Download pre-AP Electricity

Electric Charge
 Clothes
stick together when
pulled from a dryer, comb attracts
hair, rub feet on carpet- get a
shock!
 WHEN THIS OCCURS, THESE
OBJECTS ARE SAID TO BE
ELECTRICALLY CHARGED.
 Works best on dry days- high
humidity causes charges to “leak”
2 Kinds of Charge
 Positive
 Negative
 (Ben
Franklin named these
charges)
 Like charges repel
 Opposite charges attract
+
+
-
-
-
+
The Atom
e
e
Neutral when just as many + as –
 Charged when there is more of one than
the other (proton and electrons)
 We say it is positively charged when + are
more than –
 We say it is negatively charged when – are
more than +

 Protons
and neutrons stay
fixed in nucleus
 Electrons may move about
 It is this movement of
electrons that makes a
charge
–Electrons move IN =
Negative
–Electrons move OUT =
Positive
Law of Conservation of Charge
Can’t create charges.
 Can’t destroy charges.
 Can transfer charges

Charged objects always occur in
discrete amounts in nature. Always in
multiples of a fundamental unit of
charge. This fundamental unit of
charge we call e’
 therefore ±e’
±2e’ ±3e’
 e’ has a value: 1.602 x 10-19 coulomb
(C)
 C is SI unit of electric charge
 One C contains 6.25 x 1018 electrons


pg. 630 Look at table 17-1
1.6 x 10-19 Coulombs (C)
The charge of one electron or one
proton (+ proton, - electron)
 A dozen electrons have a charge of (12
x -1.6x10 -19 ) -1.92 x 10 –18 C
 A Helium (2 protons) nucleus has a charge
of (2 x 1.6x10 -19 ) 3.2 x 10 -19 C

To Move or Not to Move
Transfer of charge
 Conductors: charges move freelylots of free electrons. Cu, Au, Ag,
Hg, Al, most metals- even carbon
 Insulators: charges can NOT
move freely- electrons stay with
their atoms. wood, plastics,
ceramics, pure water. Common
among substances made of
atoms with fewer electrons.
Charging by Polarization
http://www.youtube.com/watch?v=VhWQ-r1LYXY&feature=related
http://www.youtube.com/watch?v=hoswNJZqUX0&feature=related
 Semiconductors:
In their pure
state, they are insulators; By
adding (doping) impurities
conductivity can increase.
Silicon & germanium are
examples and this topic is the
basis for electronics,
microchips, computers, ect.
 Super Conductors: Perfect
conductors (NO resistance)
when material is at or below
temperature- very cold
Electroscope A device for
studying electric charge

Electroscopes
come in many
shapes and
sizes.
Charge by contact (pg. 631): Charge
moves as a result of things touching
each other
 Charge by induction (pg632): charge
moves as a result of being attracted
or fleeing from something already
charged. Grounding plays a part.
(Earth is an infinite reservoir for
electrons- electrons move in or out of
the Earth freely and Earth maintains
a neutral charge

Charging by Contact. + charges
leave + charges
Charging by Induction. –charge
leaves +charge
Coulomb’s Law
 describes
the force of
attraction (repulsion) of two
charged objects. The distance
between them is important and
the formula very closely
resembles Newton’s Universal
Gravitation Law

electric force =Coulomb Constant x (charge1)(charge2)
(dist. between charges)2
Felectric = Kc (q1 q2 / d2)
Kc =Coulomb Constant =9 x 109
Sample problem 17A pg.
635
Practice problems are on
page 636
Electric Field

A region in space around a charged
object in which a stationary charged
object experiences an electric force due
to its charge.
Electric Field lines
Go FROM positive TO negative
 They represent both magnitude and
direction of the electric field


Electric
field lines pg.
648
electric field hockey
anyone? See phet online
http://phet.colorado.edu/en/simulation/
electric-hockey
Van de Graaff Generator pg. 651

Lightning Storm Safety:
What’s safe when its
lightning outside?
Quantity
Voltage Current Resistance
Power
Symbol
V
I
R
P
Unit
Volt
Ampere
Ohms
Watts
Unit
Symbol
V
A
Ω
W
Electricians are a little like plumbers of
electrons
We can often think about water flowing to help
us visualize how electrons flow
Voltage
 The
force that pushes
electrons through a wire
 P.d. potential difference- not
exactly, but like voltage. In this
course, we will use them
interchangeably
Think of water flowing in these pipes
Small
Potential
Small
Potential
Small Force
Large
Potential
Large
Potential
Large Force
3V
Small Force
12V
Large Force
Current
The flow of electrons. This is actually
the amount of electrons that move past
a point on a conductor in a given
amount of time.
 Rivers and streams have different
currents (rates of flow) so it is w/ wires

People often think electrons move
FAST in a wire because when you flip
a switch- you get light or sound
quickly- - - NOT THE CASE
 Charges- electrons- move slowly in a
wire. Typically, electrons drift along a
wire at .25 mm/s

 Current
can be direct
(batteries) or Alternating (wall
outlet)
AC/DC
 DC= one way street
 AC = back and forth
 Generators
type
may produce either
Resistance
the
opposition to the
flow of current in a
conductor
In very loose terms, its
like friction for electrons
Certain factors effect
resistance in a wire
Less Resistance
More Resistance
Cu
Al
cold
hot
length
Cross
sectional
area
Material
Temp
Resistance can be used to control current in
a circuit. They can also do work
Why can birds sit on power lines without
harm and we’re told not to fly kites near
them?

