February 25, 2009 Notes Electricity CW Static Electricity
Pre-Lab HW Electricity and Subatomic particles
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Electricity Chapter 11
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11.1 Electric Charge Basic facts about the atom Matter is made
of atoms ParticleLocationCharge ProtonNucleus(+) NeutronNucleusnone
ElectronCloud(-)
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Model of a Helium atom If a helium atoms has 2 protons 2
neutrons 2 electrons
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The Fundamentals Atoms consist of a small, densely packed, (+)
nucleus (p+ & n) Surrounded by a (-) cloud (e-)
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The Fundamentals Mass of a p+ = 1 amu (atomic mass unit) 1 amu
= 1.66 x 10 -27 kg! Also refered to as u Mass of a n = 1amu Mass of
an e- = 0! Therefore the nucleus contains the entire mass of the
atom! The mass of the electrons is negligible!
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The Fundamentals Electron cloud has order to it! Electrons are
arranged in orbitals. Orbitals radiate out from the nucleus
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The Fundamentals Atoms in their normal state are neutral # p+ =
# e- # p+ determines the identity of the element ( Atomic Number on
periodic table)periodic table Atoms can ONLY gain or lose e-! NEVER
PROTONS.
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The Fundamentals Ion atom with a charge. Gain 1 or more e- =
negatively charged ion = ANION (# + < # -) Only non-metals do
this! Lose 1 or more e- = positively charged ion = CATION Only
metals do this!
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Law of Conservation of Charge: Charges are neither created nor
destroyed THEY ARE MOVED! e- transferred from one object to
another. (e- never appear or disappear) Ex: by rubbing two objects
together Static electricity accumulation of a charge
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Properties of sub-atomic particles Like charges repel! (p+ p+)
(e- e-) Opposite charges attract! (e- p+) Atoms only lose or gain
electrons within their outermost orbital. Why?
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14 TriboElectric Series list that ranks various materials
according to their tendency to gain or lose electrons VERY POSITIVE
looses electrons Human hands (usually too moist though ) Rabbit Fur
Glass Human hair Nylon Wool Silk Aluminum Paper Cotton Do these
look familiar to you? Which is a conductor? Insulator?? VERY
NEGATIVE gains electrons Teflon Vinyl Scotch Tape Saran Wrap
Styrofoam Polyester fabric Gold/Platinum/Brass/Silver/Copper Hard
Rubber Amber Wood
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Agenda 26-Jan: GET OUT YESTERDAYS HW! Notes Electricity CW/HW
Electric Charge & Coulombs Law WS
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Static Electricity & Electric Charge: Charging by Friction
transfer of e- by contact. e.g. rubbing your head with a
balloon
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Static Electricity & Electric Charge: Charging by
Polarization creation of a temporary charge by bringing a charged
object near a neutral one. (e- get rearranged in atoms to create
partial positive and negative surfaces.)
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Static Electricity & Electric Charge: Polarization
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Static Electricity & Electric Charge: Charging by Induction
using one object to charge another.
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Electric Charge: Measured in Coulombs (C) 1 Coulomb represents
a HUGE amount of charge! p+ = 1.602 x 10 -19 C e- = -1.602 x 10 -19
C
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Coulombs Law: Relates charge on two objects (q 1 and q 2 ), the
distance between their centers (r), and the electrical force (F E )
F E = k x (q 1 x q 2 ) r 2 k = 9 x 10 9 Nm 2 /C 2 How does force
change with distance?
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Coulombs Law: The force between two charges acts along a line
joining their centers. Charges obey Newtons 3 rd Law of motion they
make action- reaction pairs.
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Coulombs Law: If a p+ at a particular distance from a charged
particle is repelled with a given force, by how much will the force
decrease when the distance between the two is three times greater?
Fives times greater? What is the charge on the particle in this
case?
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Agenda: March 3rd Notes Electricity Fix yours Finish Review
Questions 7-12 on p 188 Exercises 8 16 on p 189 Tomorrow Circuit
Lab!
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Electric Current Electric charges, electrons, are always
present, but may not be moving! Current the flow of charge, or
electrons in one direction, (- ) (+).
