17 Electric Charge & Electric Filed
Nature of Electric Charge
Two types of Charges: Positive and negative charges
Like charges repel each other (force?)
Unlike charge attract each other (force?)
Physical Basis of Electric Charge
All the atoms are electrically neutral
(ions, ionization & oxidization)
Same number of protons & electrons
Different number of neutrons:
Isotopes
Conductors & Insulators
If charges can move freely in a materials—conductor
If charges can’t move in a materials—insulator
Semiconductor, Limits, and Heat transfer
Metals are good conductors
What about water?
Induction & Polarization
The displacement of charge in an isolated conductor
when placed near by an electrically charged body
Separation of positive and negative charges
Polarization
Uncharged insulator
the polarized insulator
Conservation & Quantization of
Charge
Conservation: Total electric charges in any closed system is a
constant. Charge can be transferred from on object to
another.
Quantization: minimum amount of charge: e, the basic unit,
the charge of the electron or proton. Any Q = integer x e
e = 1.602x10-19 C or 1 C ~ 6x1018 protons(+) or electrons(-)!
mC, mC, nC…
Coulomb’s Law
F k
q 1 q2
r
2
k = 8.99 × 109 N m2/C2
Action & Reaction force
Units, Sign, and Direction
Superposition
Electric Field & Electric Forces

Qq '
F '  k 2 rˆ
r
Definition of Electric Field:

 F'
E
q'
Unit: N/C
Direction ?
Calculating Electric Field
Principle of Superposition
e.g., point charge and spherical charge distribution
Electric Field Lines—
Physical Meaning
Electric Field Lines—Examples
Beginning with positive charge and ending at negative or infinity
Electric Field Lines—Examples
+
+
+
+
+
+
+
+
+
+
+
+
-
*Gauss’s Law & Field Calculation
Electric Flux: E  EAcos   EA  4kQencl
Example: point charge
*Charges on Conductors
Summary: Charge, Conductors &
Insulators
Summary: Coulomb’s Law
Summary: Electric Field and Electric
Forces
Summary: Electric Filed Lines
Summary: Gauss’s Law
Summary: Charges on Conductors
Problem-Solving Strategy:
Coulomb’s Law
SET UP
1.
As always, consistent units are essential. With the value of k given earlier, distances must be in
meters, charges in coulombs, and forces in newtons. If you are given distances in centimeters,
inches, or furlongs, don’t forget to convert! When a charge is given in microcoulombs, remember
that 1 micro C = 1 mC = 10-6 C
SOLVE
2.
When the forces acting on a charge are caused by two or more other charges, the total force on
the charge is the vector sum of the individual forces. If you’re not sure you remember how to do
vector addition, you may want to review Sections 1.7 and 1.8. It’s often useful to use components
in an x-y coordinate system. As always, it’s essential to distinguish between vectors, their
magnitudes, and their components (using correct notation!) and to treat vectors properly as such.
3.
Some situations involve a continuous distribution of charge along a line or over a surface. In this
book, we’ll consider only situations for which the vector sum described in Step 2 can be
evaluated by using vector addition and symmetry considerations. In other cases, methods of
integral calculus would be needed.
REFLECT
4.
Try to think of particular cases where you can guess what the result should be, and compare your
intuitive expectations with the results of your calculations.
Problem-Solving Strategy:
Electric Field Calculations
SET UP
1.
Be sure to use a consistent set of units. Distances must be in meters, charges in coulombs.
If you are given cm or nC, don’t forget to convert.
2.
Usually, you will use components to compute vector sums. As we suggested for problems
involving Coulomb’s law, it may be helpful to review Sections 1.7 and 1.8. Use proper
vector notation; distinguish carefully between scalars, vectors, and components of
vectors. Indicate your coordinate axes clearly on your diagram, and be certain that the
components are consistent with your choice of axes.
SOLVE
3. In working out directions of vectors, be careful to distinguish between the source point S
and the field point P. The field produced by a positive point charge always points in the
.
direction from source point to field point; the opposite is true for a negative point charge.
REFLECT
4. If your result is a symbolic expression, check to see whether it depends on the variables in
the way you expect. If it is numeric, estimate what you expect the result to be and check
for consistency with the result of your calculations
If you charge a metal ball on an insulating stand/rod by
induction, which of the following happens?
A. The charge on the ball changes while the charge on the
rod stays the same.
B. The charge on the rod changes, while the charge on the
ball stays the same.
C. The charge on both the rod and the ball changes.
D. The charge does not change on either the rod or the
ball.
If you charge a metal ball on an insulating stand by
induction, which of the following happens?
A. The charge on the ball changes while the charge on the
rod stays the same.
A small object with a charge of magnitude q creates an
electric field. At a point 1.0 m to the north of the charge,
the field has a value of 2.0 N/C directed south. At a point
0.5 m to the west of the charge the field has a value of:
A. 4.0 N/C directed east
B. 4.0 N/C directed west
C. 8.0 N/C directed east
D. 8.0 N/C directed west
A small object with a charge of magnitude q creates an
electric field. At a point 1.0 m to the north of the charge,
the field has a value of 2.0 N/C directed south. At a point
0.5 m to the west of the charge the field has a value of:
C. 8.0 N/C directed east
Example 17.2: Gravity & Electric Force
Problem-Solving
Homework
Ch17: 1, 3, 5, 7, 14, 20, 31, 34, 40, 50.