Capacitor Construction

7/27/2019 Capacitor Construction

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How to Construct a Capacitor

Thomas Reddy Shyamala

Meadow Creek High School

Problem:

How to construct a capacitor and what affect does a capacitor have on a circuit?

Abstract: A paralell plate is constructed and and its capacitance is measured. The RCtime constant of a circuit is measured experimentally using different resistors and

capacitors. These results are compared with the spice simulator results.

National and State Standards:Inquiry, Process and Problem Solving

Uses science process skills in laboratory or field investigations, including observation,classification, communication, metric measurement, and prediction, inference, collecting

and analyzing data.

1.1 Designs and conducts a scientific experiment that identifies the problem,

distinguishes manipulated, responding and controlled variables, collects, analyzes andcommunicates data, and makes valid inferences and conclusions.

1.2 Evaluates procedures, data and conclusions to determine the scientific validity of

research.Learns and uses on a regular basis standard safety practices for laboratory or field

investigations.3.1 Learns and uses safety procedures specific to an investigation or research activity.

Describes the nature of potential energy in an electrical system.21.1 Describes a capacitor. Explain how a capacitor works

21.2 Defines capacitance.

21.7 Describes the relationship between work and electrical energy.21.8 Measures electrical energy stored in a capacitor.

Objectives:

To verify the dependence of capacitance on plate size and spacing for a parallelplate capacitor.

To measure the dielectric constant of a paper. To investigate how quickly the charge enters and leaves the capacitor. Verify the results with a spice simulator.

Learner Outcomes:

Students will achieve the knowledge of capacitors such as

Function Operation principle Construction Effect of dielectric

Thomas Shyamala

MHS 7/16/20061


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Measurement of capacitance Charge and discharge Time constant Usage of simulators

Capacitor Lab Rubrics

Name:_____________________ Grade:___________ Date:______________

Categories Criteria Va

Process skills 0 1 2 3 4

Analysis Nojudgment

can be made

Cannot perform theProcess skills

Accomplishment ofProcess skills with

significant errors

Accomplishment ofProcess skills without

significant errors

Accomplishment ofProcess skills fully with

minimum insignificant

errors

Investigation Nojudgment

can be made

Doesnt address thecontent knowledge

Addresses some features ofthe content knowledge

Addresses main features ofthe content knowledge

Addressee all features ofthe content knowledge

Deduction Nojudgment

can be made

Does not generate aunsupported

prediction

Generates a partiallysupported prediction

Generates a validprediction that mismatch

conclusion

Generates a validprediction that matches

conclusion

Classification No

judgment

can be made

Organizes elements

Illogically

Organizes elements

partially logic

Organizes elements

logically with no

description

Organizes elements

logically with complete

description

Induction No

judgment

can be made

Does not construct a

valid generalization

Constructs a partially valid

generalization

Constructs a valid

generalization with no

logic

Constructs a valid

generalization with logic

Decision making Nojudgment

can be made

Uses criteria that areunrelated to the

situation

Uses criteria that arepartially related to the

situation

Uses criteria that arerelated to the situation with

no explanation

Uses criteria that arerelated to the situation with

complete explanation

Compare and

contrast

Nojudgment

can be made

Uses trivialcharacteristic

elements

Uses some criticalcharacteristic elements

Uses most criticalcharacteristic elements

Uses all criticalcharacteristic elements

Assignments No

judgmentcan be made

Does not meet the

specifiedrequirements

Meets few specified

requirements

Meets most of the

specifiedrequirements

Meets all the specified

requirements

Total

Section I

Parallel Plate Capacitor

Theory:

A capacitor is a device that stores energy in the electric field created between a pair of

conductors on which electric charges of equal magnitude, but opposite sign, have beenplaced.So a capacitor can be made up of two metal surfaces separated by a certain

distance. They are used to isolate one voltage from another, trigger timely electrical

events, voltage ripple filter, etc. In electric circuits it is symbolized by a pair of verticallines of same size with two leads. The capacitance is measured in farads or submultiplesof farad.

A parallel plate capacitor is the simplest one to verify the dependence of capacitance onplate size and spacing between the plates. When a capacitor is connected across the

battery terminals, charge flows until its potential difference becomes equal to that of thebattery.

Thomas Shyamala

MHS 7/16/20062
http://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Energy


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When electric charge accumulates on the

plates, an electric field is created in the

region between the plates that isproportional to the amount of

accumulated charge. This electric field

creates a potential difference V = Edbetween the plates of this simple

parallel-plate capacitor.

The electrons within dielectric moleculesare influenced by the electric field,

causing the molecules to rotate slightlyfrom their equilibrium positions.This

effect accomdate extra charge on the

plates than usual.The air gap is shownfor clarity; in a real capacitor, the

dielectric is in direct contact with the

plates.

The amount of charge needed to produce a potential difference in the capacitor dependson

1. Area (A) of the plates.2. Distance (d) between the plates and

3. Non conducting material between the plates.

The capacitance of a parallel plate capacitor with Non conducting material between itsplates is given by

Thomas Shyamala

MHS 7/16/20063
http://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_field


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C = k0d

A

Where kis the Dielectric constant for the non conducting material and

0is the permittivity of free space which is equal to 8.8542 X 10-12

C2/ (Nm

2).

