Download Electric Potential Energy & Potential Difference

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
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
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