Physics 4B Bli: lab11 March 31 Gauss'slaw

In this class, we explore the Gauss's Law.

Question: What is the relationship between the net flux and the net charge enclosed by a two-dimensional “surface”?

Process: By using a java applet, we model the flux and the charge on computer and draw the diagram on whiteboard. By counting the flux, we find that total flux is proportional to the total charge. Then, we conclude that Electric Flux over a Closed Surface = Charge enclosed by the Surface divided by eo.

Conclusion:

Faraday cage

Each of the suspensions include a string passing over the top of the cylindrical screen with small rectangular chunks of aluminum foil, a few centimeters in size, attached to the ends of the string. Note that one end of each string is outside the cylinder and one end of the string is inside the cylinder.

Question:In particular, when the cylinder is charged, which, if any, of the aluminum foils will move away from the cylinder, either inward or outward?

Process:Turn on the charge generator, the foil outside moves outward; The foil inside keeps still.

In this case, the black cylinder is an electrical conductor. We did the experiment to prove that "if there are no moving charges inside a conductor, the electric field in the conductor must be zero."

Excess charge

Question:If the conductor has excess charge and it can’t be inside the Gaussian surface according to Gauss’ law, then what’s the only place the charge can be?

Process:we drew the diagram to show where are the charges.

Question:

You are trapped in a lightning storm in your car. What is your best course of action. Give a detailed description to support your answer.

1) Get out of the car and run to the nearest tree

2) Get out of the car and lie down flat on the ground.

3) Seek the highest point nearby and put up your umbrella

4) Seek the lowest point nearby preferably a ditch or ravine.

5) Stay in your car

Conclusion:A car is seemed as an electrical conductor. Staying inside is safer.

Professor Mason put light ball and CD inside a microwave. The light bulb lightens up and the CD is burned. We assumed the cd is an electrical conductor and has free charges on the surface. As it heat up, the electrons move rapidly and damaged the texture of the cd.

Then, we use Gauss's Law and symmetrical charge distribution to solve electrical fields.

Question:

compute the magnitude of the electric field at a distance r from the center of a uniformly charged sphere of radius R with a total charge of Q throughout its volume, where r < R

Process:To use Gauss’ Law to find the electric field, you’ll need to know the charge enclosed inside a sphere/cynlinder of radius r/cross-section area radius in which charges are distributed uniformly through the larger sphere/cynlinder of radius R

We get q=(r^2/R^2)Q for cylindrical shell, and q=(r^3/R^3)Q for sphere.

We substitute the equation E=q/e, and get the charge of outside cylinder shown in the whiteboard.