Physics 106, Summer 2012
Reading: Chapter 16 Sec 1-3
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?
Electric force is a VECTOR quantity.
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:
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
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?
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?
They will always affect each other no matter how far apart they are, but the farther apart the two charges are the weaker the affect will become.
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?
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
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?
If the charge has an initial velocity, it will be moving. It causes the force to continually change direction depending on the current location of the charge.
Look in your books for the equation of gravitational force, how does it compare to the equation for electrical force?
Masses are exchanged with charges and the Universal Gravitation constant 'G' is replaced with Coulomb's constart 'k'.
Here is another simulation for you to try:
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?
Since "opposites attract", are the yellow charges positive or 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.
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Was there anything that you didn't understand in the reading assignment?
What was confusing to you?