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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
We can calculate this value for a uniform electric field 17.1
WPE FdPE qEdPE
Electric Potential Energy & Potential Difference
Positive test charge – increases when moved against the field
Negative test charge – increases when moved with the field
Electric Potential (Potential) – electric potential energy per unit charge
17.1
q
PEV
Electric Potential Energy & Potential Difference
Only difference in potential are meaningful
Potential Difference (Electric Potential Difference) – is measureable
Measured in volts (after
Alessandro Volta)
17.1
q
PEV
q
PEV
C
JV
1
11
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
17.1
r
Electric Charge and Electric Field
17.2 Relation between Electric Potential and Electric Field
Relation between Electric Potential & Electric Field
Work
It also is
So pew
17.2
)( ab VVqW VqW
qEdW
qEdVq EdV
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)
17.4
191 1.6 10eV x J
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
Assuming that potential is zero at infinity
17.5
qV k
r
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
This process takes a short amount of time
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
17.7
Charging a Capacitor
Time for RC Circuit
VQ CVQ
Capacitance
Capacitance – Unit Farad
For a parallel plate capacitor, the capacitance depends on the area of the plates, the distance between the plates
17.7
CVQ
V
CF
1
11
d
AC o
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
17.8d
AKC o
Dielectrics
Some Dielectric Constants
17.8
Material K
Paper 3.7
Glass 5
Rubber 6.7
Mica 7
Strontium Titanate
300
Electric Charge and Electric Field
17.9 Storage of Electric Energy
Storage of Electric Energy
A charged capacitor stores energy
17.9
QVPE 21
221 CVPE
C
QPE
2
21
