## Due before class Monday, Jun 25th

Physics 106, Summer 2012

Did you complete the reading assignment?
Yes
No

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?
Yes
No

## KEY CONCEPTS:

1)  Electric force is a VECTOR quantity.

2)  Opposite charges attract.  Like charges repel.

Forces in nature are easily recognized on a large scale, but when we think of electrical forces we must think about particles much smaller than we can see. These demos will help you visualize charged particles. The first simulation is about electrical forces. Copy and paste the following link in a new window:

Open the link and play with the demo.  The small yellow charges (electrons) are free to move and the red charge (a proton) is stuck in one place.  Think about the following questions:

What happens if you place a yellow charge close to the red charge and let go?

If you release a yellow charge from the border of the box, does it move toward the red charge more quickly or more slowly than if it is released from close by?

more slowly
more quickly

How far away do you think you would need to move the yellow charge away from the red charge so that they would not affect each other?

Try "throwing" a yellow charge with a combination click and drag.  Watch the screen--charges that fly off one side can return from another.  Can you make a yellow charge orbit the red charge?

Yes
No

Try turning on the "Show Force" button.  The red arrows attached to the yellow charges are VECTOR arrows and show the direction and magnitude of the electric force by the red charge on the yellow charge. What would your textbook's notation be for this force?

Remember that velocity is also a vector quantity.  What affect does an initial velocity have on a yellow charge?

Look in your books for the equation of gravitational force, how does it compare to the equation for electrical force?

Here is another simulation for you to try:  http://www.colorado.edu/physics/2000/applets/nforcefield.html

In this simulation the charges are not free to move, but are stuck to the background until you move them around.  The force vectors point in the direction the charges would like to move if they weren't stuck, and the length of the force vectors show how much they want to move in that direction.  Drag a yellow charge closer to the negative terminal.   What happens to the electric force vector?

It gets larger.
It gets smaller.

Drag a yellow charge closer to the positive terminal. What happens to the electric force vector?

It gets larger.
It gets smaller.

Since "opposites attract", are the yellow charges positive or negative?

Positive
Negative

Try putting several yellow charges in the field by double clicking on the background and then selecting "new particle". Then grab a red or blue terminal and drag it around the field.  Watch what happens to the force vectors on the yellow charges.

Try putting 2 positive terminals and one yellow test charge in an equilateral triangle formation. Which way does the force vector point?

Between the two positive terminals.
Away from the two positive terminals.
There is no vector arrow.

This is an example of vector addition. We will practice vector addition with electric forces in class.

## FEEDBACK

The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:

1) If you have a specific question or concern and would like an individual answer you will need to come by the office or send an email.
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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?