Transcript Document

Electricity Concepts
Key Objectives
1. Define the term electric current.
2. Define the term potential difference and electrical
potential.
3. Define the term resistance
4. State the SI units for measuring current, voltage
and resistance.
5. Solve problems using current, charge and time.
6. Relate the resistance of a material to its length,
cross sectional area, resistivity and temperature.
7. Solve problems that relate resistivity, cross
sectional area and length.
The Coulomb is the SI unit of charge.
An elementary charge is the amount of charge on one
electron or proton in Coulombs.
An elementary charge is a very tiny unit of charge.
Since it is so small it is an inconvenient unit to measure
typical amounts of charge. Bigger units are needed.
1 elementary charge
=
(the charge on 1 electron)
1.6x10-19 Coulomb
On the other hand, a coulomb is an incredibly
large unit of charge.
1 Coulomb = 6.3x1018 electrons or elementary charges
All matter is made up of positive charges and
negative charges.
The positive charges have mass and are not
usually free to move.
The negative charges have virtually no mass and
are free to move through conductors.
Metals are the best conductors of electricity.
All metals are composed of positively charged atoms
immersed in a sea of movable electrons.
Negative charges are attracted to positive charges the
same way mice are attracted to cheese.
Any time there is a natural attraction between two
things we can use it to make the objects do work.
If there is a path, the negative charges (mice) will
gladly do work in order to get to the positive charges
(cheese).
In order to bring two like charges near each other work
must be done.
In order to separate two opposite charges, work must be
done.
Remember that
whenever work
gets done, energy
changes form.
Voltage and Potential
Difference
As the monkey does work on the positive charge, he
increases the energy of that charge. The closer he
brings it, the more electrical potential energy it has.
When he releases the charge, work gets done on the
charge which changes its energy from electrical
potential energy to,kinetic energy. Every time he brings
the charge back, he does work on the charge.
If the monkey brought the charge closer to the other
object, it would have more electrical potential energy.
If he brought 2 or 3 charges instead of one, then he
would have had to do more work so he would have
created more electrical potential energy.
If you place a charge in an electric field and release it,
the charge will begin to accelerate from an area of high
potential energy, to one of low potential energy.
This is because there is an electrostatic force acting on
the charge.
No work is done if the charge from a position of high potential
energy to low potential energy (the same direction as the
electric field).
In the diagram above, the arrows represent the direction of
the electric field. If the positive test charge moves from B to A,
it is moving in the same direction as the electric field and no
work is done.
When no work is done on a positive test charge to move it
from one location to another, potential energy increases and
voltage increases.
Electric potential energy and voltage are greatest at point B.
If you want to move the charge from a position of low to high
potential energy (against the electric field), you must do work on
the object against the electric field.
When work is done on a positive test charge by an external
force to move it from one location to another, potential energy
increases and voltage increases.
If the positive test charge moves from A to B, work must be
done to move the charge against the field.
Electric potential energy and voltage are greatest at point B.
Electrical Potential is also known as Voltage or
Potential Difference.
The potential difference (voltage) is the amount of
energy per unit of charge, or the work that each
charge will do as it goes through a circuit.
The formula for calculating potential difference is:
V = Voltage
PE = electrical potential energy
q = units of charge
Measured in volts
Measured in joules
Measured in Coulombs
Potential difference is measured in Joules per Coulomb which
has been defined as a volt.
Problem
What is the potential difference between two points if 1000 J
of work is required to move 0.5 C of charge between the two
points.
In this example the amount of work done by the person
is 30J.
This is also the amount of electrical potential energy
that is possessed by all three charges together.
At the original position of the charges they have no
energy, so they also have no electrical potential or
0 volts.
Once they are pulled apart, they have an electrical
potential of 10 volts.
Potential Difference
Think of the mouse as a charge trying to move through an
electric field to get to the cheese. When the mouse crosses
the turnstile, it uses some of its energy to do work on the
turnstile.
The mouse’s energy has decreased.
A
B
The mouse has more energy per charge before it
crosses the turnstile than after it crosses it.
In this case the potential difference represents a decrease
in the amount of energy per charge (voltage drop)
from point A to point B.
The potential difference between two points is equal to the
energy change between those two points.
For batteries, we specify the potential difference of the charges
within the battery.
A "D-cell" has a rating of 1.5 volts. The potential difference of the
battery is 1.5 v, which means that for every Coulomb of charge
that moves from the negative side of the cell to the positive side
will do 1.5 Joules worth of work.