Apparatus:

1. Parallel plate capacitor (need to make seven different capacitors with Aluminumfoil for each part of the experiment).

2. Card stock paper.3. Clothes pins.4. Digital multimeter.5. Ruler.

Procedure:

Part I:

To verify of the dependence of capacitance on plate size for a parallel plate

capacitor.

Take the aluminum foil cut it in rectangular shape (as shown in the figure) in pairsand place required number of Card stock paper between them. This arrangement

now acts like a capacitor.

Make seven such capacitors of different sizes .The thickness must be uniform andconstant for all capacitors.

Then measure the area of each capacitor and corresponding capacitance for allcapacitors. Now record the data in table1.

Graph Capacitance versus Area. Find the dielectric constant of the material in each case.

Thomas Shyamala

MHS 7/16/20064


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Table 1

Spacing (Thickness) between the plates = _______________meter.

Area (A) of the plate

( meter )

Capacitance (C)

(Farad)

K = (C d ) / (0 A)

Part II:

To verify the dependence of capacitance on spacing between the plates for a parallel

plate capacitor:

Follow the first step of part I to make a capacitor. Make seven capacitors of same area with different thicknesses. The number of

Card stock paper between the plates changes the thickness of the capacitor.

Then measure the thickness of each capacitor and corresponding capacitance forall capacitors. Now record the data in table2.

Graph Capacitance versus Thickness. Find the dielectric constant of the material in each case.

Table 2

Area of the plate = _______________meter.

Spacing (d) between the plates

( meter )

Capacitance (C)

(Farad)

K = (C d ) / (0 A)

Thomas Shyamala

MHS 7/16/20065


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Assessment:

1. What is the function that describes the relationship between Capacitance and Areaof the capacitor?

2. What is the function that describes the relationship between Capacitance andSpacing (thickness) of the capacitor?

3. Graph Capacitance versus the reciprocal of the plate Spacing and find out theslope.

4. Determine the dielectric constant k of the material using the measured value ofthe slope (Capacitance versus the reciprocal of the plate Spacing), the measured

plate area in part II and the known value of the permittivity of free space(0=8.8542 X 10

-12C

2/ (Nm

2)).

5. Find the percentage error in k with respect to experimental average.6. Describe the sources of error in the experiment?

Section II

RC Time Constant:

Capacitor:

It is a two-terminal electrical component that stores energy in the electric field created

between a pair of conductors on which electric charges of equal magnitude, but oppositesign, have been placed.

Charge

When a battery is connected to a series resistorand capacitor, the initial current is high asthe battery transports charge from one plate of the capacitor to the other. The charging

current exponentially approaches zero as the capacitor becomes charged up to the battery

voltage. Charging the capacitor stores energy in the electric field between the capacitor

plates. The rate of charging is typically described in terms of a time constant RC.

Here the voltage (V) across the capacitor is given by

V = Vo (1et / ) (1)

Where Vo is the battery voltage and is the product RC.

When t is equal to infinity the above equation yields V=V0.

Thomas Shyamala

MHS 7/16/20066
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_chargehttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/filter.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electric/filter.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html#c1http://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Electricity


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Discharge

While discharging the voltage (V) across the capacitor is given by

V = Vo et /

(2)

When t is equal to infinity the above equation yields V= 0.thus the voltage decreases by a

factor 1/e every seconds.

Equations 1 and 2 reveal that the larger the time constant, the slower the charging or

discharging

Resistor:

It is a two-terminal electrical component that resists an electric current by producing avoltage drop between its terminals in accordance with Ohm's law (V= I R ).Hence it

dissipates energy.In series with a capacitor,it delay the charge or discharge when a switch

is opend or closed.,producing(for charge) an exponential variation of the voltage acroos

the capacitor.

Procedure:

Construct the circuit as shown below.

Case 1:

When the switch S is placed in position A, the charge flows from the battery into the

capacitor until the capacitor is fully charged.Case 2:

When the switch is placed in B, the capacitor discharges through the resistor.

Thomas Shyamala

MHS 7/16/20067
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Ohm%27s_lawhttp://en.wikipedia.org/wiki/Ohm%27s_lawhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electricity


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Record at least 10 values of voltage (v) and corresponding time (t) in each case.

Battery Voltage = __________.

Charging of a capacitor

Voltage(v) Time(s)

Discharging of a capacitor

Voltage(v) Time(s)

Assessment:

1. How much time (approximately) does it take the capacitor to charge anddischarge?

2. Double the resistance and measure the time to charge and discharge.3. Double the battery voltage. How much time does it take to charge and discharge?4. Double the capacitance and measure the time to charge and discharge.5. Compare your measurements to the values found from the equations (1 & 2) and

SPICE simulator (download the Evaluation Versionsimulator form

http://www.spectrum-soft.com/download.shtm . It is a free limited version. )

6. Graph voltage versus time in both cases.7. Fit the discharge curve to obtain value (fit).8. Calculate using R and C values.9. Find the percentage error of.10.What is the maximum current while the capacitor is charging?11.What is the minimum current while the capacitor is discharging?12.Calculate the maximum energy stored in the capacitor.13.Calculate the energy supplied by the battery for maximum charging?

Thomas Shyamala

MHS 7/16/20068
http://www.spectrum-soft.com/download.shtmhttp://www.spectrum-soft.com/download.shtm

Capacitor Construction

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