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Coulomb’s Law
Apparatus
balloons
long string (2-3 meters)
Goal
To see how much charge you can rub onto a balloon,
or to place an upper bound on the amount of charge
placed on the balloon.
Introduction
A Situation A: Balloons don’t touch
Sometimes, when you rub a balloon on your hair,
your hair clings to the balloon. The force is much
stronger than a gravitational force – it is electrostatic
in origin, and occurs because some electrons from
your hair have been ripped off your head via friction and transferred to the balloon. The positivelycharged hair is then attracted to the negatively-charged
balloon.
If you take two such balloons that have been rubbed
on two peoples’ heads, the balloons will each pick
up a negative charge. We would expect these two
balloons to repel one another, with magnitude given
by Coulomb’s Law:
kq1 q2
r2
In this lab, we will try to measure Fe , and infer the
charge on each balloon.
Fe =
Observations
Blow up two balloons and weigh them. Take the
average and call it “m” — we’ll use this value of m for
each balloon (our final result is very approximate).
Try to recreate the image shown below with two balloons and a long piece of string. Tape the string to
the ceiling so that L is about 2 meters.
Now, have one person rub his/her hair all over one
balloon. As we will see later in the class, as long as
the charge is spread evenly over the surface of the
(spherical) balloon, then we can treat the balloon
as a single point charge at the center of the balloon. Have another person rub the other balloon on
his/her head. Let the balloons come to equilibrium.
Here, the electrostatic force between the two balloons is strong enough to counteract the horizontal
component of the tension in the string. Do the balloons stay the same equilibrium distance over time,
or do they get closer together? (after waiting, say,
a couple minutes.) What does this imply about the
charge on the balloons?
Draw a free-body diagram for one balloon (there are
three forces acting on the balloon). By setting q1 =
q2 = Q, and using other measurements you can make
in the lab, solve for Q, the charge on a balloon.
B Situation B: Balloons touch
In this case, the electrostatic force between the two
balloons is not enough to balance the horizontal component of the tension in the string: an additional
normal force from the other balloon is required as
well. Draw a free-body diagram for one balloon
(there are four forces acting on the balloon). By
setting q1 = q2 = Q, and using other measurements
you can make in the lab, give an upper bound for
the amount of charge on one balloon. That is, your
final answer should say, “The charge on the balloon
must be less than about
.” If the charge
was more than this, there wouldn’t need to be any
normal force from the other balloon to counteract
the horizontal component of the tension.
Analysis
In your write-up, clearly state the charge on one balloon, with error, in your abstract. Your calculations
and reasoning section should show how you arrived
at your answer (include a free-body diagram).
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