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.
• At this time in your academic
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.
• http://faraday.physics.utoronto.ca/GeneralInterest/Harrison/Flash/EM/Co
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.