Electrostatics Review Packet (with solutions…online )

  1. Two, identical metal (conducting) spheres have charges of +8 x 10-7 C and -3 x 10-7 C on them. They are touched together. What is the remaining charge on each?

2. Explain in words what happens when a positively charged insulating (plastic) rod is brought near, touched, and then removed from a neutral electroscope.

3. A metal sphere has a net charge of -3.4 x 10-8 C on it. How many electrons must be taken away (or added) to give the sphere this charge?

  1. Explain in detail how you could take a charged object and use it to charge another object with the same sign charge. (Hint: would you use friction, contact, or induction?)
  1. Sketch, or describe in words, how you would charge a neutral metal rod to become negative by method of induction. Make sure to show (or explain) all steps clearly.
  1. Two metal spheres are near each other. One sphere has 3 x 108 excess electrons and the other is lacking 2 x 108 electrons. What is the force between them when they are 4 cm apart?
  1. What would happen to the force in #6 above if one charge were tripled, the other was quartered, and the distance between them was reduced to one-third its original amount?
  1. If one of the metal spheres from problem #6 above was touched by a neutral sphere of the same size (and then this sphere was removed), what would the force between the spheres be? What would the force between the spheres be if this process were repeated (every time with a neutral sphere touching the originally charged sphere) 4 times?
  1. A charge of -6 x 10-6 C feels a force of 2 x 10-8 N acting downward. What is the magnitude and the direction of the electric field that the charge must be sitting in?
  1. What is the magnitude and direction of the electric field at a point 4cm below an electron? How much electric potential energy is stored in the electric field at this point if another electron is sitting there?
  1. Two like charges, each of magnitude 6C, are held 4 mm apart. How much electric potential energy is stored in the electric field between the charges?
  1. A charge of 3.2 x 10-6 C is placed at the origin of a number line. Another charge of -8.0 x 10-6 C is placed on the number line at x = 6 cm. Determine the electric field at a point x = 10 cm on the number line due to the charges.
  1. A charge of -3.2 x 10-6 C is placed on the upper left vertex of a square with sides 2cm. Another charge of value 8 x 10-6 C is placed in the lower right hand corner of the same square. Determine the net electric field at a point in the lower, left-hand corner of the square.
  1. Label the direction of force acting on a proton if it were placed in the electric field below at point A, B, or C. Then, label the force acting on an electron if it were placed at points D, E, or F shown below.
  1. Draw the electric field created by two charges of like sign that are near each other. Assume that the charges have the same magnitude.
  1. Draw the electric field created by two charges of different sign that are near each other. Assume that the positive charge is twice as large as the negative charge. Where is the electric field the strongest? Explain.

17. One model of the structure of the hydrogen atom consists of a stationary proton with an electron moving in a circular path around it, of radius 5.3 x 10-11 m. The masses of a proton and an electron are 1.67 x 10-27 kg and 9.1 x 10-31 kg, respectively.

a)What is the electrostatic force between the electron and the proton?

b)What is the gravitational force between them?

c)Which of these two forces is responsible for the electron’s centripetal acceleration?

d)Calculate the velocity and the period of the electron’s orbit around the proton.

18. Assume that a single, isolated electron is fixed at ground level. How far above it, vertically, would another electron have to be so that its mass is supported by the force of electrostatic repulsion between them?

19. A charged oil droplet of unknown mass is balanced between the parallel plates shown below by an electric field of strength 4,000 N/C. If the magnitude of the charge on the droplet is 6 x 10-16 C, find the droplet’s mass AND the sign of the charge (positive or negative).

20. An electron (me = 9.11x10-31 kg) is held at rest between two parallel plates, with opposite charges. It I then released. The strength of the electric field is a uniform 200 N/C. Find:

a)the potential difference as the electron moves 20 cm closer to the positive plate.

b) the potential energy difference between these two points.

c) the velocity the electron would attain while moving these 20 cm.