Reading Quiz 3 Key

Reading Quiz 3 Key

Due before class Monday, Jun 25th

Physics 106, Summer 2012

Reading: Chapter 17 (All)

Did you complete the reading assignment?

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?


1)  Electric potential, or voltage, is the potential energy per unit of charge of an electric field.

2)  Electric potential is a scalar quantity.

3)  A capacitor is a device that stores charges.

4)  The separation distance, dielectric material, and area of the plates determine the capacitance.

Electric potential has units of volts. What units make up a volt?

Newtons * Coulombs
Newtons / Coulombs
Joules * Coulombs
Joules / Coulombs

Can electric potential exist with just one charge?


What happens when a negative charge is placed in a negative electric potential?

Exercise 1

Visually, you can think of electric potential as a geographical contour map. A contour map separates altitudes with loops, each loop representing the same change in altitude. You can think of this as being levels of gravitational potential energy; as loops on a contour map come closer together, the gravitational potential energy experienced by an object going through that area changes more quickly. The same can be said for the electric potential of charges in an electric field.

Go to the following website and watch the simulations on it:

Does bringing two opposite charges closer together (like in the first simulation) change the electric potential between them?


Compare two opposite charges that are far apart with two opposite charges that are close together. Will the electric potential for a particle moving from one charge to the other be faster for the far charges or the close charges?

Charges that are far apart.
Charges that are close together.

What do you know about the electric potential an equal distance from two opposite charges with the same magnitude of charge?

Exercise 2

Click on the 'Go to exercise 2' link on the bottom left of the website. Then click on the 'Start SimPhysics' button on the new page to open the applet. On the applet, select 'Electric Potential Field' from the display menu and click on the 'camera' tab at the top and select 'show plot' to see the electric potential of the charges. Delete one of the charges by right clicking on it give both remaining charges the same charge. Changing charge value can be done by double clicking on the charge and changing the value of q. You can move these charges by clicking on them and dragging them to the location you would like.

Compare two similar charges that are far apart with two similar charges that are close together. Will the electric potential for a particle moving from one charge to the other be faster for the far charges or the close charges?

Charges that are far apart.
Charges that are close together.

Does bringing two similar charges very close to each other change the maximum electric potential produced by the charges?


Exercise 3

Capacitance is the ability of a body to hold an electrical charge or the measure of the amount of electric charge stored for a given electric potential. One way to store electricity is to collect a bunch of charges and then when you need a little boost you can let these charges go. Electrically, we call this a capacitor. When placed in a circuit it can collect charge as the electrons flow. When electrons reach one side of the capacitor, they cannot easily jump across the gap. Instead they collect on one side. The electrons on the other plate can still feel the electric force pushing them away, so they leave the plate and flow further down the wire leaving a positive plate where the electrons left, and a negative plate where they collect.

Go to this link and play with the capacitor:

Change the values of the plate area and distance between plates. Also change the dielectric material used between the plates.

Does capacitance increase or decrease as the separation distance of the plates increases?


Why does distance effect capacitance in this way?

Does the capacitance increase or decrease as the plate area increases?


Why does the plate area effect capacitance in this way?

Which dielectric material yields the greatest capacitance? Why?

The units of capacitance are measure in Farads, what units make up a Farad?

Volts * Coulombs

In the activity, the units on the capacitors you built were given in picofarads (a picofarad is 1 x 10-12 Farads or 1 trillionth of a Farad). Televisions and computer monitors need capacitors that are about 100 microfarads (1 microfarad is 1 x 10-6 Farads). Why should you never open up a TV or computer monitor to try and fix an electrical problem, even if it is unplugged?


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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?

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