electricity concepts key objectives 1.define the term electric current. 2.define the term potential...

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.

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.

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.

All metals are composed of positively charged atoms immersed in a sea of movable electrons. Metals are the best conductors of electricity.

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.

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:

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 mouses energy has decreased.

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.

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.

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.

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.

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 q = coulombs of charge passing through T = timeMeasured in seconds

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)

The unit of current is the ampere, which is named for French scientist Andr Ampre (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.

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.

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.

RMS Graphic Comparison of AC and DC Circuits

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

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.

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 (the Greek letter Omega). Any device (resistor) that asks the charge to do work will slow it down.

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.

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.

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.

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. 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.

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.

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: 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)

The formula shows that resistance is directly proportional to length and inversely proportional to cross-sectional area.

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)

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 (Faradays Law of Induction). This current effectively forms an electromagnet that repels the magnet.

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.

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. Charge carriers in the wires of electric circuits are electrons. These move very slowly. False

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/2009/07/1 1/google-honors-nikola-tesla/ Music Frankenstein The Edgar Winter Group Electricity Midnight Star

electricity concepts key objectives 1.define the term electric current. 2.define the term potential...

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