# Purpose of this Minilab - PowerPoint PPT Presentation

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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. +. -.

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Purpose of this Minilab

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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

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### What is a “Capacitor”?

Relationship between charge and voltage

Total charge on one plate: Q

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Capacitance

of the capacitor

Voltage V

### Getting the Charge on the Capacitor

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

### Getting the Charge on the Capacitor

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.

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e

### Getting the Charge on the Capacitor

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Capacitor is fully charged

 No more current flows

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I = 0

e

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t

Q

V

t

### Discharging the Capacitor

Removing battery and closing the loop:

Voltage across capacitor is decreasing as charge is leaving.

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Current decreases

as charge decreases

on the capacitor.

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### The Discharging Process …Mathematically

I

Note: Current flows in

opposite direction

compared to charging.

t

Q

V

t

### 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…..

### Measuring the “RC time constant” During the Discharge Process

V

Vinitial

Vinitial*0.368

t

RC

Note: Your initial voltage can be at ANY starting point.

### 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…

### Measuring the “RC time constant” During the Charging Process

Vcapacitor

Vfinal

Vfinal*0.63

t

RC

Note: Your initial voltage must be 0.

### Using the Function Generator to Automate Charging and Discharging

C

R

V

Must use thesquare wave of function generator

C

R

V

Oscilloscope

### Connections

Oscilloscope

C

R

Function

Generator

The black cables clips

must be located as shown!

Power remains OFF on breadboard in this lab.

To

Oscilloscope

To

Function

Generator

A Close View

To Oscilloscope

Capacitor

Resistor

Both black clips

must be attached

to the same point

in circuit.

To Function

Generator

### Choosing a Good Frequency on Function Generator

Period (T)

Function

Generator

Voltage

time

Capacitor

discharges

Capacitor

charges

Capacitor

discharges

Capacitor

charges

• 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)

Oscilloscope

(Vcapacitor)

time

Capacitor

discharges

Capacitor

charges

Capacitor

charges

Capacitor

discharges

### Frequency too High …..

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

### Choose Oscilloscope Channel 1 Mode = DC

Then select coupling

With this button

(must be “DC”).

Vmax

### 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

### The Capacitance Meter

Small knob can

be turned to

“zero” capacitance

meter.

### Using Capacitance Meter with Clips

• where you want them.

• 2) “Zero” capacitance

• meter without

• capacitor attached.

• 3) Attach capacitor but

• try to move leads as

• little as possible.

### Using Capacitance Meter without Clips

• 2) “Zero” capacitance

• meter without

• capacitor attached.

• 3) Insert capacitor into

• slits as shown.

### Making a Resistor from Paper and Carbon

White cardboard

Use alligator clips

to make good

electric contact.

Draw with pencil

(apply thick layer).

### Measuring R

If you deposit a generous

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.

### 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.

### Making a Capacitor

8”x11” paper

Aluminum foil smaller than

paper (but not much smaller).

Keep Aluminum foil as flat as

possible.

Leave overhang for clips

### 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.

### 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).

### 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!!