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Physics Pre-lab 212P-1
Exploring Static Electricity
Name:__________________________
Section:_____
Date:__________
(Read this & answer the questions before coming to lab)
Summary of relevant concepts:
 Fundamental properties of electric charge:
(a) There exist two types of electric charge: + (positive) and - (negative);
(b) Electric charge is conserved;
(c) Electric charge is quantized in units of 1.6 x 10-19 C;
(d) There exists an electric force between point charges which obeys the following rules:
 It acts along the line joining the two points and is repulsive for like charges and attractive
for unlike charges;
 The magnitude of the force is given by Coulomb’s Law:
Fk

q1q2
9
2 2
2 , where k = 8.99 x 10 N m /C
R
The atomic picture of matter:
 All matter is made of atoms containing a nucleus of neutrons (no charge) + protons
(charge = +1.6 x10-19 C) surrounded by a “cloud” of electrons (charge = -1.6 x 10-19
C). Note that the number of electrons and protons in an atom are the SAME. In solid
matter, the atomic nuclei might vibrate but do not otherwise move; the electrons on
the other hand can move. A useful microscopic picture of a solid is given below:
Gray: electron clouds
Black: nuclei (relative
size greatly exaggerated)


Insulators are materials in which electrons are only free to move around the
neighborhood of their respective atomic nuclei: note that the electron cloud around
each atom in the material can be "distorted" if an external charge is brought nearby.
So, in the picture above, imagine that the clouds of electrons can change shape.
Conductors are materials in which many electrons can freely move over long
distances. You’ll learn later that these “conduction electrons” come from the outer
regions of the atom. So, in the picture shown above, you could imagine the gray
electron clouds merging into a “sea” of mobile electrons.
Pre-lab Questions:
In Physics 211, you learned that all masses exert a gravitational force on each other obeying
Newton’s Universal Law of Gravitation.
Q1. List the similarities and the differences between the force of gravity and the electric force.
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Consider the example of the Hydrogen atom; a simplified (but very useful) model for this atom
consists of a single electron going in a circular orbit around a single proton. The parameters of
the Hydrogen atom are as follows:
Electron mass: 9.1 x 10-31 kg; electron charge: -1.6 x 10-19 C;
Proton mass: 1.7 x 10-27 kg; proton charge: + 1.6 x 10-19 C;
Radius of electron orbit: 1 x 10-10 m
Q2. What is the magnitude of the electrical force on the electron from the proton? Is it attractive
or repulsive?
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Q3. What is the magnitude of the gravitational force on the electron from the proton? Is it
attractive or repulsive?
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Q4. What is the ratio of the gravitational force on the electron to electrical force?
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Q5. If gravitational forces are so much weaker than electrical forces, why do we directly
experience the effects of gravity more readily in Nature than the effects of electric forces?
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Q6. A rule of thumb calculation (no calculators!): The Coulomb force between an electron and
a proton in an atom is given to be 10 nN (“n” = “nano” or 10-9). If the distance between these
charges is tripled, what now is the magnitude of the electrical force between them?
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Q7. How much is a “coulomb”? The concepts you encountered in Physics 211 were somewhat
familiar from everyday life – e.g. we all have a physical feeling for the meaning of a “meter”,
“kilogram” and “second.” But how much is a “coulomb”? Imagine that you and a friend each
held +1 C of charge at a distance of 1m from each other. Calculate the force of repulsion that you
would experience. Compare this force with your weight.
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Q8. Physics 211 Review: Static Equilibrium, Vector Addition & Trigonometry
The figure below shows a mass M suspended from a vertical string. A horizontal force F is
applied to the mass as shown, so that it is deflected by an angle  from the vertical. The entire
system is in static equilibrium. Draw the free body diagram for the forces acting on the mass and
determine an expression that relates F to the mass M and the angle 

F