Download Electric Potential

S-113
A proton is placed 4 cm to the right of a 1.2 nC
charge and 2 cm to the left of a -2.4 nC
charge. What is the acceleration of the
proton?
Squirrel Jedi
S-117
Four charges, each of 1.2 nC are placed on
the corners of a square 2.5 cm per side.
What is the magnitude and direction of
the electric field on the upper right
corner? What is
the force on the
charge in that
corner?
Electric Potential
AP Physics
Chapter 17
Electric Charge and Electric Field
17.1 Electric Potential Energy and
Potential Difference
Electric Potential Energy & Potential Difference
Electricity can be viewed in terms of energy
The electrostatic force is conservative
because it depends on displacement
Now
PE  W
PE  Fd
PE  qEd
We can calculate this value for a uniform
electric field
17.1
Electric Potential Energy & Potential Difference
Positive test charge – increases when moved
against the field
Negative test charge – increases when moved
with the field
PE
V
q
Electric Potential (Potential) – electric
potential energy per unit charge
17.1
Electric Potential Energy & Potential Difference
Only difference in potential are meaningful
Potential Difference (Electric Potential
Difference) – is measureable
PE
PE
V 
qq
Measured in volts (after
Alessandro Volta)
1J
1V 
1C
17.1
Electric Potential Energy & Potential Difference
If we want a specific potential value at a point,
we must pick a zero point.
That point is usually either
A. The ground
B. At an infinite distance r  
17.1
S-118
A 1.2 mC charge is moved 3 cm into a 231
N/C electric field.
A. What is the change in potential energy?
B. If the particle has a mass of 4.4 g, what will
be its velocity when it has moved back 3
cm (assume all energy is converted to
kinetic energy)
Electric Charge and Electric Field
17.2 Relation between Electric
Potential and Electric Field
Relation between Electric Potential & Electric Field
Work
WW q(V
q
VVa )
b 
It also is
W  qEd
So pew
qV
V Ed
qEd
17.2
Electric Charge and Electric Field
17.3 Equipotential Lines
Equipotential Lines
Equipotential Line or Equipotential Surface –
one at which all points are at the same
potential
17.3
Equipotential Lines
Must be perpendicular to the electric field
Constant Field
17.3
Equipotential Lines
Must be perpendicular to the electric field
Point Charge
17.3
Equipotential Lines
Must be perpendicular to the electric field
Dipole
17.3
Electric Charge and Electric Field
17.4 The Electron Volt, a Unit of
Energy
The Electron Volt
A unit of energy – for electrons, atoms, or
molecules
Electron Volt (eV)
1eV  1.6 x10
19
J
17.4
Electric Charge and Electric Field
17.5 Electric Potential Due to
Point Charge
Electric Potential Due to Point Charge
Using calculus it can be shown that the
electric potential a distance r from a single
point charge q is
q
V k
r
Assuming that potential is zero at infinity
17.5
Electric Charge and Electric Field
17.7 Capacitance
Capacitance
Capacitor – device that stores electric charge
In RAM,
Camera
Flash,
17.7
Capacitance
Simple capacitors consist of
two plate
The symbol for a capacitor
is
The symbol for a cell is
The symbol for a battery is
17.7
Capacitance
When a potential difference is placed across a
capacitor it becomes charged
Charging a Capacitor
This process takes a short amount of time
Time for RC Circuit
The charge on each plate is the same, but
opposite charge
The amount of charge is proportional to the
potential difference
A constant C (Capacitance) gives
Q CV
V
17.7
S-120
Two electrons are moved from an infinite distance to
a location 2 cm from each other.
A. What is the electric field produced by one at the
location of the other?
B. What is the force between them?
C. What is electric potential energy due to their
positions relative to
each other?
cat gone to dark side
Capacitance
Capacitance – Unit Farad
1C
1F 
1V
For a parallel plate capacitor, the capacitance
depends on the area of the plates, the
distance between the plates
A
C  o
d
Q  CV
17.7
Electric Charge and Electric Field
17.8 Dielectrics
Dielectrics
Most capacitors have an insulator between
the plates
Called a Dielectric
Increases the
capacitance by
a factor K
Called the dielectric
constant
o
A
C  K
d
17.8
Dielectrics
Some Dielectric Constants
Material
K
Paper
3.7
Glass
5
Rubber
6.7
Mica
7
Strontium
Titanate
300
17.8
Electric Charge and Electric Field
17.9 Storage of Electric Energy
Storage of Electric Energy
A charged capacitor stores energy
PE  QV
1
2
PE  CV
1
2
2
2
Q
1
PE  2
C
17.9
S-116
A 1.7 F capacitor in a camera is charged
across a 120 V socket. What is the energy
stored in the capacitor?
Combinations of Capacitors
Combinations of Capacitors
Parallel – more than one pathway
For a parallel set of
capacitors – the total
charge is the sum of
the individual charges
QT  Q1  Q2  ..Qn
In all parallel circuits – the potential across
each branch is the same as the total
VT  V1  V2  ..Vn
Combinations of Capacitors
The equivalent capacitance is the value of one
capacitor that could replace all those in the
circuit with no change in charge or potential
Since
Q  Q  Q  ..Q
T
1
2
And
We combine and get
Q  CV
CeqVC

C1C
V1  C2V2 ..C..nCnVn
T eq
n
Combinations of Capacitors
Series – components of a circuit are in one
pathway
The magnitude of the charges is the same on
each plate Q  Q  Q  ..Q
T
1
2
n
Combinations of Capacitors
The total potential is the sum of the potential
drops across each capacitor
VT  V1  V2  ..Vn
We then use that equation and the equation
for capacitance
Q
V
We get
C
Q
Q1T Q
11 Q
12
1n
 
 ..
Ceq C1 C22
Cnn
S-121
3 10F capacitors are placed in a circuit
with a 6 volt source of EMF.
A.What is the equivalent capacitance,
potential drop and charge if they are in
series?
B.What if they are in parallel?
S-118
Peace
Love
Happiness
Test
S-118