Download Welcome to 1161 Principles of Physics II

Chapter 20
Electric Potential and Electric
Potential Energy
Dr. Jie Zou
PHY 1161
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Outline
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Electrical potential energy, U
Electrical potential, V
Connection between electric field E and
electric potential V
Energy conservation
Electrical potential of point charges
Dr. Jie Zou
PHY 1161
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Electrical Potential Energy, U
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Definition for U, the
change in electrical
potential energy of a
charge:
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U = Uf - Ui = -W
The work done by a
conservative force (e.g.,
an electric force and
force of gravity) is equal
to the negative of the
change in potential
energy.
SI units: Joules (J)
Dr. Jie Zou
PHY 1161
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Electrical Potential, V
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Definition for V, the change in electric
potential:
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Relationship between U and V:
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V = Vf – Vi = U/q0 = (-W)/q0
V is the change in electric potential energy per
charge.
SI units: Joules/Coulomb (J/C) = Volt (V)
U = q0 V
Both are scalar quantities.
Another commonly used unit of energy is the
electron volt (eV):
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1 eV = (1.60x10-19 C)(1 V) = 1.60x10-19 J
Dr. Jie Zou
PHY 1161
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Exercise 20-1
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Find the change in electric potential
energy, U, as a charge of (a) 2.20 x
10-6 C or (b) -1.10 x 10-6 C moves from
a point A to a point B, given that the
change in electric potential between
these points is V = VB – VA = 24.0 V.
Dr. Jie Zou
PHY 1161
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Connection between Electric
Field and Electric Potential
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Connection between the
electric field and the electric
potential:
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E = -V/s
V = -Es
The electric field depends on the
rate of change of the electric
potential with position.
The electric potential decreases
as one moves in the direction of
the electric field.
SI units for E: 1 N/C = 1 V/m
Dr. Jie Zou
PHY 1161
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Example 20-1: Plates at
Different Potentials
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A uniform electric field is
established by connecting
the plates of a parallel-plate
capacitor to a 12-V battery.
(a) If d = 0.75 cm, what is
the magnitude of the electric
field in the capacitor?
(b) A charge of +6.24x10-6 C
moves from the positive
plate to the negative plate.
Find the change in electric
potential energy.
Dr. Jie Zou
(In electrical systems, we
shall assume that gravity
can be ignored, unless
specifically instructed
otherwise.)
PHY 1161
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Energy Conservation
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Energy conservation:
For a charged object in an
electric field, its total
energy must be conserved.
KA + UA = KB + UB, or
2
2
 (1/2)mvA + UA = (1/2)mvB
+ UB
Example 20-2: What is (a) the
mass of the charge and (b) its
final kinetic energy?
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Dr. Jie Zou
PHY 1161
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Example 20-2: From Plate to
Plate
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Dr. Jie Zou
Suppose a charge q =
+6.24 x 10-6 C is released
from rest at the positive
plate and that it reaches
the negative plate with a
speed of 3.4 m/s.
(a) What is the mass of the
charge?
(b) What is its final kinetic
energy?
PHY 1161
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The Electric Potential of Point
Charges
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Electric potential V produced by a
point charge q at a distance r:
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Electric potential energy U for point
charges q and q0 separated by a
distance r:
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Conventionally, choosing the electric
potential to be zero at infinity, V = kq/r
“+”
charge
U = q0V = kq0q/r
Exercise 20-2: Find the electric
potential by a point charge of
6.80x10-7 C at a distance of 2.60 m.
Dr. Jie Zou
PHY 1161
“-”
charge
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Homework #2
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Chapter 20, P. 717-718, Problems:
#2, 4, 19 (Physics, Walker, 4th edition).
Dr. Jie Zou
PHY 1161
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