Transcript Document

General Physics (PHY 2140)
Lecture 5
 Electrostatics
 Electrical energy
 potential difference and
electric potential
 potential energy of charged
conductors
 Capacitance and capacitors
http://www.physics.wayne.edu/~apetrov/PHY2140/
Chapter 16
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1
Lightning Review
Last lecture:
Q
1. Flux. Gauss’s law.
 net   EA cos  
o
 simplifies computation of electric fields
PE
2. Potential and potential energy
V  VB  VA 
q
 electrostatic force is conservative
 potential (a scalar) can be introduced as potential
energy of electrostatic field per unit charge
Review Problem: Perhaps you have noticed sudden
gushes of rain or hail moments after lightning strokes in
thunderstorms. Is there any connection between the
gush and the stroke or thunder? Or is this just a
coincidence?
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Vcloud
E
FC  qE
mg
2
16.2 Electric potential and potential energy
due to point charges
Electric circuits: point of zero potential is defined by
grounding some point in the circuit
Electric potential due to a point charge at a point in
space: point of zero potential is taken at an infinite
distance from the charge
With this choice, a potential can be found as
q
V  ke
r
Note: the potential depends only on charge of an object,
q, and a distance from this object to a point in space, r.
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Superposition principle for potentials
If more than one point charge is present, their electric
potential can be found by applying superposition
principle
The total electric potential at some point P due to several
point charges is the algebraic sum of the electric
potentials due to the individual charges.
Remember that potentials are scalar quantities!
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Potential energy of a system of point
charges
Consider a system of two particles
If V1 is the electric potential due to charge q1 at a point P,
then work required to bring the charge q2 from infinity to P
without acceleration is q2V1. If a distance between P and
q1 is r, then by definition
q2
P
q1
r
A
q1q2
PE  q2V1  ke
r
Potential energy is positive if charges are of the same
sign and vice versa.
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Mini-quiz: potential energy of an ion
Three ions, Na+, Na+, and Cl-, located such, that they
form corners of an equilateral triangle of side 2 nm in
water. What is the electric potential energy of one of the
Na+ ions?
Cl?
qNa qCl
qNa qNa
qNa
PE  ke
 ke
 ke
 qCl  qNa 
r
r
r
but : qCl  qNa !
Na+
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Na+
qNa
PE  ke
 qNa  qNa   0
r
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16.3 Potentials and charged conductors
Recall that work is opposite of the change in potential
energy,
W  PE  q VB VA 
No work is required to move a charge between two points
that are at the same potential. That is, W=0 if VB=VA
Recall:
1. all charge of the charged conductor is located on its surface
2. electric field, E, is always perpendicular to its surface, i.e. no work is
done if charges are moved along the surface
Thus: potential is constant everywhere on the surface of a
charged conductor in equilibrium
… but that’s not all!
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Because the electric field is zero inside the conductor, no
work is required to move charges between any two
points, i.e.
W  q VB VA   0
If work is zero, any two points inside the conductor have
the same potential, i.e. potential is constant everywhere
inside a conductor
Finally, since one of the points can be arbitrarily close to
the surface of the conductor, the electric potential is
constant everywhere inside a conductor and equal to its
value at the surface!
Note that the potential inside a conductor is not necessarily zero,
even though the interior electric field is always zero!
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The electron volt
A unit of energy commonly used in atomic, nuclear and
particle physics is electron volt (eV)
The electron volt is defined as the energy that electron
(or proton) gains when accelerating through a potential
difference of 1 V
Relation to SI:
Vab=1 V
1 eV = 1.6010-19 C·V = 1.6010-19 J
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Problem-solving strategy
Remember that potential is a scalar quantity
Superposition principle is an algebraic sum of potentials due
to a system of charges
Signs are important
Just in mechanics, only changes in electric potential are
significant, hence, the point you choose for zero electric
potential is arbitrary.
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Example : ionization energy of the electron
in a hydrogen atom
In the Bohr model of a hydrogen atom, the electron, if it is in the
ground state, orbits the proton at a distance of r = 5.2910-11 m. Find
the ionization energy of the atom, i.e. the energy required to remove
the electron from the atom.
Note that the Bohr model, the idea of electrons as tiny balls orbiting the nucleus, is not
a very good model of the atom. A better picture is one in which the electron is spread
out around the nucleus in a cloud of varying density; however, the Bohr model does
give the right answer for the ionization energy
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In the Bohr model of a hydrogen atom, the electron, if it is in the ground state,
orbits the proton at a distance of r = 5.29 x 10-11 m. Find the ionization energy,
i.e. the energy required to remove the electron from the atom.
The ionization energy equals to the total energy of the
electron-proton system,
Given:
r = 5.292 x 10-11 m
me = 9.1110-31 kg
mp = 1.6710-27 kg
|e| = 1.6010-19 C
Find:
E=?
E  PE  KE
e2
v2
, KE  me
with PE  ke
r
2
The velocity of e can be found by analyzing the force
on the electron. This force is the Coulomb force;
because the electron travels in a circular orbit, the
acceleration will be the centripetal acceleration:
me ac  Fc
or
v2
e2
e2
2
me
 ke 2 , or v  ke
,
r
r
me r
Thus, total energy is
e2 me  ke e2 
e2
E   ke  
 2.18 1018 J  -13.6 eV
   ke
r
2  me r 
2r
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16.4 Equipotential surfaces
They are defined as a surface in space on which the potential
is the same for every point (surfaces of constant voltage)
The electric field at every point of an equipotential surface is
perpendicular to the surface
convenient to represent by drawing
equipotential lines
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16.6 The definition of capacitance
Capacitor: two conductors (separated by an insulator)

