LHWHS Physics
Unit 6 - Electricity NOTES
NAME_____________________
I. History
Lightening - people have observed (negative) charges moving from the
bottom of clouds in an attempt to 'balance' the net
charge of the cloud
- flash of lightening is accompanied by loud sound wave
~1730's - Ben Franklin - accurately described the phenomena of
lightening in terms of discharging static electricity
Static Electricity - 'non-moving' charges that build up and are stored on
the surface of materials
~1745 - Leyden jar- developed at University of Leyden in Netherlands,
early type of capacitor
~1760's James Watt - Scottish engineer who made important
improvements to steam engines, unit of power
named after him
watts - joules / second
~1780's Charles Coulomb - measured how electric charges behave by
observing the attracting and repelling forces
the charges exerted
Coulomb's law Newton's law Fe = k q1q2 / r2
Fg = G m1m2 / r2
k = coulomb's constant
q = amount of charge
r = distance between charges
G = newton's constant
m = mass r = distance between mass
(mass and distance determine
force due to gravity)
(charge and distance determine
force due to electricity)
coulombs (C) - standard unit of charge
6.24 × 10 18 electrons have the charge of 1 coulomb
~1799 Alessandro Volta - designed the 'voltaic pile' which was the first
practical battery using metals and salt solution
volts (V) - unit for electric potential difference
joules / coulomb
~1820's Andre'-Marie Ampere - demostrated the relationship between
moving charges (current) and the strength of the
magnetic field created
amps (A)- unit of current
coulombs / sec
~1826 Georg Ohm - developed the mathematical model for the
relationship between current , voltage, and resistance
ohms (Ω) - unit for resistance
~1850 James Joule - studied how energy can be transferred
between different systems......
........heat, mechanical, and electrical systems
joules (J) - unit of energy
II. Electrostatics
A. Definition - study of electric charges, forces, and
fields
static electricity-electricity at rest, 'non-moving'
electrons can 'sit' on a surface
B. Materials- glass, rubber, clothing
C. Electrostatics of metals
electrons spread out on metals, metal atoms have 'loose' hold on electrons
1. explains conductivity
2. explains shiny appearance and malleability
3. explains ability to reflect electromagnetic radiation
(light)
D. Like charges - repel
E. Opposite charges - attract
F. Insulator - high resistance, does not allow charge to move easily
(rubber, plastic, etc...)
G. Conductor - low resistance, allow charge to move easily
(metal)
III. Electric Field .........................similiar to _Gravitational_______field
1. introduce electrons into an electric field - the electrons begin to
move (accelerate)
2. introduce a ball into a gravitational field - the ball begins to move
(accelerate)
3. Potential energy - energy based on position in the field
electric = determined by the distance between charged
objects (Coulomb's law)
gravity = determined by the distance (height) from Earth
(Newton's law)
4. When accelerating ===> FORCE ===> WORK (ENERGY) ==> POWER
IV. Electric Potential -represents a difference in potential energy of each charge in the
electric field (Joules / Coulomb = Voltage)
A. Volt - defined as electric potential difference ,
-a measure of the work done to move a charge from one location
to another,
- electromotive force (EMF)
- measure electric field strength,
- named for Alessandro Volta
JOULES / COULOMB
V. Capacitance - measure of the amount of charge stored on conductors for a
given potential difference (voltage)
Capacitors - device that stores charge on conductors separated by an
insulator ,
ex: Leyden jar, clouds in lightening storm, nerve
membranes, flash chargers in camara, on virtually all circuit
boards in electronic devices
A. Farad (F)- unit for capacitance, named for Michael Faraday
- typically use µF ( 10-6 F)
COULOMB / VOLT
VI. Circuits - closed loop or pathway that allows electric charge to flow
A. Current- amount of charge that flows through an area in a given
amount of time
1. How to produce a current....How to move electricity
a. static charge will move to metal
Direct
Current
(DC)
b. BATTERY - chemical reaction between metal
and salt solution (acid)
c. using a magnet and coil of wire - produces an alternating current
(AC)
2. amps (A) - unit for current , named for Ampere
COULOMBS / SECOND
B. Resistance- an opposition to the flow of charge
- property that determines how much current will flow
- equal to voltage divided by current
- resistance of a wire depends on material used, length and
cross-sectional area of the wire
small diameter==> high resistance
large diameter===> low resistance
temperature....hotter wire==> more resistance
1. ohms (Ω) - unit for resistance, named for Georg Ohm
C. Ohm's law -
V= IR
V = volts
I = current
R = resistance
D. Power - rate that energy is transferred
1. Watt - joules / sec
mechanical systems
electrical systems
P = Force × distance P = Current × Voltage
time
= Newton × meters
sec
= amps × volts
= coul. / sec
×
= joules / sec
= joules / sec
= watts
= watts
E. Series circuits
joules/coul.
1. example
2. resistance in series
3. voltage in series
4. current in series
F. Parallel circuits
1. example
2. resistance in parallel
3. voltage in parallel
4. current in parallel
G. Direct current vs. Alternating current (DC vs
electrons move in the same direction
AC)
electrons change direction (back and forth)
1. Electromagnetism
• Moving electricity generates a magnetic field
•
Moving a magnet causes electricity to move
(induces a current)
2. Moving Charges
History (1700's - early 1800's)
DIRECT CURRENT
•
Lightening - static charges are discharged or moved
toward metal (and ultimately the ground)
•
Chemical Reactions - metals and salt solutions (acid)
can move electrons from one metal to another
•
Coil of Wire with Magnet (~1820's) produces an
alternating current (AC)
VII. Power usage- most outlets are 120 volts in home, amps vary with appliance
1. power bill - electric meter@ home measures power usage
kw hrs = total kilowatts × total hours
(1000 watts = 1 kilowatt)
2. high wattage appliances - typically 'lose' a lot of energy to heat
(ex: toaster, microwave, refridgerator, space heater, some types of lights)
voltage - relatively high
current - relatively high
resistance - relatively low
3. efficiency - ratio of power in / power out