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 electron or proton is the amount of charge on one
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)
On the other hand, a coulomb is an incredibly large unit of charge.
1 Coulomb = 6.3x10
18 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
are
free
charges have virtually no mass and 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
the charge .
acting on
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
Potential Difference.
Voltage
or 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 Measured in
volts
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
.
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 AA cell.
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 high power 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
Δq
= coulombs of charge passing through Measured in
amperes 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 terminal to the
negative 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 Omega
).
(the Greek letter 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
in a conductor. factors that influence the resistance
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:
m 2
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 to cross-sectional area.
inversely
proportional
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