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Lab 4. Capacitors

A capacitor is any two conductors separated by an insulator. If a potential difference, V, is established between the conductors, charges +q and –q appear on the conductors. The relationship between q and V is q = VC, where the constant C is the capacitance of the capacitor measured farads, F, (1 F = 1 C/V). The capacitance depends on size, shape, and location of the conductors as well as the insulating material. In this lab, we first build a parallel plate capacitor and then investigate combinations of capacitors as well as the charging/discharging characteristics of capacitors. A parallel plate capacitor consists of two flat conducting plates placed on top of each other but separated by an insulating layer. The capacitance of a parallel plate capacitor is given by

where is the dielectric constant of the insulator, 0 is the permittivity of free space, A is the area of either plate, and d is the separation distance between the plates.

To study the charging/discharging properties of a capacitor, we’ll build an RC circuit with a capacitor, resistor, and battery. We’ll connect a charged capacitor to the resistor and monitor the charge flowing off one of the plates, through the resistor, and onto the other plate.

Capacitor Simulation Link: https://ophysics.com/em5.html

Preliminary Questions: (1-3)

Procedure Part 1: (Simulation)

1. Parallel plate capacitor. Adjust the separation tab, and plate area to their maximum values, then fill in the data table with your values.

2. To explore the relationship of capacitance to the separation distance, change the distance four more times, then fill in the data table with your values.

3. To explore the relationship of capacitance to area, reduce the plate area values 5 times then fill in the data table with your values.

Analysis Part 1: (1-5)

6. Using the slope of this graph, together with the distance between the plates, determine the dielectric constant.

Analysis Part 2: (1-2)

Procedure Part 3: (Simulation)

1. Set the resistance to 10 and voltage to 5 V. Set the separation to the lowest value, and the plate area to whatever value you prefer.

2. Close the switch and let the capacitor charge until the VCAP reaches 5 V.

3. Set the voltage to 0 V.

4. Close the switch and with a stopwatch record how long it takes for the capacitor to reach a current of 0 A.

5. Do the same procedure for the separation at its maximum.

Analysis Part 3:

1. How does the rate of the current dissipating differ between the two?

Note: Show all equations, calculations and very clear screenshots for the graphs with fits to receive full credit.

Data Table

Part 1

Area = (m2)

Separation (m)

Capacitance (F)

Separation = (m)

Area (m2)

Capacitance (F)

Part 2

Capacitance (µF)

{Measured}

Capacitance (µF)

{Theoretical}

C1

C2

C3

Series Combination

Parallel Combination

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Lab 4. Capacitors A capacitor is any two conductors separated by an insulator. If a potential difference, V, is established between the conductors, charges +q and q appear on the conductors. The relationship between q and V is q= VC, where the constant C is the capacitance of the capacitor measured farads, F, (1 F = 1 C/V). The capacitance depends on size, shape, and location of the conductors as well as the insulating material. In this lab, we first build a parallel plate capacitor and then investigate combinations of capacitors as well as the charging/discharging characteristics of capacitors. A parallel plate capacitor consists of two flat conducting plates placed on top of each other, but separated by an insulating layer. The capacitance of a parallel plate capacitor is given by

d A

C 0=

where is the dielectric constant of the insulator, 0 is the permittivity of free space, A is the area of either plate, and d is the separation distance between the plates. To study the charging/discharging properties of a capacitor, we build an RC circuit with a ca aci , e i , a d ba e . We c ec a cha ged ca aci the resistor and monitor the charge flowing off one of the plates, through the resistor, and onto the other plate. OBJECTIVES Build and investigate the capacitance of a parallel plate capacitor. Investigate the capacitance of capacitors connected in series and in parallel Measure an experimental time constant of a resistor-capacitor circuit. Compare the time constant to the value predicted from the component values of the

resistance and capacitance. MATERIALS

aluminum foil resistor meter stick power supply multimeter current probe capacitors Labquest Mini

Vernier caliper computer

PRELIMINARY QUESTIONS 1. A good analogy to charging a capacitor would be filling a scuba tank with compressed air.

What would be the quantities equivalent to q, V, and C in the relation q = VC?

2. In simple terms, why would you expect the capacitance of a capacitor to be proportional to the plate area?

3. Why would you expect the capacitance of a capacitor to be inversely proportional to the distance between the plates?