Week 24 (Feb. 5 - 9)

Reading: Electric fields (Chap. 23)

Key Topics:
Electric charge, triboelectricity, insulators and conductors, coulomb's law, electric fields

Homework Problems: Due Sunday, February 18.
  1. Proton and electron: Suppose that a proton and an electron are separated by a distance of 1 nanometer. (a) How hard does the proton pull on the electron? (b) How hard does the electron pull on the proton? (Answer: each of them pulls on the other with a force of 230 pico-newtons). (c) how would your answers change if the distance between them were doubled? (Answer: if the distance were doubled, the force would be reduced by a factor of 4.)
  2. Electron accelerator: Suppose that two large parallel plates are connected to a 1 kilo-volt power supply so that the left plate is maintained at an electric potential of +500 volts and the right plate is maintained at an electric potential of -500 volts. The plates are separated by distance of 1 mm. (a) First, sketch the electric field in the region between the plates. (b) What is the strength of the electric field between the plates? (Answer: 1000 volts / 1 mm = 1,000,000 volts/meter) (c) If an electron is placed very near the +500 volt plate, what is the force exerted on the electron by the electric field? (Answer: the force is just the electric field times the electron charge. This gives 1.6 e -13 Newtons) (d) How much work does it take to move the electron from the +500 volt plate over to the -500 volt plate? (Answer: this is the force times the distance. It is 1.6 e-16 joules) (e) If the electron, placed near the -500 volt plate is released, what will be its acceleration toward the +500 volt plate? (Answer: the acceleration is the force divided by the electron mass. it is a =1.76e17 meters/second^2) (f) what will be the electron's speed just before striking the +500 volt plate (Answer: using conservation of energy, we can set the work equal to the change in kinetic energy. This gives v =1.88 e 7 m/s )?
General College Physics