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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. +. -. - PowerPoint PPT Presentation
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Physics 2225 : Resistors, Capacitors, and RC Circuits
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”.
Physics 2225 : Resistors, Capacitors, and RC Circuits
What is a “Capacitor”?
Capacitor = an object that can store electric charge Q.
Example: Two metal plates
++
+
++ ++
+--
--
----
Physics 2225 : Resistors, Capacitors, and RC Circuits
What is a “Capacitor”?
Relationship between charge and voltage
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+
++ ++
+--
--
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-
Total charge on one plate: Q
Voltage V
VCQ
Capacitanceof the capacitor
Physics 2225 : Resistors, Capacitors, and RC Circuits
Getting the Charge on the Capacitor
Current (movement of charge) must occur: Applying a voltage willcause current to flow when switch is closed.
battery/power supply
switch
e
not charged…yet
resistance of circuitR
C
Physics 2225 : Resistors, Capacitors, and RC Circuits
Getting the Charge on the Capacitor
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+
+
-
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e
I I
I I Current decreasesas charge increaseson the capacitor.
Voltage across capacitor is increasing as more charge is deposited.
CQVcapacitor
has opposite polaritycompared to e.
capacitorV
Physics 2225 : Resistors, Capacitors, and RC Circuits
Getting the Charge on the Capacitor
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+
+
-
---
e
I = 0
Capacitor is fully charged No more current flows
capacitorV ecapacitorV
+++
+ -
-
-
-
Physics 2225 : Resistors, Capacitors, and RC Circuits
The Charging Process …MathematicallyI
RCt
eR
I
e
t
V
RCt
eCQ 1e
t
Q
RCt
eCQV 1e
Physics 2225 : Resistors, Capacitors, and RC Circuits
Discharging the Capacitor
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+
+
-
---
I I
I I Current decreasesas charge decreaseson the capacitor.
Voltage across capacitor is decreasing as charge is leaving.
capacitorV
Removing battery and closing the loop:
Physics 2225 : Resistors, Capacitors, and RC Circuits
The Discharging Process …MathematicallyI
RCt
eR
I
e
t
V
RCt
eCQ
e
t
Q
RCt
eCQV
e
Note: Current flows inopposite directioncompared to charging.
Physics 2225 : Resistors, Capacitors, and RC Circuits
The “RC time constant”
The product RC has units of time: It is called the “time constant of the RC circuit”.
RCt
capacitor etV
e)(
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…..
eee 3678.0)( 1 eeV RC
RC
RCtcapacitor
Physics 2225 : Resistors, Capacitors, and RC Circuits
Measuring the “RC time constant” During the Discharge Process
V
t
Vinitial
Vinitial*0.368
RC
Note: Your initial voltage can be at ANY starting point.
Physics 2225 : Resistors, Capacitors, and RC Circuits
The “RC time constant”
For the charging process we saw:
and…after a time t=RC has elapsed…..
eee 6322.011)( 1
eeV RCRC
RCtcapacitor
RCt
etV 1)( e 0)( 0 tV
And the final (maximum) voltage is reached after a “long” time…
eee
011)( eeV RCtcapacitor
Physics 2225 : Resistors, Capacitors, and RC Circuits
Measuring the “RC time constant” During the Charging Process
Vcapacitor
t
Vfinal
Vfinal*0.63
RC
Note: Your initial voltage must be 0.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Using the Function Generator to Automate Charging and Discharging
V
Must use the square wave of function generator
CR
Physics 2225 : Resistors, Capacitors, and RC Circuits
Measuring Vcapacitor(t) with the Oscilloscope
V
CR
Oscilloscope
Physics 2225 : Resistors, Capacitors, and RC Circuits
Connections
CR
Oscilloscope
FunctionGenerator
The black cables clipsmust be located as shown!
Physics 2225 : Resistors, Capacitors, and RC Circuits
Power remains OFF on breadboard in this lab.
ToOscilloscope
ToFunctionGenerator
Physics 2225 : Resistors, Capacitors, and RC Circuits
Capacitor
Resistor
To FunctionGenerator
Both black clipsmust be attachedto the same pointin circuit.
To OscilloscopeA Close View
Physics 2225 : Resistors, Capacitors, and RC Circuits
Choosing a Good Frequency on Function Generator
FunctionGeneratorVoltage
time
Capacitorcharges
Capacitordischarges
Capacitorcharges
Capacitordischarges
Theoretical charge time = RCPeriod (T) should be approximately 10*RC so that capacitor can fully charge and discharge.frequency (=1/T) should be approximately 1/(10*RC)
Period (T)
Physics 2225 : Resistors, Capacitors, and RC Circuits
If Frequency is Chosen Well ….Oscilloscope Will Show ….
Oscilloscope(Vcapacitor)
time
Capacitorcharges
Capacitordischarges
Capacitorcharges
Capacitordischarges
Physics 2225 : Resistors, Capacitors, and RC Circuits
Frequency too High …..
Oscilloscope(Vcapacitor)
time
Not enough time for proper charging and discharging: Looks like sawtooth
Physics 2225 : Resistors, Capacitors, and RC Circuits
Frequency too Low Harder to Measure RC
Oscilloscope(Vcapacitor)
time
Capacitorcharges
Capacitordischarges
Capacitorcharges
Capacitordischarges
Physics 2225 : Resistors, Capacitors, and RC Circuits
Choose Oscilloscope Channel 1 Mode = DC
First push “Ch1 Menu” Button
Then select coupling With this button (must be “DC”).
Physics 2225 : Resistors, Capacitors, and RC Circuits
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.
Vmax0.63 Vmax
RC
Physics 2225 : Resistors, Capacitors, and RC Circuits
The Capacitance Meter
Small knob canbe turned to “zero” capacitancemeter.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Using Capacitance Meter with Clips
1) First position leads where you want them.
2) “Zero” capacitance meter without capacitor attached.
3) Attach capacitor but try to move leads as little as possible.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Using Capacitance Meter without Clips
1) Remove leads.
2) “Zero” capacitance meter without capacitor attached.
3) Insert capacitor into slits as shown.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Making a Resistor from Paper and Carbon
White cardboard
Draw with pencil(apply thick layer).
Use alligator clipsto make goodelectric contact.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Measuring R
If you deposit a generousthickness of carbon, aboutas large as shown, you shouldget approx. 100kW resistance.
If you get a few MW, you needto apply carbon more thicklywith the pencil.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Varying Length (L) and Area (A) of Resistor
LALR
Simply attach clips at different positions to vary L.
W
Varying W effectivelychanges A (A=W*Thickness of carbon) Draw broader W for more A.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Making a Capacitor
8”x11” paper
Leave overhang for clips
Aluminum foil smaller thanpaper (but not much smaller).Keep Aluminum foil as flat aspossible.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Step-by-Step
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.
Physics 2225 : Resistors, Capacitors, and RC Circuits
Varying the Capacitance
dAC e
Area of capacitor (in ourcase this is the area of overlap of the two aluminum foils).
Distance between the aluminum foils( = thickness of paper).
Physics 2225 : Resistors, Capacitors, and RC Circuits
Varying the Capacitance
dAC e
Change “d” by inserting 1, 2, 3…sheets of paper between the twofoils (doubles, triples, etc. “d”).
You can simply move one sheet tochange the area of overlap.
Effective area of overlap.
Make sure to cover with book again!!
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