Download Homework Problems on Electric Fields

Homework Problems due Monday 2/4/08 in class
PH308 Bennett
Explain completely your answers to all questions, using the definition of electric field.
1. The Earth has an electric field whose magnitude near Earth’s surface is about 150 N/C.
The direction of the field is towards the center of the Earth. Let’s assume that this field is
made by charges that are spread uniformly over Earth’s surface.
a. Determine the sign and magnitude of the charge on Earth’s surface. Explain completely,
using the definition of electric field. A diagram showing Earth’s electric field lines is
required as part of your explanation. You will need to find out what Earth’s radius is in
order to answer this question.
b. Suppose there is a charged particle (maybe a positive or negative ion) in the atmosphere
near Earth’s surface. The mass of particle is 10-25 kg. The only forces acting on the
particle are gravity and an electric force. If the particle is in equilibrium, determine what
the sign and magnitude of the charge of the particle must be.
2. A negatively charged oil drop whose mass is 10-15 kg and whose charge is 6.4 10-19
Coulombs remains stationary under the influence of the Earth’s gravitational field and a
constant external electric field made by some charged parallel plates. Ignore the
influence of Earth’s electric field on the oil drop.
a. Determine the magnitude and direction of the external electric field (not the electric field
of the oil drop !). A diagram showing the external electric field and the charged parallel
plates is required – make your field lines clearly show that the magnitude and direction of
the electric field are constant. Explain completely.
b. How many excess electrons does the drop have ?