http:/www.youtube.com/watch?v=GRY28R0UUdQ
What brings these 3 ideas
together?? How do they relate
to each other??
OHM’S LAW
V = IR
I = V/R
R = V/I
Sample problem 19B pg. 702
Practice Problems 19b, pg 703 #1-6
Practice
1. When a 12 V battery is connected to a light bulb as
shown, 6 amps flow through the ammeter. Find the
resistance of the bulb. Show Work
2. A motor which has 30 Ohms of resistance will draw 3
amps of current while it runs. Find the voltage of the
battery.
Relationships between quantities
 Direct
(linear) Volts-Current : Volts-Resistance
 Inverse Resistance - Current
 Graphs…
V
Slope of V – I graph?
I
Resistance
I
R
Power
 The
rate of conversion of
electrical energy
Power = work/ time = ΔPE/ Δt =
P = Current x Voltage = VI
V = IR
P = I 2R
P = V2/R
Sample problem 19C pg. 710
Practice problem 19c pg.710 #1-4
 Electric
Companies measure
consumption of energy in kWh:
Kilowatt- Hours
 E = Pt
– Energy used= Power x time used
– Energy used = kW x hr

Electric companies will multiply a fee to your
kWh and that is what is on your bill, fees
range from 9¢ to 20¢ per kWh
 Texas
is quite cheap for electricity
use ( why do you think that is?)

Electric companies bill you depending
on your Energy Consumption.
Power=energy / time
 Energy = Power x time
 Watt seconds are too small a unit.
 kW*hrs is a better unit of Energy for
household consumption.
 Power companies multiply their rate by
the energy you use each month. This
determines what your bill is.
 Money you owe = P (t) rate(set by e company)

Take a look at a bill

electric bill.JPG
Power practice
Most household appliances receive a
120 volt supply. As such what is the
current running through a 60 watt bulb.

How much money would it cost to
leave this bulb on for 24 hours? (the
company rate is $0.16 per kWhr).

Power Practice

A certain home runs an air conditioner
for 11 hours a day in the summer. The
power demand for this unit is
4500Watts. The electric company
charges $0.18 per kWhr. Calculate the
daily cost of running the unit. How
much $ per month?
Which would cost less money, Watching a 2.5 hour
movie at home or popping a few bags(3) of popcorn
in the microwave to eat during the movie?

The tv, dvd, stereo has a combined power of 400W. Your
electric company charges 20cents per kWhr. Your
microwave is 1100watts and the cooking time is 7
minutes.
 To
minimize power loss, electric
company power lines have huge
voltages and relatively small
currents
 Because the lengths of the wire
are long, resistance would be a
BIG factor in energy loss. To get
the same power to your home
P=VI
 Either a big V or a big I will work.
Because P = I2R it is cheaper to
run electricity to homes under
large voltages
 Power
lines can have 765,000
volts in them
 Neighborhood lines: 4000 volts
 to your house: 240 volts
 transformers
are a device that
alter voltage, more on them
later.
Circuit Symbols
Resistor
Voltmeter
Ammeter
Battery
Light bulb
Switch
Series Circuits
Series Practice Problems
1. Two 5Ω resistors are connected in series to a 12V
battery. Calculate the voltage drop and current
across each resistor.
5Ω
R
R
I
V
V = 12V
1
2
Total
5Ω
Series Practice Problems
2. A 5Ω and 10Ω resistor are connected in series to a
15V power supply. Calculate the voltage drop and
current across each resistor.
5Ω
R
1
2
Total
R
I
V
V = 15V
10 Ω
Parallel Circuits
Parallel circuit analogy
Comparing resistance
Comparing series and
parallel circuit simulation
simulation of a circuit breaker
Parallel Circuits
• More than 1 path for current to flow.
• IT in and out of battery is = to the sum of the
current in each branch so the total current leaving
the battery is equal to the amount of current
entering your battery
• 1/RT = 1/R1 + 1/R2 + 1/R3 …
• As the number of resistors in parallel increases
the total resistance decreases!
Parallel Circuit Problems
1. Two 5Ω resistors are connected in parallel to a 12 V battery.
Calculate the voltage drop and current across each resistor.
R
1
2
Total
R
I
V
V= 12V
5Ω
5Ω
Parallel Circuit Problems
2. A 5Ω and a 10 Ω resistor are connected in parallel to a 12 V
power supply. Calculate the voltage drop and current across
each resistor.
R
1
2
Total
R
I
V
V= 12V
5Ω
10Ω
Comparing Series and Parallel Circuits
Series
Parallel
As the # of resistors
increase
RT…
RT…
As the # of resistors
increase
IT…
IT…
Voltage drop across VBattery =
each resistor or bulb
Path for current
Disadvantages /
advantages
VBattery =
8
8
8
8
16
4
Find the total resistance and the total current for this
circuit.
V1 = 120V
R1 = 20W

R2 = 20W
R3 = 20W
R4 = 10W
R5 = 10W
Which schematic has the largest
total resistance?
What is the total resistance in the circuit?
What current does the battery provide?
Which graph best displays the relationship
between power and voltage? P=V2/R
How come many people get shocked by house
current but they aren’t burned badly? How come
others are killed?
Dry Skin is very high in resistance[100,000W], wet skin
is much lower in resistance [1,000W].
Figure out the current that flows when your skin is dry
and the voltage is 110V
Figure out the current that flows when your skin is wet
and the voltage is 110V