Electric Charge
Electric Charge
• A fundamental unit like mass
• Glass rods, plastic tubes, silk, and fur can be used to demonstrate
the movement of electrons and how their presence or absence make
for powerful forces of attraction and repulsion.
• Electrostatic interactions – interaction of electric
charges _____________.
• Electric – Greek word elektron meaning amber
• Two kinds of electric charges:
_____________ and _____________
• Like charges _____________
• Opposite charges ______________
EF 152 Spring, 2010 Lecture 4-1
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EF 152 Spring, 2010 Lecture 4-1
The photocopier
How is the atom arranged?
• The world may have come to take copiers for granted, but they
are amazing devices. They use charge to hold fine dust in
patterns until the pattern may be transferred to paper and made
permanent with heat.
• Visualize a football stadium as an
atom. Electrons would be garden
peas in the highest seats with
charge of −1. Protons would be
basketballs or melons with charge
of +1, and neutrons would reside
near the protons with no charge.
All of the protons and neutrons
could be in a small basket on the
50-yard line.
2
• Charges of electron and proton are
equal in _______________
• Easiest to add or remove
_______________
EF 152 Spring, 2010 Lecture 4-1
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EF 152 Spring, 2010 Lecture 4-1
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Conservation and electric charge
Electrostatic force law
• The ____________ sum of all the electric charges in any
_____________ is constant.
• Coulomb’s Law allows the calculation
of electrostatic attraction or repulsion
_________ for _____________ in a ________.
• Electric charges are not created or destroyed; they are
_______________from one body to another.
•
• The magnitude of charge of the __________________
is a natural unit of charge.
q1q2
r2
• k = 8.98755 ·109 N·m2/C2
k = c2(10-7 N s2/C2)
(related to speed of light)
• The magnitude of an electric charge is always an
_____________ value. (quantized)
• F=
When you rub a plastic rod with fur, the plastic rod becomes
negatively charged and the fur becomes positively charged.
As a consequence of rubbing the rod with the fur,
EF 152 Spring, 2010 Lecture 4-1
F =k
1
q1q2
4πε 0 r
2
k=
1
4πε 0
ε0 = 8.854 ·10-12 C2/(N·m2)
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EF 152 Spring, 2010 Lecture 4-1
Units and magnitude
Force between two particles
• Coulomb (C) is the SI unit of charge
(no USC unit)
• Book Examples 21.1, 21.2
• Determine the force on each of the two particles
• Charge of one electron (or proton)
e = 1.6022 ·10-19 C
q1 = +3 nC
5 cm
• Two 1 C charges separated by 1 meter would result in
a force of 9 ·109 N (~1 million tons)
q2 = -2 nC
• nC (___) and µC (___) are more often used for
electrostatic charges.
• Example 21-1: Electrostatic force between two alpha
particles (helium nuclei) is 1035 times as large as the
gravitational force.
EF 152 Spring, 2010 Lecture 4-1
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EF 152 Spring, 2010 Lecture 4-1
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Superposition of Forces
Q21.4
• Given a set of charged particles, the net force on a single particle is
the vector sum of the forces between that particle and each of the
other particles.
Charge #2
+q
Charge #2
–q
• Book Examples 21.3, 21.4.
Charge #1
+q
Charge #1
+q
• Determine the force on particle 3
y
y
–q
–q
x
q1 = -5 µ C
1. The net electric force
that Charges #2 and #3
exert on Charge #1 is in
3 cm
q3 = +6 µC
4 cm
q2 = -2 µ C
EF 152 Spring, 2010 Lecture 4-1
x
Charge #3
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Charge #3
2. The net electric force
that Charges #2 and #3
exert on Charge #1 is in
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EF 152 Spring, 2010 Lecture 4-1
Electric fields
Electric fields — the point charge
• Measure the force on a test charge (B) at all points relative to
another charge (A), an electric field may be mapped.
• Fields of force may be sketched for different arrangements of charge.
• Reference Examples 21.5 and 21.6
F=
1
q1q2
4πε 0 r 2
F
1 q
E= =
rˆ
q 4πε 0 r 2
rˆ - unit vector point from
• Can’t do this experimentally – a “thought” experiment or a
“gedanken experiment”.
• F0 = q0 E
• E = F0 / q0
the charge to the location
Field Direction
Positive charge: _______________
Negative charge: _______________
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EF 152 Spring, 2010 Lecture 4-1
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Electric field for a point charge
Uniform Electric field — charges in motion
• Find E at A, B, and C for the given charge and coordinate system
• Uniform field constant force constant acceleration
• Just like ______________ motion
B @(1,6.6)cm
An electron is shot with an initial horizontal velocity of 50·105 m/s through a
uniform electric field of 1.8·103 N/C. How far has it traveled in the horizontal
direction when it has dropped 4.2 mm in the vertical direction?
A @(-6,3)cm
y
C @(1,2.3)cm
x
-12 nC @ (1,-2)cm
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Electric field lines map out regions of equivalent force
Electric fields add as vectors
• E = E1 + E2 + E3 + … _____________________
• Reference Example 21.9
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