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Electric Charge
Electric charge
• Fundamental property of matter
• Positive or negative
• Determines electric force
The Electric Force
Coulomb’s Law
F=
kq1q2
d2
2
Nm
k = 8.992  109 2
C
C = coulomb (unit of electric charge)
Coulomb’s Law
F=
kq1q2
d2
Force is attractive for opposite charges
Force is repulsive for like charges
Discussion Question
The electric force is like the gravitational
force because
a. Opposites attract.
b. Both decrease as 1/d2.
c. Both involve charge.
d. Both were identified by Isaac Newton.
e. All of these.
Discussion Question
The electric force is different from the
gravitational force because
a. Gravity is only attractive.
b. The electric force is much stronger.
c. Electric charges can cancel.
d. All of the above.
Example Problem
The electron has a charge of –1.6  10-19 C
and the proton has a charge of 1.6  10-19 C.
Their average separation in a hydrogen
atom is 5.3  10–11 m. What is the force
between them at this distance?
Quick Question
What is the force between the electron and proton
of a hydrogen atom when the electron is twice as
far away?
a. Twice as much.
b. The same.
c. Half as much.
d. One fourth as much.
Charge Polarization
tells us something about matter
Scenario
A bag contains equal numbers of positive
and negative charges. The charges can
move around inside the bag, but they cannot
leave the bag. The bag is placed near a
very large, immobile + charge.
+
+
–
–
–
+
–
+
+
Quick Question
What sort of force exists between the +
charges in the bag and the large + charge?
A. The + charges are attracted to the charge.
B. The + charges are repelled by the charge.
C. The + charges are neither attracted nor repelled.
+
+
–
–
–
+
–
+
+
Quick Question
What sort of force exists between the –
charges in the bag and the large + charge?
A. The – charges are attracted to the charge.
B. The – charges are repelled by the charge.
C. The – charges are neither attracted nor repelled.
+
+
–
–
–
+
–
+
+
Quick Question
In which direction do the + charges in the
bag accelerate due to the large + charge?
A. Toward the charge.
B. Away from the charge.
C. The + charges will not accelerate.
+
+
–
–
–
+
–
+
+
Quick Question
In which direction do the – charges in the
bag accelerate due to the large + charge?
A. Toward the charge.
B. Away from the charge.
C. The – charges will not accelerate.
+
+
–
–
–
+
–
+
+
Poll Question
After the charges re-distribute, which force
to the external + charge will be stronger?
A. The attraction to the – charges.
B. The repulsion to the + charges.
C. The attraction and repulsion will exactly cancel.
+
+
+
+
–
–
–
–
+
Poll Question
What sort of force exists between the bag
overall and the large + charge?
A. The bag is attracted to the charge.
B. The bag is repelled by the charge.
C. The bag is neither attracted nor repelled.
+
+
+
+
–
–
–
–
+
Group Poll Question
What sort of force on the bag will exist if the
external charge is negative?
A. The bag is attracted to the charge.
B. The bag is repelled by the charge.
C. The bag is neither attracted nor repelled.
+
+
–
–
–
+
–
+
–
Coulomb’s Torsion Balance
Principle of Superposition
Force from several charges = sum of forces
from individual charges
c
b
Fb
Fc
a
Fd
F = Fb + Fc + Fd
Fc
d
Fb
Fd
F
Vector Fields
Electric field
• Direction is the direction of the force
exerted on a positive charge
• Vector arrows point away from positive
charges and toward negative charges
• Magnitude is the force in N on a +1 C
charge
Electric Field
Unit = N/C
Visualizing a Field
• Magnitude of the force on a charge is
greater where field lines are close together
• Direction of the force is parallel to field
lines
– Force on a positive charge is along field lines
– Force on a negative charge is opposite field
lines
Field Lines
Question
Particles A–D have
the same charge.
Which experiences
the greatest force
from the field?
(Green arrows are
field lines.)
C
A
B
D
Example
Draw field lines for the electric field about
these two objects with opposite charges.
+
-
Group Work
Draw field lines for the electric field about
these two objects with equal charges.
+
+