Electricity Theory VIR PIV and Capacitors!!!
Dec 28, 2015
Electricity TheoryVIR PIV and Capacitors!!!
Energy When an object is at
some height in a gravitational field it is said to have gravitational potential energy, PEg
PEg
Energy Like gravitational fields causing masses to
have potential energy, Electric Fields cause charges to have electric potential energy, PEE
PEE is a type of mechanical energy
MEtotal = KE + PEg + PEs + PEE
Energy To give something PE you must do work
(apply force over a distance) on the something (raising up in g-field)
For PEE to occur a FE must be applied by either
a. An E-Field (uniform)
b. A pair of charges
EnergyUniform E-field
AB
Line Color
Red: E-Field
Black: Equipotential lines
Blue: charge displacement
E
W PE Fd
F Eq
PE qEd
Energy Pair of Charges
1 22
1 2
c
E c
W PE Fd
q qF k
rq q
PE kr
Electric Potential
Any point in an electric field is said to have Electric Potential, V. However, only a Difference in PE is measurable (remember zero point) so we talk of electric potential difference AKA potential difference, ΔV.
EPEV
q
PEV
q
unit Volt, V
J1V=1
C
Potential Difference
Potential Difference
Potential Difference Back to the zero point
A convenient zero point to chose in a circuit or any electric system is the “ground”
Battery (cells) A battery produces
electricity by transforming chemical energy into electrical energy
BatteryCarbon Electrode
Zinc Electrode
Sulfuric Acid
+
Capacitor A capacitor is a storehouse of charge and energy that
can be reclaimed when needed for a specific application
A capacitor will only charge to the potential difference between the terminals of the battery
Capacitance Capacitance, C: The ability of a conductor to
store energy in the form of electrically separated charges
Capacitance is the ratio of charge to potential difference
QC
V
unit Farad, F
C1F=1
V
Capacitance Capacitance depends on size and shape
0
AC
d
2-12
0 2permittivity of free space, 8.85x10
Area of one plate
d distance between plates
C
NmA
Capacitor Energy stored in a
capacitor
21 1
2 2U energy QV CV
Electric Current Movement of electric charge Rate of charge movement
QI
t unit Ampere, A
C1A=1
s
Charge Movement
Charge Movement
Circuit Analogy
Types of Current AC Alternating current charges
continuously change direction forward and back at 60 Hz
Example: outlets (approx 120 V)
DC Direct current charges move in one direction
Example: batteries
AC-DC Debate births the Electric Chair
Resistance Resistance is the impedance of the motion of
charge through a conductor The ratio of potential difference across a
conductor to the current it carries
VR
I
2
unit ohm,
V Js1 1 1
A C
Ohm’s Law
V IR
Resistance Depends on: Length, cross sectional area,
material, and temperature
LR
A
2
resistivity, m
L length, m
A cross sectional area, m
Resistance and Temp
Resistance and Thickness
Resistor An electronic element
that provides a specified resistance.
A current or voltage REGULATOR
Power (it’s Electric!) Power: Rate at which work is done. OR Rate
at which energy is transformed Electric Power: The rate at which charge
carriers convert PEE into non-mechanical energy
P IVunit watt, W
J1 W = 1
s
Reading and Homework Read Chapter 17
pp 593 - 625
HW due on test day:p 599 1-3; p 601 2, 3, 5-9;
p 607 1 – 4 (B); p609 1 – 5
p 615 1 – 6; p 616 2-4, 7,9
p 621 1 – 5
Extra Practicep 626 – 628 11, 20 – 54