“PHLYZICS”
Last chapter, we learned that masses attract each other. We learned that the attractive force
between two masses could be found using Newton’s Universal Law of Gravitation (Newton’s
ULOG),
Fg
GmM
R2
, where R is the distance between the centers of the objects. “G” was simply a
constant, and its units could easily be found by solving the above equation for G.
~~~
When dealing with charged objects, we also talk of forces between them. Unlike last chapter,
these forces can be EITHER attractive (for unlike charges) OR repulsive (for like charges). To
find the force between charged objects, we can use Coulomb’s Law, which is
2
d
kqQ
Fe
. In this
equation, “d” is the distance between the objects, q and Q are the charges on the charged objects,
and k is a constant equal to
k8.99 109N m2
C2
. This law is similar in form and structure to
Newton’s ULOG, and the relationships that we used last chapter still apply. For example,
Feq
(ex: If you double the charge on an object, you double the force)
FeQ
(ex: If you quarter the charge on an object, you quarter the force)
FeqQ
(ex: If you double both charges, you quadruple the force)
2
1
d
Fe
(ex: If you double the distance between the charges, you quarter
the force. If you divide the distance between the charges by 3,
you multiply the force by 9 times).
~~~~
NEVER, EVER plug signs into Coulomb’s law. If two charged objects have opposite signs,
plug only their magnitudes into the equation. At the end, make sure to account for the fact that
they have different signs by saying that the forces were either “repulsive”. If the charges had
opposite signs, you account for this by saying that the charges
~~~~~
It is important to see that we can easily use
qne
(or
Qne
) and
2
d
kqQ
Fe
together in
problem solving. If you are told how many electrons an object gains or loses, you can easily
calculate “q” or “Q” to plug into Coulomb’s Law.
~~~~
Similarly to Newton’s ULOG, Coulomb’s Law is a vector law. When multiple charges are
involved, make sure to draw the forces acting on each charged object (as vector arrows). Again,
do not put signs into Coulomb’s law, as the vector arrows provide sufficient direction. Once the
object has all the forces draw on it (free-body diagram style), then the forces can be added as
vectors.
Coulomb’s Law
