oct. 8, 08 blog posted by Sung

Hey folks! Here's a blog for today's lecture. Since I personally don't like reading a long, long writing, so I will summerize as much as can.

Quiz
An equilateral triangle with a side of 10 cm. One charge with 2nC on the top, and two charges with 1nC on the bottom.
(A) Electrical field at the center?
Use geometry to find distances from each charge to the middle point. Also, the x-components cancel out because of the symmetry. So, add each E in y direction, and it comes out to be about -3000N/C
(B) Eletrical force on the top charge?
Again, by symmetry x-components of force cancel out. The net force directs the positive Y axis. I'd find a force by one of the bottom charges in Y direction and double it since both bottom charges have same charges and y distances.
(C) Now the bottom charges are replaced with a long rod. How does the force change compared to part B?
Use integral. Please refer to the book chapter 21, in which it discusses about eletric field in a thin, long rod. I used trig sub to integrate. So the total eletric field comes out to be 8.2*10^-6 N, which is bigger than that of partB.

Lecture.
There is much similarities between gravitational force and eletrical force. The man difference is that gravitational force is neglectable when we calculate gravitational force. Also, the eletrical force can repel. By the analogy, we learned a new equation, which deals with its potential energy, more specifically "eletrical potential energy."
Ue = q0 * E * d
or by simply substituting E = k*q/r^2
Ue = k*q*q0/r (d = r, for point charges)
When there are more than two charges,
Ue = q0*k*sumof(qi/ri) (qi, ri for each "source" particles)
If there is a eletrical potential energy, can we not find work done by the eletric field too?
Knowing the relationship between work and energy as W = mgh1 - mgh2,
We = q0Ed1 - q0Ed2
Or using integral
We = integral(q0*E*ds, from A to B)
*By the way, if you got stuck at problem 4 on today's homework, you are going to use this equation basically. Know the relationship between work done by the electrical field, and energy, which will also give you much aid!
One important concept of work is that when the force and the displacement are perpendicular, there is no work. A good example is the normal force, which is always perpendicular to the surface, and the displacement!
Voltage is the eletrical potential energy difference per unit charge. This notion has some similarity with eletrical force and eletrical field. Simply divide deltaU by q0:
V = q0*E*deltad / q0 = E*deltad J/C
or using integral
V = integral(-E*ds, from A to B)

Ask Sung for anything you need. I will try to update if you COMPLAIN anything about my BLOG. Let's all go to the speech on friday! it is going to be exciting. ahhh Prof. Mason, may I ask you a favor? I am a semi-vegeterian, and I only eat cheese pizza. So please make sure to have cheese pizza for Sung >_< We love you mason and i am having so much fun in the class !!!

P.S. Does Joy really smell ? +_+ JOyyyyyy >_<