from Newton's law of universal
gravitation. It states that every matter which
has a mass attracts other matters with a force that
is directly proportional to the product of their
masses and inversely proportional to the square of
the distance between the centers of gravity of the two matters.
We already studied about gravitational force of an
object on earth, which is F = m*g, where
"m" is mass of the object and "g"
is the gravity of the earth. Then, we can say that .
Therefore, gravity (g) of the earth is , where
"me" is the mass of the earth
and "d" is its radius (we are talking about
gravitational force on the surface of the earth.).
However, electric field E is a little bit
different from gravitational field g. Gravitational
force depends on mass, whereas electric force does
not depend on mass. Instead, electric force depends
on charges on both objects.
By rearranging the formula, we get:
are the magnitude and direction of the
electric field 1.5 m away from a positive
charge of 2.1*10-9 C?
(e.g. 1.0 N/C)
(e.g. "outward" or
PE = qEd (see right)
Voltage is also related to force.
V = Ed = (F/q)*d = Fd/q=
(W = Fd -- force
times displacement in the direction of force is work
A high voltage means that each individual charge
is experiencing a large force. A low voltage means
that each individual charge is experiencing a small
the diagram of A and B above, rA
is 0.0005 m and rB
is 0.0003 m. What is the force acting on q at
A and B, if the charge of q is 1.2 * 10-11
C and the charge of Q is 1.5 * 10-11
(e.g. 1.00*10^-10 N) A =
(e.g. 1.00*10^-10 N) B =
B has high potential energy
because these particles want to come together. It is
the same priciple as the gravity.
D has lower
potential energy compare to B
for the same reason.
Try Chapter 12 Quiz and see how much
[Ch 11] - [Ch 12] - [Ch 13] - [Ch 14] - [Ch 15]
Colegio Franklin Delano
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