Download Electricity

February 25, 2009
• Notes – Electricity
• CW – Static Electricity Pre-Lab
• HW – Electricity and Subatomic
particles
Electricity
Chapter 11
3
11.1 Electric Charge
Basic facts about the atom
• Matter is made of atoms
Particle
Proton
Neutron
Electron
Location
Nucleus
Nucleus
“Cloud”
Charge
(+)
none
(-)
Model of a Helium atom
If a helium atoms has…
• 2 protons
• 2 neutrons
• 2 electrons
The Fundamentals…
Atoms consist of…
• a small, densely packed, (+)
nucleus (p+ & n)
• Surrounded by a (-) cloud (e-)
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!
The Fundamentals…
Electron cloud has order to it!
• Electrons are arranged in
orbitals.
• Orbitals radiate out from the
nucleus
The Fundamentals…
Atoms in their “normal” state are
neutral…
• # p+ = # e• # p+ determines the identity of
the element (– “Atomic Number”
on 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!
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
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?
TriboElectric Series
list that ranks various materials according to their
tendency to gain or lose electrons
VERY POSITIVE – looses electrons
VERY NEGATIVE – gains electrons
•Human hands
(usually too moist though)
•Teflon
•Rabbit Fur
•Scotch Tape
•Glass
•Saran Wrap
•Human hair
•Styrofoam
•Nylon
•Polyester fabric
•Wool
•Gold/Platinum/Brass/Silver/Copper
•Silk
•Hard Rubber
•Aluminum
•Amber
•Paper
•Wood
•Vinyl
•Cotton
Do these look familiar to you? Which is a conductor? Insulator??
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Agenda 26-Jan:
• GET OUT YESTERDAY’S HW!
• Notes – Electricity
• CW/HW – Electric Charge &
Coulombs Law WS
Static Electricity &
Electric Charge:
• Charging by Friction – transfer
of e- by contact. e.g. rubbing
your head with a balloon
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.)
Static Electricity &
Electric Charge:
• Polarization
Static Electricity &
Electric Charge:
• Charging by Induction – using
one object to charge another.
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
Coulomb’s Law:
• Relates charge on two objects (q1
and q2), the distance between their
centers (r), and the electrical force
(FE)
FE = k x (q1 x q2) ÷ r2
k = 9 x 109 Nm2/C2
How does force change with
distance?
Coulomb’s Law:
• The force between two charges
acts along a line joining their
centers.
• Charges obey Newton’s 3rd Law
of motion – they make actionreaction pairs.
Coulomb’s 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?
Agenda: March 3rd
• Notes – Electricity
• Fix yours…
• Finish Review Questions
• 7-12 on p 188
• Exercises
• 8 – 16 on p 189
• Tomorrow – Circuit Lab!
Electric Current
• Electric charges, electrons, are
always present, but may not be
moving!
• Current – the flow of charge, or
electrons in one direction, ()(+).
Electric Current
• Measured in Amperes (Amps, A)
• 1 Amp = 1 Coulomb per second
How many electrons per second
is that?
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.
Electric Current
• Voltage is therefore…
Watts ÷ Amps, or…
Joules (PE) ÷ Coulombs (charge)
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 it’s 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.
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.
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.
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?
Agenda March
th
4
• Notes – Resistance & Circuits
• CW – Electrical Resistance WS
• http://www.moourl.com/circuit
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
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.
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?
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...
Electrical Resistance
Resistance is measured in OHMS,
represented by Greek symbol omega (Ω).
• 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)
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.
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 Ω (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...
Electrical Resistance
Practice (EASY!)
• How much current will flow
through a lamp that has a
resistance of 100 ohms when
voltage is 25V?
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?
Agenda March 5th:
• Get out HW
• Notes on Series and Parallel
Circuits
• HW – Series and Parallel
Circuits
Electrical Circuits
• Circuit – path along which eflow.
Voltage source – battery
Conductor – wire
Device – electrical device that
changes electrical E to another
type of E.
Electrical Circuits
Series Circuit
• Only one path through which eflow.
• (I) is the same at all points in
the circuit.
• If there is a break ANYWHERE
in a series circuit, all devices
stop.
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…
Rtotal = R1 + R2 + R3…
Electrical Circuit
You try…
A series circuit contains a 12-V
battery and three bulbs with a
resistance of 1Ω, 2Ω, 3Ω. What
is the current in the circuit?
Electrical Circuits
Devices convert Electrical E to
another form of E.
• As current passes through each
device it’s power is reduced.
• Remember V = P ÷ I, or W per A.
• If P ↓ after each device, then V
↓, too! Voltage drop
Electrical Circuits
Kirchhoff’s Law: the sum of all V
drops in a system must be equal
to the V battery.
Electrical Circuits
You try:
A circuit contains a 9-V battery,
1-Ω bulb, and a 2-Ω bulb.
Calculate the circuit’s total R
and I. Then find each bulb’s
voltage drop.
Electrical Circuits
Parallel Circuit
• More than one path through
which e- flow; branch.
• (I) splits at one or more
branches.
Electrical Circuits
(I) & (V) in a Parallel Circuit:
• Kirchhoff’s Law…
• I in a parallel circuit = sum of I
for individual branches.
Itotal = I1 + I2 + I3…
• V is the same in every branch!
• I in each branch is determined
by R in each branch.
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.
Electrical Circuits
Devices decrease resistance in a
parallel circuit!
• More devices = more branches.
• Remember, Iparallel = sum of I per
branch.
• Therefore, more branches =
more current.
Electrical Circuits
All of the electrical outlets in
Jonah’s 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 Ω),
stereo (150 Ω) and an air
conditioner (10 Ω)?
Electrical Circuits
Resistance & Electricity:
• Resistance converts Electrical
E to Thermal E
• Short Circuits = parallel braches
are formed with no device
(resistance) present.
• Rwire = 0.001 Ω. 1.5 V battery
produces an I = 1500 Amps!!
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).
Agenda Jan-29:
• Submit HW
• Notes – Electricity
• HW- SerParallel Circuits Packet
• Circuits Galore Lab; Test Next
Week!
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
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
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.
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!
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?
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 60x’s)
Electricity & Power
Distributing Electricity:
• Household electricity comes from
power plants via high tension wires.
• V = IR. I is VERY BIG, & R is VERY
SMALL (Rwire tiny).
• Electrical Power transmitted at a
VERY HIGH I prevents Electrical E
lost due to R. MOST efficient!
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.
Agenda Jan-30:
• Submit HW
• Circuits Galore Lab!
• HW – Part 2: Circuits Galore
Lab; Test Next Week!
Agenda Feb-2:
• Submit HW
• Review
• HW – finish study guide; Test
Wednesday
Agenda Feb-3:
• Submit HW
• Review
• Test Tomorrow~