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.
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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 = 1.6x10 -19 Coulomb (the charge on 1 electron) 1 Coulomb =
6.3x10 18 electrons or elementary charges On the other hand, a
coulomb is an incredibly large unit of charge. The Coulomb is the
SI unit of charge.
Slide 7
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.
Slide 8
All metals are composed of positively charged atoms immersed in
a sea of movable electrons. Metals are the best conductors of
electricity.
Slide 9
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).
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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.
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Voltage and Potential Difference
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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.
Slide 13
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.
Slide 14
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).
Slide 15
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.
Slide 16
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.
Slide 17
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.
Slide 18
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.
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V = Voltage Measured in volts PE = electrical potential energy
Measured in joules q = units of charge Measured in Coulombs
Potential difference is measured in Joules per Coulomb which has
been defined as a volt. The formula for calculating potential
difference is:
Slide 20
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.
Slide 21
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.
Slide 22
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.
Slide 23
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 mouses energy has decreased.
Slide 24
The mouse has more energy per charge before it crosses the
turnstile than after it crosses it. A B 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.
Slide 25
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. For batteries, we specify the potential difference of the
charges within the battery.
Slide 26
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 AA- cell.
Slide 27
220-240 V is commonly used for most high- power electrical
appliances (ovens, furnaces, dryers, large motors, etc.). The
voltage used for lighting and small appliances is 120V an average
(called the RMS average). Electric utilities typically deliver
electricity, under standard conditions, at 240 volts and 120
volts.
Slide 28
Alessandro Volta (1745-1827) Italian physicist who invented the
voltaic pile which was the first electric battery. The volt is
named for him.
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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.
Slide 33
Note: current means the flow of electric energy at any moment
not over a certain period of time.
Slide 34
To calculate electric current use the formula: I = Current
Measured in amperes q = coulombs of charge passing through T =
timeMeasured in seconds
Slide 35
Problem What is the electric current in a conductor if 240
coulombs of charge pass through it in one minute? (* remember time
is in seconds)
Slide 36
The unit of current is the ampere, which is named for French
scientist Andr Ampre (1775 1836).
Slide 37
Currents are established and maintained through a conductor by
the application of a potential difference (voltage) across the
conductor.
Slide 38
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.
Slide 39
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.
Slide 40
The 60 Hz oscillations are obtained by making the generator go
around at that speed. 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.
Slide 41
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.
Slide 42
RMS Graphic Comparison of AC and DC Circuits
Slide 43
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. A Bit OF
History
Slide 44
Nikola Tesla The inventor of alternating current. (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.
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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.
Slide 49
The unit for resistance is the Ohm. The symbol for resistance
is (the Greek letter Omega). Any device (resistor) that asks the
charge to do work will slow it down.
Slide 50
Ohm was the scientist who defined the fundamental relationship
among voltage, current and resistance, known as Ohms Law. We will
discuss Ohms law when we get to electrical circuit analysis. The
Ohm is named for Georg Simon Ohm (16 March 1789 6 July 1854), a
German physicist.
Slide 51
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. An
example of electrical resistance is shown in a simple light
bulb.
Slide 52
As the charges move through the filament (resistor) they do
work on the resistor and as a result, they lose energy.. When the
charges move across the filament, some of the electrical energy is
converted to heat and light.
Slide 53
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.
Slide 54
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.
Slide 55
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.
Slide 56
4. Resistivity The quantity that measures how well a substance
resists carrying a current. For example, gold would have a lower
resistivity than lead or zinc, because it is a better conductor.
The resistivity only depends on the material being used. Metallic
conductors for example have very low resistances.
Slide 57
Silver and Copper are the best metallic conductors and thus
have the lowest resistivity. The table lists the resistivities of
some common materials. Nichrome wire has such high resistance that
it is used to convert electrical energy into heat. Many heating
elements are made from nichrome.
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The formula for calculating resistance relates the cross
sectional area, length, and resistivity of a conducting
material.
Slide 60
The formula that relates cross sectional area, length, and
electrical conductivity (resistivity) to the resistance of the wire
is: the resistance of the conductor Unit: Ohms is the cross
sectional area Unit: m 2 l is the length of the wire Unit: meters
is the resistivity of the material Unit: Ohm(meters) R A (the Greek
letter rho)
Slide 61
The formula shows that resistance is directly proportional to
length and inversely proportional to cross-sectional area.
Slide 62
Problem 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. (* the resistivity of aluminum is 2.65 x 10 -8 m)
Slide 63
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.
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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.
Slide 66
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
(Faradays Law of Induction). This current effectively forms an
electromagnet that repels the magnet.
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However, the electric field in the wire is established at close
to the speed of light. Electrons inside the wires move very slowly.
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.
Slide 70
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.
Slide 71
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.
Slide 72
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.
Slide 73
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. Charge carriers in
the wires of electric circuits are electrons. These move very
slowly. False
Slide 74
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.
Slide 75
The Science Joy Wagon The Physics Classroom Youtube Videos
WikePedia http://ghostradio.wordpress.com/2009/07/1
1/google-honors-nikola-tesla/ Music Frankenstein The Edgar Winter
Group Electricity Midnight Star