Gauss' Law 3/24/2014

Today, we started with how Electric Field has impact on electrons.  We warmed up with a small demonstration of how old CRTube monitors work as the motion of a charged particle in a uniform electric field is being attracted across the display.

a CRT thing tube

Prof Mason tweaking the monitoring machine

We then were introduced to Electric Dipole in an Electric Field.  This dipole moment is defined as the product of the magnitude of charge and the distance of separation between the charges.  These are all a preparation for the main attraction-  ELECTRIC FLUX.

Our class experiment involves hanging some foils both the outside and inside of a metal cylinder.  We tested the reaction when the van de Graff generator is turned on, putting large negative charge onto the cylinder conducting onto the foils.  The result??

Only the outer foils will move away from the cylinder because the electric charge moves to the outside of the cage, leaving no net charge on the inside. Therefore no effect is felt by the foils on the inside of the cage.

The nail bed apparatus to determine how many uniformly spaced flux lines will pass through an imaginary surface area as a function of the angle between the direction of the flux lines and the normal vector representing the surface area.

The nails represent the electric field and the flux is perpendicular to it.  (area of the flux surface made by the wire hanger). Using a ruler and tangent, we measured the angle made with the surface and graphed how the angle changed with respect to the amount of nails covered by the enclosed wired surface.  We used a wire square as it contained 49 nails, the angle of the normal vector in this case was obviously 0 degrees. We then angled the wire so that it contained 42 nails and determined the angle. We continued to do this in decreasing increments of 7 nails until we reached zero and then used symmetry to determine the angles of up to -49 nails.

We constructed a graph of electric flux vs. the angle of the normal vector. These points were plotted in LoggerPro and a graph was constructed.  The graph is similar to the cosine function. This graph makes sense because the electric flux through a surface can be expressed as EAcosTheta.


Finally, we wrapped up the day with some Active Physics.  We conclude that the

In the simulation, the yellow ring is the surface across which we determine the electric flux. The flux is caused by the electric field, the green lines, caused by one or two electric charges.