PHY112/262 On-Line Lab #7
Electrical Capacitance and Capacitors
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In this lab we will be examining the concept of Electrical Capacitance and the basic physics of Capacitors.
Capacitors are passive electrical components with two terminals that store electromagnetic energy in the form of an electric field. Terminals are connected to two conductive plates that have air, another non-conductor, or as in most cases, a dielectric material between them. They store an electric charge on their conductive plates. The charged plates, separated by the dielectric material, create an electric field. A simple illustration of a basic capacitor is shown below:
Modern capacitors take on a variety of forms, shapes, and sizes. They can be of the constant capacitance type or can come with variable capacitance. For this lab we will be discussing the simplest type, the Parallel Plate Capacitor.
The symbol for a basic capacitor in an electrical
circuit is simple and intuitive. Here it is to the right.
Capacitance is a parameter of a capacitor that indicates the amount of stored electric charge for a given voltage applied to its terminals. It is measured in farads (F). Doubling the capacitance will double stored electric charge (for a given constant voltage at the terminals).
An equation for the unit of Capacitance, which is a Farad (F) can be written:
C = coulomb
(do not confuse this with the C for capacitance) Eq. #1
V = volt
A Farad is therefore defined as one Coulomb of charge stored per one Volt of applied voltage.
When we apply an electrical voltage to the capacitor, the capacitor then stores the electrical energy in the form of electrical charge. So, we can write:
Q= charge Eq. #2
V= Voltage or potential difference
Based on the physical characteristics of the capacitor, the capacitance formula is,
k= relative permittivity (of the dielectric)
ε0 = permittivity of free space
A = surface area of plates Eq. #3
d=distance between plates
The important part to remember about this last equation is that capacitance gets larger as the surface area of the plates increases, and it gets smaller as the distance between the plates increases.
This lab is composed of several tasks which utilize a PhET (Physics Education technology) simulation.
Capacitor Lab: Basics
Once you have the program up, click on the Capacitance icon.
In the capacitance simulation check all the available boxes.
By using the arrows, try adjusting the both the separation of the capacitor plates and the plate area.
Pay attention to what happens to the capacitance as you make your changes.
1) What did you notice about how the capacitance changed while adjusting those two variables?
One thing you’ll note in this simulation, the units are in Picofarads, Picocoulombs, and Picojoules, and not Farads, Coulombs, and Joules.
2) How much smaller than a Farad is a Picofarad?
Now, reset the simulation. (You will have to re-check all the boxes.)
Slide the voltage leaver on the battery back and forth between +1.5V and -1.5V several times. There will be several things changing as you do this.
3) Describe in detail below what you see happening. Specifically, changes in capacitance, plate charge, electric field, stored energy, and current direction.
Now slide the voltage leaver all the way to +1.5V.
As you did before, by using the arrows, adjust both the separation of the capacitor plates and the plate area.
This time pay particular attention to what happens to the plate charge and the stored energy as you make your changes.
4) Record your observations below:
Now, reset the simulation. Re-check all the boxes. Slide the voltage leaver all the way to +1.5V. Using the arrows as before, minimize both the plate separation and the plate area.
Next disconnect the capacitor from the battery
Once you have done this, adjust the plate area and the plate separation. Pay attention what happens to capacitance, plate charge, and stored energy as a function of these variables.
5) Describe in detail your observations. Include what has changed from when the battery was connected:
6) Explain why you think the plate charge remained constant.
7) Explain your observations of how the capacitance changed when you adjusted the plate area , in terms of what you would expect from Equation #3 above.
8) Explain your observations of how the capacitance changed when you adjusted the plate separation , in terms of what you would expect from Equation #3 above.
Go back to the main screen and click on the Light Bulb Lab icon.
This simulation is nearly identical to the previous one, but with the added feature of being able to discharge the capacitor through a light bulb.
Again, check all of the boxes before starting. Take some time charging up the capacitor while it is in different configurations and then discharging the stored energy through the light bulb.
1) Write down and briefly discuss as much as you can about what you observe as the capacitor discharges in these various configurations. Pay particular attention to what you observe about the rate of capacitor discharge.
Now, connect the battery, minimize the plate separation and maximize the plate area. As illustrated to the right.
Next, disconnect the battery placing the circuit in the “neutral” position. Now maximize the plate separation and minimize the plate area. As illustrated to the right.
This will provide you with the maximum possible stored energy or voltage potential.
Finally, connect to the lightbulb and discharge the capacitor.
2) What do you observe about the rate of discharge of the capacitor even with maximum energy storage?
3) Describe how you will need to configure the simulated capacitor to achieve the maximum capacitance?
Now, set your capacitor up in that configuration. Charge it up, and then note how long it takes to discharge through the light bulb.
4) Describe what you observed.
5) What one variable seems to be the most important in determining how quickly the capacitor will discharge?
[Side note: Adding additional components to a circuit, in particular resistor(s), can prevent the overly rapid discharge of the capacitor that you observed.]
Watch the following YouTube Video on capacitors. You should find it very instructive.
Capacitors and Capacitance: Capacitor physics and circuit operation
( https://youtu.be/f_MZNsEqyQw )
After watching the above video, answer the following:
6) Use your own words to describe the how a capacitor is able to hold an electric charge.
7) What are three ways that you can increase the capacitance of a capacitor?
Briefly explain how each of these methods works.
Finally, do an internet search for the term:
“ Fast-charging supercapacitor technology ”
8) In one or two sentences explain why this new technology might offer some very exciting possibilities for the future.
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