movement of charge) must occur: Applying a voltage will
cause current to flow when switch is closed.
C
not charged…yet
switch
battery/power supply
R
resistance of circuit
e
Voltage across capacitor is increasing as
more charge is deposited.
has opposite polarity
compared to e.
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+

+

I
I
Current decreases
as charge increases
on the capacitor.

+
I
I
e
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+

+

+

+

+
+

Capacitor is fully charged
No more current flows

+
I = 0
e
Removing battery and closing the loop:
Voltage across capacitor is decreasing as charge is leaving.
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+

+

I
I
Current decreases
as charge decreases
on the capacitor.

+
I
I
I
Note: Current flows in
opposite direction
compared to charging.
t
Q
V
t
The product RC has units of time:
It is called the “time constant of the RC circuit”.
For the discharging process we saw:
initial voltage of the capacitor at time t=0 (start of discharge)
so…after a time t=RC has elapsed…..
V
Vinitial
Vinitial*0.368
t
RC
Note: Your initial voltage can be at ANY starting point.
For the charging process we saw:
and…after a time t=RC has elapsed…..
And the final (maximum) voltage is reached after a “long” time…
Vcapacitor
Vfinal
Vfinal*0.63
t
RC
Note: Your initial voltage must be 0.
C
R
V
Must use thesquare wave of function generator
Oscilloscope
C
R
Function
Generator
The black cables clips
must be located as shown!
A Close View Discharging
To Oscilloscope
Capacitor
Resistor
Both black clips
must be attached
to the same point
in circuit.
To Function
Generator
Period (T)
Function
Generator
Voltage
time
Capacitor
discharges
Capacitor
charges
Capacitor
discharges
Capacitor
charges
can fully charge and discharge.
frequency (=1/T) should be approximately 1/(10*RC)
Oscilloscope
(Vcapacitor)
time
Capacitor
discharges
Capacitor
charges
Capacitor
charges
Capacitor
discharges
Oscilloscope
(Vcapacitor)
time
Not enough time for proper charging and discharging:
Looks like sawtooth
Oscilloscope
(Vcapacitor)
time
Capacitor
discharges
Capacitor
charges
Capacitor
discharges
Capacitor
charges
First push “Ch1 Menu” Button
Then select coupling
With this button
(must be “DC”).
V Dischargingmax
Hints for Measuring RC with Oscilloscope* Use the cursor functions to measure voltages and time differences.
* Expand x and y axes to get good resolution.
0.63 Vmax
RC
White cardboard
Use alligator clips
to make good
electric contact.
Draw with pencil
(apply thick layer).
If you deposit a generous
thickness of carbon, about
as large as shown, you should
get approx. 100kW resistance.
If you get a few MW, you need
to apply carbon more thickly
with the pencil.
L
Simply attach clips at
different positions to
vary L.
W
Varying W effectively
changes A
(A=W*Thickness of carbon)
Draw broader W for more A.
8”x11” paper
Aluminum foil smaller than
paper (but not much smaller).
Keep Aluminum foil as flat as
possible.
Leave overhang for clips
1 2 3 4 5
The two aluminum foils must not touch each other anywhere.
(Separate them with a sheet of paper).
Put a heavy book on top to keep aluminum foil as flat as possible.
Use the overhangs to make electric connection with alligator clips.
Area of capacitor (in our
case this is the area of overlap
of the two aluminum foils).
Distance between the aluminum foils
( = thickness of paper).
Change “d” by inserting 1, 2, 3…
sheets of paper between the two
foils (doubles, triples, etc. “d”).
You can simply move one sheet to
change the area of overlap.
Effective area of overlap.
Make sure to cover with book again!!