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Electric Current Measured in Amperes (Amps, A) 1 Amp = 1
Coulomb per second How many electrons per second is that?
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Electric Current Just like water flows downhill (from high GPE
to low), electrons flow from high electrical potential (pressure)
to low electrical potential. Voltage (V) a measure of the
difference in electrical potential between two parts of a circuit,
or the joules of electrical work done per second (Watts) by the
current.
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Electric Current Voltage is therefore Watts Amps, or Joules
(PE) Coulombs (charge)
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Electric Current Batteries make use of chemical reactions to
create a difference in electrical potential energy, or pressure
When an e- leaves a battery, it carries E. The e- gives up its E to
the electrical device it powers. The e- travels back to the battery
to get more E. This cycle continues until all of the chemicals in
the battery are used up and the reaction stops.
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Electric Current Conductors: materials through which electrons
move easily Metals are good conductors Why? Atoms of metal bond by
sharing electrons, and in turn act like 1 huge molecule. Valence
electrons (those furthest away from the nucleus) are free to move
anywhere.
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Electric Current Insulators: materials through which electrons
DO NOT move easily Rubber, glass & wood are good insulators
Why? Atoms/molecules of these material stick together through
surface area interaction, because the electrons within these atoms
of these molecules are fixed within chemical bonds.
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Questions to answer: What power does a 1.5 V battery give to
each amp of current in a circuit? What flows when there is an
electric current in the wire? Why is it easy to create a current in
a conductor and not in an insulator?
Electrical Resistance Resistance - a measure of how strongly a
wire or other substance resists current flowing through it.
conductors are low resistance materials, electrical flow is fast
insulators are high resistance materials, electrical flow is
slow
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Electrical Resistance Total resistance determines Total
current. In a circuit each device attached adds to the total
resistance in the system. As you increase resistance you decrease
current.
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Electrical Resistance You and a friend are trying to illuminate
a space that has very poor lighting. You both have several feet of
wire, two batteries and several light bulb systems to work with.
Your friend strings together three light bulbs in a series. You
decide set up a one bulb circuit. Whose set up provides the room
with more light?
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Electrical Resistance In a wire... TO decrease resistance use
thicker wire lower the temperature decrease the length of the wire
TO increase resistance do the opposite of above...
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Electrical Resistance Resistance is measured in OHMS,
represented by Greek symbol omega (). We already know that
resistance results in current... George Ohm relates current,
voltage and resistance... V and I are directly related, and are
inversely related to R. OHMS LAW: Current (I) = voltage (V)
/resistance (R)
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Electrical Resistance Uses of resistance: Electrical devices
utilize resistance to control flow of current through resistors.
Specifically, devices are designed with resistors that cause the
right amount of current to flow when connected to the proper
voltage.
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Electrical Resistance Why can we receive a fatal shock from a
wall socket, but handle a 9 V battery safely? Ohms Law...
9V/100,000 (R skin) = 0.00009 amps. We can feel an I of 0.005
amps... Wall sockets provide a V = 120 V 120 V/100,000 ohms =
0.0012 amps. Wet skin has an even lower resistance, because of
water's high conductivity... Lower resistance means greater
current, which increases the possibility for a fatal electric
shock...
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Electrical Resistance Practice (EASY!) How much current will
flow through a lamp that has a resistance of 100 ohms when voltage
is 25V?
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Electrical Resistance More Practice At 100,000 ohms, how much
current will flow through your body if you touch the terminals of a
12 V battery? If your skin is covered with salt water reducing the
resistance to 100 ohms, how much current will flow through your
body if you touch the terminals of a 12 V battery? Will you feel
it?
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Agenda March 5th: Get out HW Notes on Series and Parallel
Circuits HW Series and Parallel Circuits
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Electrical Circuits Circuit path along which e- flow. Voltage
source battery Conductor wire Device electrical device that changes
electrical E to another type of E.
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Electrical Circuits Series Circuit Only one path through which
e- flow. (I) is the same at all points in the circuit. If there is
a break ANYWHERE in a series circuit, all devices stop.