A "AA-cell" also has a potential difference of 1.5 volts, so each
Coulomb of charge that moves from one side to the other will also
do 1.5 joules worth of work.
The difference between the D-cell and the AA-cell is
that the D-cell has more Coulombs worth of
charge (more energy), so it will last longer.
As a result of having more charge, the D-cell has more
energy and can do more work, but it will still do work at
the same rate (or has the same power) as the AAcell.
Electric utilities typically deliver electricity, under
standard conditions, at 240 volts and 120 volts.
The voltage used for lighting and small appliances is
120V – an average (called the RMS average).
220-240 V is commonly used for most highpower electrical appliances (ovens, furnaces,
dryers, large motors, etc.).
Alessandro Volta (1745-1827)
Italian physicist who
invented the voltaic pile
which was the first electric
battery.
The volt is named for him.
Current:
Electric current means flow of charge.
Current – The number of charges passing a
point per second. The rate of flow of charges.
ampere – the SI unit of current.
The symbol used to represent current is I.
1 Ampere is equivalent to 1 coulomb of
charge passing a fixed point each second.
Note: current means the flow of
electric energy at any moment — not
over a certain period of time.
To calculate electric current use the formula:
I
= Current
Measured in amperes
Δq = coulombs of charge
passing through
T = time
Measured in seconds
Problem
(* remember time is in seconds)
What is the electric current in a conductor if 240
coulombs of charge pass through it in one minute?
The unit of current is the ampere, which is
named for French scientist André Ampère
(1775 – 1836).
Currents are established and maintained through a
conductor by the application of a potential
difference (voltage) across the conductor.
An electric current that flows in a conductor has a
number of effects:
Heating – Current causes friction that heats up
the wire. The greater the current, the more heat
is generated.
Magnetic Effect – A magnetic field is generated
around any conductor when an electric current
flows through it.
Alternating Current
AC or Alternating Current is
commonly used for residential
and commercial power
sources.
The current in AC electricity
alternates in direction. The
current switches direction
with a frequency of 60 times
every second (60 Hz).
The voltage can be readily
changed, thus making it more
suitable to long distance
transmission than DC
electricity.
Alternating current is created by an AC generator,
which determines the frequency. A picture of a
generator Is shown below.
1.5% at the transformer.
The 60 Hz oscillations are obtained by making the
generator go around at that speed.
Direct Current
1. DC or direct current
means the electrical
current is flowing in only
one direction in a circuit.
2. Batteries are a good
source of direct current
(DC).
3. The circuit has polarity.
In other words, electrons
flow from the negative
terminal to the positive
terminal of a battery.
Graphic Comparison of AC and DC Circuits
RMS
A Bit OF History
The original voltage was actually about 90 volts direct
current (VDC) which was Thomas Edison's plan.
Nicola Tesla proposed that the electrical grid be
alternating current (AC) and competed with Edison for
the first generating plant to be built in the State of
New York at Niagara Falls.
Edison proposed a DC system and Tesla an AC system.
As history tells us Tesla won the competition.
Nikola Tesla
(10 July 1856 - 7 January
1943) was an inventor,
mechanical engineer, and
electrical engineer.
He was born in Croatia
and later became an
American citizen.
The inventor of alternating current.
Resistance:
1. Resistance is the opposition to the flow of charge.
2. Resistance is friction that electricity experiences
while flowing through something.
3. When electrons move against the opposition of
resistance, friction is generated. The friction
manifests itself as heat and light.
4. Resistance or lack of resistance is used in circuits to
control the flow of the current.
5. Conductors have low resistances and insulators
have high resistances.
The unit for resistance is the Ohm.
The symbol for resistance is Ω (the Greek letter
Omega).
Any device (resistor) that asks the charge to do
work will slow it down.
The Ohm is named for
Georg Simon Ohm (16
March 1789 – 6 July 1854),
a German physicist.
Ohm was the scientist who defined the
fundamental relationship among voltage,
current and resistance, known as Ohm’s
Law.
We will discuss Ohm’s law when we get to
electrical circuit analysis.
An example of electrical resistance is shown in a
simple light bulb.
Electrons move relatively freely through
the conducting wire.
When the electrons work their way
through the filament they encounter
more opposition to motion (friction) than
the would in the conducting wire.
The electrons can get through, but not
as easily as they can through the wire.
The work done overcoming the
resistance causes the filament to heat
up and to give off light.
When the charges move
across the filament, some
of the electrical energy is
converted to heat and
light.
As the charges move
through the filament
(resistor) they do work on
the resistor and as a result,
they lose energy..
There are four factors that influence the resistance
in a conductor.
