Download There are only two charges, positive and negative.

When one object is
given a net electric
charge by placing
it in contact with a
charged object it is
called charging by
contact.
Ch 18
If a charged object is held
close to another object,
and the second object is
temporarily grounded before
the first object is removed;
the second object is left
charged opposite the initial
charge.
This is charging by induction.
Charging by contact makes
both objects the same
charge. Charging by
induction will make the two
objects opposite charges.
F = k•q1•q2/
2
r
k is a proportionality
constant whose value is
9
2
2
k = 8.99 x 10 N•m /C
A small point charge,
called a test charge,
may be used to
determine the extent to
which the surrounding
charges generate a
force.
The electric field E that exists
at a point is the electrostatic
force F experienced by a
small test charge q0 placed
at that point divided by the
charge itself:
E = F/q0
The electric field is a
vector, and its direction
is the direction of of the
force on the charge.
The unit is the newton
per coulomb (N/C).
From Coulomb’s law, the
force exerted on a test
charge q0 by a charge q is
2
F = k•q•q0/ r . Since E = F/q0,
E is also equal to k•q•q0/ r2
divided by q0. q0 cancels out,
and we are left with
2
E = k•q/ r .
A parallel plate capacitor
has two plates of different
charge with a space
between them. The
charges are distributed
uniformly over each plate.
The electric field
points from the
positive plate to the
negative and is
perpendicular to
both.
In a parallel plate capacitor, the
field has the same value at all
places between the plates. The
field does not depend on the
distance from the charges, as it
does in a field created by an
isolated point charge.
Electric field lines are
always directed away
from positive charges and
toward negative charges.
Where lines are closest
together, the electric field
is strongest.
At equilibrium
under electrostatic
conditions, any excess
charge resides on the
surface of a conductor.
Free electrons within the
conductor are not moving,
so no electric field exists
there. At equilibrium under
electrostatic conditions, the
electric field at any point
within a conducting material
is zero.
Gauss’ law
The electric flux through a
Gaussian surface is equal to the
net charge q enclosed by the
surface divided by the ε0 , the
permittivity of free space:
FE = ∑(E cosf)∆A = q/ε0.
The SI unit of electric flux: N•m2/C
Ch 19
Ch 18