# Notes: Coulomb`s Law ```Coulomb’s Law
The Apparatus
• This device is called
a torsion balance or
a Coulomb’s Law
apparatus. When
both spheres are
charged the
apparatus measures
the angle of twist on
the suspended
sphere. Force can
be found from this.
endeavors, you are probably
quite aware that there are two
types of electric charges,
positive and negative. (Besides
credit cards!!!.)
• Like charges repel each other
and opposites attract.
• To further characterize the
force F between two point
charges q1 and q2 separated by
a distance R, Coulomb’s Law
states the magnitude of F:
• F is the magnitude of the
electrical force between
the two point charges,
• k is Coulomb’s constant,
• q1 is the charge of the first
point in Coulombs,
• q2 is the charge of the
second point in Coulombs,
• r is the distance between
the two point charges in
meters.
So what is a Coulomb?
• A coulomb is a specific
quantity of charges.
The charge of 1
•
electron has been
determined to be…
•
• -1.6x10-19 C = 1 e-,
(Coulombs) which we
call an elementary
charge. A proton is
this same value, only
positive.
So how many charges
make up one coulomb
of charge?
1 C / (1.6x10-19 C/e-) =
6.25x1018 electrons (or
protons) per coulomb.
How Strong is Electrical Force?
• Electrical Force is a very large - HUGE - force
compared to the force of gravity.
• If we could scrunch up 10 grams of
electrons into a little ball and 10 grams of
protons into another little ball and place
them about a pinkie width apart, the force
between them would be the same as the
gravitational force between the Earth and
the moon.
ulomb/Coulomb.html
Compare Coulomb’s Law to
Newton’s Universal Law of Gravitation!
• Note that this equation
looks like Coulomb’s
Law Equation!
• Substitute in Mass for
charge and a wimpy,
weak gravitational
constant G, and you can
11
2
2
G = 6.67 x 10 Nm /kg
solve for the
gravitational force
between two objects!
What? Why? Try….
• What does it mean to have a negative
force in Coulomb’s Law? …Positive
force?
• Why is the force in Newton’s
Gravitational Law mathematically
always positive?
• Try out the questions on the next slide
and write down the answers on your
note paper.
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