Electricity and Magnetism


1. Which of the following effects are fundamentally electrical in nature? Briefly discuss the origin of each





a. tension in a spring,
b. “crackles” when you take clothes off,
c. “crackles” from walking on dry leaves,
d. the spiral structure of galaxies,
e. nerve conduction,
f. nuclear fission,
g. the auroras,
h. pressure in a gas.
2. In a simple (but not very accurate) model of the helium atom, two electrons (each of charge = -e) orbit a
nucleus consisting of two protons (charge = +2e) and two neutrons (charge = 0). Is the magnitude of the force
exerted on the nucleus by one of the electrons less than the force exerted on the electron by the nucleus?
Explain your answer.
B. Activity Questions:

1. Tape Charge
Stick two strips of tape on the desk, then peel them off.
Hang them close to each other and see what happens. Explain your observations.
2. Electroscope and electrophorus
Charge up the plate using the electrophorus, by first rubbing the lower plate with the rubber glove, then placing
the metal plate on the lower plate. Before the upper plate is removed, touch the top of the metal plate with
your finger.
Explain how the metal plate becomes charged.
One can separate the electroscope leaves by both touching and not touching the electroscope with the metal
Explain how.
3. Charged rods
Charge up the rods using different materials. How do the items get charged?
Balance a charged rod on a watch glass. How can you accelerate it without touching or blowing on it?
C. Quantitative Questions:
1. Newton’s law of gravitation says that the magnitude of the force between any two objects with mass is
proportional to the masses of the objects and decreases with the square of the distance between them:
FG = 2
1 2
Gm m

a. How is Newton’s law of gravitation similar to Coulombs law? How is it different?
In a simple (but not very accurate) model of the hydrogen atom, an electron orbits the nucleus at a mean
distance of 5.29 × 10-11 m. The nucleus (a proton) has a mass of 1.67 × 10-27 kg and the electron has a mass of
9.11 × 10-31 kg.
b. What is the ratio of the gravitational force to the electrostatic force acting on the electron due to the nucleus?

a. Write an expression for the flux through the Gaussian surface shown.
b. Which of the charges contribute to the electric field at point P?
Suppose you have a Gaussian surface which contains no net charge.
c. Does Gauss’ law require that there is no electric field at all points on the surface? Draw a diagram to help
explain your answer.
d. Is the converse necessarily true, i.e. that if the electric field is zero everywhere then the charge contained
must be zero?

2. A “Faraday cage” is a metal cage used to shield devices from electromagnetic fields. These are often used in
precision experiments when a very small voltage needs to be measured. For example, a Faraday cage is usually
used when measuring potential changes across nerve membranes, otherwise these changes cannot be detected
due to “noise” from nearby electrical wiring.
a. Using gauss’ law, explain how a conducting wire cage can shield its contents from external electric fields.
It is regularly proposed that Faraday cages be placed around sources of electric fields, such as mobile phones
and large transformers, to protect people from possible effects of exposure to electric fields.
b. Would this shield nearby people? Explain why or why not.

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