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Electrical Circuits (I) & (R) in a Series Circuit: R in a
series circuit = sum of R for individual devices. R for the wire is
TINY so we ignore it R total = R 1 + R 2 + R 3
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Electrical Circuit You try A series circuit contains a 12-V
battery and three bulbs with a resistance of 1, 2, 3. What is the
current in the circuit?
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Electrical Circuits Devices convert Electrical E to another
form of E. As current passes through each device its power is
reduced. Remember V = P I, or W per A. If P after each device, then
V , too! Voltage drop
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Electrical Circuits Kirchhoffs Law: the sum of all V drops in a
system must be equal to the V battery.
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Electrical Circuits You try: A circuit contains a 9-V battery,
1- bulb, and a 2- bulb. Calculate the circuits total R and I. Then
find each bulbs voltage drop.
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Electrical Circuits Parallel Circuit More than one path through
which e- flow; branch. (I) splits at one or more branches.
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Electrical Circuits (I) & (V) in a Parallel Circuit:
Kirchhoffs Law I in a parallel circuit = sum of I for individual
branches. I total = I 1 + I 2 + I 3 V is the same in every branch!
I in each branch is determined by R in each branch.
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Electrical Circuit Parallel Circuit Ads: Each device has a V
drop = V battery. Each device may be turned off independently
without affecting other devices in circuit.
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Electrical Circuits Devices decrease resistance in a parallel
circuit! More devices = more branches. Remember, I parallel = sum
of I per branch. Therefore, more branches = more current.
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Electrical Circuits All of the electrical outlets in Jonahs
living room are on one parallel circuit. The circuit breaker cuts
off the current if it exceeds 15 A. Will the breaker trip if he
uses a light (240 ), stereo (150 ) and an air conditioner (10
)?
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Electrical Circuits Resistance & Electricity: Resistance
converts Electrical E to Thermal E Short Circuits = parallel
braches are formed with no device (resistance) present. R wire =
0.001 . 1.5 V battery produces an I = 1500 Amps!!
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Electrical Circuits Circuit breaker switch that opens circuit
when I gets too great. Fuse piece of metal that melts when I gets
too great (openning circuit).
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Agenda Jan-29: Submit HW Notes Electricity HW- SerParallel
Circuits Packet Circuits Galore Lab; Test Next Week!
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Electricity & Power Remember that Voltage tells us the
amount of electrical power per Amp of current Therefore we can
calculate electrical power simply Power = Voltage x Current
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Electricity & Power Electrical Power is the rate at which
electrical energy is transferred by a current. Electrical Power is
measured in Watts (W) or kilowatts (kW). 1000 Watts = 1 kW
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Electricity & Power A 12-V battery is connected in series
to two identical light bulbs. The current in the circuit is 3 A.
Calculate the power output of the battery.
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Electricity & Power Buying Electricity: Electricity
Companies charge by kWh (kilowatt hours). 1 kWh = 1 kilowatt of
Electrical power has been used in 1 hour. kWh are units of Energy,
not Power!
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Electricity & Power How much does it cost to run an
electric stove for 2 hours if the electric stove using 3000 W per
hour, and the electric company charges $0.15 per kWh?
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Electricity & Power Electricity comes from two sources:
Batteries Direct Current (DC) current always flows in one
direction. Power Plants Alternating Current (AC) direction of
current changes (usually 60xs)
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Electricity & Power Distributing Electricity: Household
electricity comes from power plants via high tension wires. V = IR.
I is VERY BIG, & R is VERY SMALL (R wire tiny). Electrical
Power transmitted at a VERY HIGH I prevents Electrical E lost due
to R. MOST efficient!
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Electricity & Power Transformers: Voltage of electricity
produced at plants to VERY HIGH levels. Voltage of electricity
carried by transmission lines closer to your home eventually down
to 120 V or 240 V.
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Agenda Jan-30: Submit HW Circuits Galore Lab! HW Part 2:
Circuits Galore Lab; Test Next Week!
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Agenda Feb-2: Submit HW Review HW finish study guide; Test
Wednesday