usually oppositely charged
a
b
+Q
-Q
The capacitance, C, of a capacitor is defined as a ratio of
the magnitude of a charge on either conductor to the
magnitude of the potential difference between the
conductors
Q
C
V
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1. A capacitor is basically two parallel
conducting plates with insulating
material in between. The capacitor
doesn’t have to look like metal
plates.
2. When a capacitor is connected to an
external potential, charges flow
onto the plates and create a
potential difference between the
plates.
3. Capacitors in circuits
symbols
analysis follow from
conservation of energy
conservation of charge
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Capacitor for use in
high-performance
audio systems.
-
+16
Units of capacitance
The unit of C is the farad (F), but
most capacitors have values of C
ranging from picofarads to
microfarads (pF to F).
1 F 1C V
Recall, micro 10-6, nano 10-9,
pico 10-12
If the external potential is
disconnected, charges remain on
the plates, so capacitors are good
for storing charge (and energy).
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16.7 The parallel-plate capacitor
The capacitance of a device
depends on the geometric
arrangement of the conductors
A
A
C  0
d
where A is the area of one of
the plates, d is the separation,
0 is a constant called the
permittivity of free space,
+Q
d
A
-Q
ke 
1
4 0
0= 8.8510-12 C2/N·m2
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Problem: parallel-plate capacitor
A parallel plate capacitor has plates 2.00 m2 in area, separated by a
distance of 5.00 mm. A potential difference of 10,000 V is applied
across the capacitor. Determine
the capacitance
the charge on each plate
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A parallel plate capacitor has plates 2.00 m2 in area, separated by a distance of 5.00 mm. A
potential difference of 10,000 V is applied across the capacitor. Determine
the capacitance
the charge on each plate
Solution:
Given:
V=10,000 V
A = 2.00 m2
d = 5.00 mm
Find:
C=?
Q=?
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Since we are dealing with the parallel-plate capacitor,
the capacitance can be found as
2
A
2.00
m
C   0  8.85 1012 C 2 N  m2
d
5.00 103 m
 3.54 109 F  3.54 nF


Once the capacitance is known, the charge can be
found from the definition of a capacitance via charge
and potential difference:


Q  C V  3.54 109 F 10000V   3.54 105 C
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16.8 Combinations of capacitors
It is very often that more than one capacitor is used in an
electric circuit
We would have to learn how to compute the equivalent
capacitance of certain combinations of capacitors
C2
C1
C3
C2
C5
C3
C1
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C4
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a. Parallel combination
Connecting a battery to the parallel
combination of capacitors is equivalent to
introducing the same potential difference for
both capacitors,
a
C1
V=Vab
+Q1 C2
+Q2
Q1
Q2
V1  V2  V
A total charge transferred to the system
from the battery is the sum of charges of
the two capacitors,
b
Q1  Q2  Q
By definition,
Q1  C1V1
Thus, Ceq would be
Ceq 
Q2  C2V2
Q Q1  Q2 Q1 Q2 Q1 Q2





V
V
V
V
V1 V2
CCeqeq CC11CC22
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Q1  Q2  Q
V1  V2  V
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Parallel combination: notes
Analogous formula is true for any number of capacitors,
Ceq  C1  C2  C3  ...
(parallel combination)
It follows that the equivalent capacitance of a parallel
combination of capacitors is greater than any of the
individual capacitors
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Problem: parallel combination of capacitors
A 3 F capacitor and a 6 F capacitor are connected in parallel
across an 18 V battery. Determine the equivalent capacitance
and total charge deposited.
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A 3 F capacitor and a 6 F capacitor are connected in parallel across an 18 V
battery. Determine the equivalent capacitance and total charge deposited.
a
Given:
C1
V=Vab
V = 18 V
C1= 3 F
C2= 6 F
Find:
Ceq=?
Q=?
+Q1 C2
+Q2
Q1
Q2
b
First determine equivalent capacitance of C1 and C2:
C12  C1  C2  9  F
Next, determine the charge


Q  C V  9  106 F 18V   1.6  104 C
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b. Series combination
Connecting a battery to the serial
combination of capacitors is equivalent to
introducing the same charge for both
capacitors,
a
V=Vab
Q1
c
C2
b
Q1  Q2  Q
+Q1
C1
+Q2
Q2
A voltage induced in the system from the
battery is the sum of potential differences
across the individual capacitors,
V  V1  V2
By definition,
Q1  C1V1 Q2  C2V2
Thus, Ceq would be
1 V V1  V2 V1 V2 V1 V2
 
   
Ceq Q
Q
Q Q Q1 Q2
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11 11 1 1
  
CCeqeq CC
1 1 CC
2 2
Q1  Q2  Q
V1  V2  V
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Series combination: notes
Analogous formula is true for any number of capacitors,
1
1 1
1
 
  ...
Ceq C1 C2 C3
(series combination)
It follows that the equivalent capacitance of a series
combination of capacitors is always less than any of the
individual capacitance in the combination
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Problem: series combination of capacitors
A 3 F capacitor and a 6 F capacitor are connected in series
across an 18 V battery. Determine the equivalent capacitance.
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A 3 F capacitor and a 6 F capacitor are connected in series across an 18 V
battery. Determine the equivalent capacitance and total charge deposited.
a
Given:
V=Vab
V = 18 V
C1= 3 F
C2= 6 F
Find:
Ceq=?
Q=?
+Q1
C1
Q1
c
C2
+Q2
Q2
b
First determine equivalent capacitance of C1 and C2:
C1C2
Ceq 
 2 F
C1  C2
Next, determine the charge


Q  C V  2 106 F 18V   3.6 105 C
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