Download Electricity Theory

Electricity Theory
VIR PIV and Capacitors!!!
PEg
Energy

When an object is at
some height in a
gravitational field it is
said to have
gravitational potential
energy, 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
Energy
Uniform E-field
W  PE  Fd
F  Eq
PEE  qEd
B
A
Line Color
Red: E-Field
Black: Equipotential lines
Blue: charge displacement
Energy

Pair of Charges
W  PE  Fd
q1q2
F  kc 2
r
q1q2
PEE  kc
r
Electric Potential

PEE
V
q
Any point in an electric field
is said to have Electric
Potential, V.
However, only a Difference V  PE
in PE is measurable
q
(remember zero point) so we
talk of electric potential
unit  Volt, V
difference AKA potential
J
difference, ΔV.
1V=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
Battery
Carbon Electrode
Sulfuric Acid
+
Zinc Electrode
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
Q
C
V
unit  Farad, F
C
1F=1
V
Capacitance

Capacitance depends on size and shape
A
C  0
d
 0  permittivity of free space, 8.85x10
A  Area of one plate
d  distance between plates
2
-12
C
2
Nm
Capacitor

Energy stored in a
capacitor
1
1
2
U  energy  QV  CV
2
2
Electric Current


Movement of electric charge
Rate of charge movement
Q
I
t
unit  Ampere, A
C
1A=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
V
R
I
unit  ohm, 
V
Js
1  1  1 2
A
C
Ohm’s Law
V  IR
Resistance

Depends on: Length, cross sectional area,
material, and temperature
L
R
A
  resistivity, m
L  length, m
A  cross sectional area, m2
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  IV
unit  watt, W
J
1W=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 Practice
p 626 – 628 11, 20 – 54