Download Chapter 23 Electric Fields

Chapter 23
Electric Fields
23.1 Properties of Electric Charges
23.3 Coulomb’s Law
23.4 The Electric Field
23.6 Electric Field Lines
23.7 Motion of Charged Particles
in a Uniform Electric Field
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23.1 Properties of Electric Charges
• There are two kinds of
electric charges in
nature:
– Positive
– Negative
• Like charges repel one
another and Unlike
charges attract one
another.
• Electric charge is
conserved.
• Charge is quantized
q=Ne
e = 1.6 x 10-19 C
N is some integer
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23.3 Coulomb’s Law
From Coulomb’s experiments, we can
generalize the following properties of the
electric force between two stationary
charged particles.
The electric force
• is inversely proportional to the square of the
separation r between the particles and directed
along the line joining them.
• is proportional to the product of the charges q1
and q2 on the two particles.
• is attractive if the charges are of opposite sign
and repulsive if the charges have the same sign.
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• Consider two electric charges: q1
and q2
• The electric force F between these
two charges separated by a
distance r is given by Coulomb’s
Law
• The constant ke is called Coulomb’s
constant
•  is the permittivity of
free space
0
2
1
12 C
k
where  0  8.85  10
4  0
Nm 2
• The smallest unit of
charge e is the charge on
an electron (-e) or a
proton (+e) and has a
magnitude e = 1.6 x 10-19
C
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Example 23.1 The Hydrogen Atom
The electron and proton of a hydrogen
atom are separated (on the average)
by a distance of approximately 5.3 x
10-11 m. Find the magnitudes of the
electric force.
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• When dealing with Coulomb’s
law, you must remember that
force is a vector quantity
• The law expressed in vector
form for the electric force
exerted by a charge q1 on a
second charge q2, written F12,
is
•
where rˆ is a unit vector directed from q1 toward q2
• The electric force exerted by q2 on q1 is
equal in magnitude to the force exerted
by q1 on q2 and in the opposite direction;
that is, F21= -F12.
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• When more than two charges are present,
the force between any pair of them is
given by Equation
• Therefore, the resultant force on any one
of them equals the vector sum of the
forces exerted by the various individual
charges.
• For example, if four charges are present,
then the resultant force exerted by
particles 2, 3, and 4 on particle 1 is
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• Double one of the charges
– force doubles
• Change sign of one of the charges
– force changes direction
• Change sign of both charges
– force stays the same
• Double the distance between charges
– force four times weaker
• Double both charges
– force four times stronger
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Example:
• Three point charges are aligned
along the x axis as shown. Find the
electric force at the charge 3nC.
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Example 23.2 Find the Resultant Force
• Consider three
point charges
located at the
corners of a right
triangle, where
q1=q3= 5.0μC, q2=
2.0 μC, and a=
0.10 m. Find the
resultant force
exerted on q3.
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