Physics 1809: 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 1809: Resistors, Capacitors, and RC Circuits What is a “Capacitor”? Capacitor = an object that can store electric charge Q. Example: Two metal plates

Physics 1809: Resistors, Capacitors, and RC Circuits What is a “Capacitor”? Relationship between charge and voltage Total charge on one plate: Q Voltage V Capacitance of the capacitor

Physics 1809: Resistors, Capacitors, and RC Circuits Getting the Charge on the Capacitor Current (movement of charge) must occur: Applying a voltage will cause current to flow when switch is closed. battery/power supply switch  not charged…yet resistance of circuit R C

Physics 1809: Resistors, Capacitors, and RC Circuits Getting the Charge on the Capacitor  I I I I Current decreases as charge increases on the capacitor. Voltage across capacitor is increasing as more charge is deposited. has opposite polarity compared to .

Physics 1809: Resistors, Capacitors, and RC Circuits Getting the Charge on the Capacitor  I = 0 Capacitor is fully charged  No more current flows

Physics 1809: Resistors, Capacitors, and RC Circuits The Charging Process …Mathematically I t V t Q

Physics 1809: Resistors, Capacitors, and RC Circuits Discharging the Capacitor I I I I Current decreases as charge decreases on the capacitor. Voltage across capacitor is decreasing as charge is leaving. Removing battery and closing the loop:

Physics 1809: Resistors, Capacitors, and RC Circuits The Discharging Process …Mathematically I t V t Q Note: Current flows in opposite direction compared to charging.

Physics 1809: 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”. 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…..

Physics 1809: Resistors, Capacitors, and RC Circuits Measuring the “RC time constant” During the Discharge Process V t V initial V initial *0.368 RC Note: Your initial voltage can be at ANY starting point.

Physics 1809: Resistors, Capacitors, and RC Circuits The “RC time constant” 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…

Physics 1809: Resistors, Capacitors, and RC Circuits Measuring the “RC time constant” During the Charging Process V capacitor t V final V final *0.63 RC Note: Your initial voltage must be 0.

Physics 1809: Resistors, Capacitors, and RC Circuits Using the Function Generator to Automate Charging and Discharging V Must use the square wave of function generator C R

Physics 1809: Resistors, Capacitors, and RC Circuits Measuring V capacitor (t) with the Oscilloscope V C R Oscilloscope

Physics 1809: Resistors, Capacitors, and RC Circuits Connections C R Oscilloscope Function Generator The black cables clips must be located as shown!

Physics 1809: Resistors, Capacitors, and RC Circuits Power remains OFF on breadboard in this lab. To Oscilloscope To Function Generator

Physics 1809: Resistors, Capacitors, and RC Circuits Capacitor Resistor To Function Generator Both black clips must be attached to the same point in circuit. To Oscilloscope A Close View

Physics 1809: Resistors, Capacitors, and RC Circuits Choosing a Good Frequency on Function Generator Function Generator Voltage time Capacitor charges Capacitor discharges Capacitor charges Capacitor discharges Theoretical charge time = RC  Period (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 1809: Resistors, Capacitors, and RC Circuits If Frequency is Chosen Well …. Oscilloscope Will Show …. Oscilloscope (V capacitor ) time Capacitor charges Capacitor discharges Capacitor charges Capacitor discharges

Physics 1809: Resistors, Capacitors, and RC Circuits Frequency too High ….. Oscilloscope (V capacitor ) time Not enough time for proper charging and discharging:  Looks like sawtooth

Physics 1809: Resistors, Capacitors, and RC Circuits Frequency too Low  Harder to Measure RC Oscilloscope (V capacitor ) time Capacitor charges Capacitor discharges Capacitor charges Capacitor discharges

Physics 1809: 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 1809: 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. V max 0.63 V max RC

Physics 1809: Resistors, Capacitors, and RC Circuits The Capacitance Meter Small knob can be turned to “zero” capacitance meter.

Physics 1809: 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 1809: 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 1809: Resistors, Capacitors, and RC Circuits Making a Resistor from Paper and Carbon White cardboard Draw with pencil (apply thick layer). Use alligator clips to make good electric contact.

Physics 1809: Resistors, Capacitors, and RC Circuits Measuring R If you deposit a generous thickness of carbon, about as large as shown, you should get approx. 100k  resistance. If you get a few M , you need to apply carbon more thickly with the pencil.

Physics 1809: Resistors, Capacitors, and RC Circuits Varying Length (L) and Area (A) of Resistor 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.

Physics 1809: Resistors, Capacitors, and RC Circuits Making a Capacitor 8”x11” paper Leave overhang for clips Aluminum foil smaller than paper (but not much smaller). Keep Aluminum foil as flat as possible.

Physics 1809: Resistors, Capacitors, and RC Circuits Step-by-Step 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 1809: Resistors, Capacitors, and RC Circuits Varying the Capacitance 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).

Physics 1809: Resistors, Capacitors, and RC Circuits Varying the Capacitance 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!!