Lecture for Payap University General Science Course

1. Electrostatics Solving problems involving stationary electric fields

3. Electrostatics Electric Flux: The amount of electric field passing through a surface area (The angle  is the angle between a normal to the surface and the electric field) A E

6. A Gaussian surface A Gaussian surface that completely surrounds a point charge intercepts the same number of field lines regardless of its shape. For a positive charge, the lines exit the surface; for a negative one they enter it.

7. Gauss ’s Law for Electric Fields If a greater amount of charge is enclosed, more field lines cross the surface.

9. Gauss ’s Law for Electric Fields The net number of electric field lines passing through an imaginary closed surface is proportional to the amount of net charge enclosed within that surface. This can be used to show that excess charge on a conductor must reside on the surface: E is 0 inside, so electric flux through Gaussian surface just inside conductor is 0, so no net charge is enclosed.

10. Using Gauss’ Law Can we use the law to calculate electric field from a point charge? This is equation for field from a point charge, so it works! We know field from positive point charge points away from charge. Put Gaussian sphere around it, with radius r. What’s the flux? Gauss: So:

12. One last bit on electric fields Let’s look at the strength of the electric force: electrons in the human brain if we took away all the protons. There are roughly 4.2 x 10 26 electrons in your brain. Let’s assume the brain is a sphere with radius 10 cm, calculate force on one electron near the surface: Putting in numbers, we get about 10 Newtons of force on one electron. Considering the mass of electron is only 9.1x10 -31 kg, acceleration would be pretty severe: F= ma…

13. One last bit on electric fields Let’s assume the electrons are uniformly distributed on the surface of the brain (as if the brain were a conductor). What’s the force per unit area? (F/A is called pressure ) When I stand on the ground, what pressure do I exert? Roughly, So pressure from un-neutralized charge in the brain = 30,000,000,000,000 men standing on your head