1. Length - The longer the length of the conductor,
the higher its resistance.
The length of a conductor is similar to the length of
a hallway. A shorter hallway would allow people to
move through at a higher rate than a longer one.
2. Cross Sectional Area of the wire) (Thickness)
The bigger the cross sectional area, the lower the
resistance.
The animation below demonstrates the comparison
between a wire with a small cross sectional area (A) and
a larger one (A).
The electrons seem to be moving at
the same speed in each one but
there are many more electrons in the
larger wire. This results in a larger
current and lower resistance.
3. Temperature - The higher the temperature the
higher the resistance.
As a conductor heats up, the protons start vibrating
and moving slightly out of position.
As their motion becomes more erratic they are more
likely to get in the way and disrupt the flow of the
electrons.
4. Resistivity – The quantity that measures how
well a substance resists carrying a current.
The resistivity only depends on the material
being used. Metallic conductors for example
have very low resistances.
For example, gold would have a lower resistivity
than lead or zinc, because it is a better
conductor.
The table lists the resistivities
of some common materials.
Silver and Copper are the
best metallic conductors
and thus have the lowest
resistivity.
Nichrome wire has such high
resistance that it is used to
convert electrical energy into
heat. Many heating elements
are made from nichrome.
The formula for calculating resistance relates the
cross sectional area, length, and resistivity of a
conducting material.
The formula that relates cross sectional area, length, and
electrical conductivity (resistivity) to the resistance of the wire is:
R
A
l
ρ
the resistance of the conductor
Unit: Ohms Ω
is the cross sectional area
Unit: m2
is the length of the wire
Unit: meters
is the resistivity of the material
Unit: Ohm(meters) (the Greek letter rho)
The formula shows that resistance is directly
proportional to length and inversely proportional
to cross-sectional area.
Problem
(* the resistivity of aluminum is 2.65 x 10-8 Ω·m)
Calculate the resistance at 20° C of an aluminum wire
that is 0.200 meter long and has a cross-sectional
area of 1.00 x 10-3.
In general it is important to realize that:
1. If you double the length of a wire, you will
double the resistance of the wire.
2. If you double the cross sectional area of a
wire you will cut its resistance in half.
Superconductors
Superconductors are materials lose all resistance
at low temperatures, a phenomenon known as
superconductivity.
In a superconductor the resistance drops abruptly
to zero when the material is cooled below its
critical temperature.
An electric current flowing in a loop of
superconducting can persist indefinitely with no
power source.
A magnet levitating above a superconductor, cooled with
liquid nitrogen.
Persistent electric current flows on the surface of the
superconductor, acting to exclude the magnetic field of
the magnet (Faraday’s Law of Induction).
This current effectively forms an electromagnet that
repels the magnet.
Electrons inside the wires move very slowly.
However, the electric field in the wire is established at close
to the speed of light .
The action of electricity
over distance using wires
is fast because the
electrons are already in
the wire waiting to move
and move through the
entire circuit at once.
Misconceptions: True of False
When an battery no longer works, it is out of charge
and must be recharged before it can be used again.
False
When a battery dies, it is out of energy, not
charges. The charges (electrons) come from the
wire in the circuit.
Misconceptions: True of False
A battery can be a source of charge in a circuit. The
charge which flows through the circuit originates in the
cell.
False
The charges (electrons) come from the wire in the
circuit not the battery.
Misconceptions: True of False
Charge becomes used up as it flows through a
circuit. The amount of charge which exits a light bulb
is less than the amount which enters the light bulb.
False
The charges are not used up. The charges
are still in the wire. It is the energy that the
charges carry that gets used up.
Misconceptions: True of False
Charge flows through circuits at very high speeds.
This explains why the light bulb turns on
immediately after the wall switch is flipped.
False
Charge carriers in the wires of electric circuits
are electrons. These move very slowly.
Misconceptions: True of False
The local electrical utility company supplies millions
and millions of electrons to our homes everyday.
False
The fact is that the mobile electrons which are in the wires of
our homes would be there whether there was a utility company
or not. The electrons come with the atoms that make up the
wires of our household circuits.
The utility company simply provides the energy which causes
the motion of the charge carriers within the household circuits.
And when they charge us for a few hundred kilowatt-hours of
electricity, they are providing us with an energy bill.
The Science Joy Wagon
The Physics Classroom
Youtube Videos
WikePedia
http://ghostradio.wordpress.com/2009/07/
11/google-honors-nikola-tesla/
Music
Frankenstein – The Edgar Winter Group
Electricity – Midnight Star