Unit 1 Ccurrent Electricity


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Unit 1 Ccurrent Electricity

  1. 1. Current Electricity
  2. 2. Current Electricity
  3. 3. Static Electricity to Current Electricity <ul><li>You have seen that electric charge will move when electrostatic attraction or repulsion is applied. In order for the charge to move, however, some conditions must exist. These conditions will be explored in the section dealing with moving charge. </li></ul>Unit 1 – The Nature of Electricity
  4. 4. Static Electricity to Current Electricity <ul><li>Electrostatic electricity can be described as &quot;packets&quot; of electricity. Static electricity results from the movement of negative charges (electrons) off one material to another material. </li></ul><ul><li>Whenever an electric charge is allowed to leave a charged object, a path called a conductor is regularly used. A conductor is most often a metal wire made of copper or aluminum, but it may have been an ionized liquid or gaseous solution. </li></ul>Unit 1 – The Nature of Electricity
  5. 5. Static Electricity to Current Electricity <ul><li>When the electric charge traveled through a conductor, it is relatively small and the transfer takes place almost instantaneously. To transfer more charge, objects need to be rubbed together again. A charge builds up, and then a conductor is used to transfer another packet of charge. These packets of electricity travel through a conductor only irregularly because of the need to recharge the object after each transfer of electricity. </li></ul>Unit 1 – The Nature of Electricity
  6. 6. A Detailed Description of Charge Movement Unit 1 – The Nature of Electricity <ul><li>- + </li></ul><ul><li>- + </li></ul><ul><li>+ </li></ul><ul><li>+ </li></ul><ul><li>+ </li></ul>+++++++ _ _ _ _ _ _ _ _ - - - - - - - - - - - - Enlarged foil ball
  7. 7. A Detailed Description of Charge Movement <ul><li>A charge separation occurs on the ball because of the charged plates (see enlargement) </li></ul><ul><li>The foil ball moves to the right and picks up electrons. </li></ul><ul><li>The negative plate (and electroscope) lose some electrons to the ball and the ball becomes negative. </li></ul><ul><li>The negatively charged ball and the right plate repel. </li></ul><ul><li>The ball is attracted to the positive left plate. </li></ul>Unit 1 – The Nature of Electricity
  8. 8. A Detailed Description of Charge Movement Unit 1 – The Nature of Electricity <ul><li>- + </li></ul><ul><li>+ </li></ul><ul><li>+ </li></ul><ul><li>+ </li></ul><ul><li>+ </li></ul>+++++++ _ _ _ _ _ _ _ _ - - - - - - - - - - - - Enlarged foil ball
  9. 9. A Detailed Description of Charge Movement <ul><li>The foil ball touches the left plate and releases some electrons to the plate. </li></ul><ul><li>These new negative charges reduce some of the positive charge on the left plate. </li></ul><ul><li>The ball is once again attracted to the right plate and the entire process continues. </li></ul><ul><li>Eventually both plates reduce their charge until they are neutral. </li></ul><ul><li>As this happens there is a charge flow from the negative to the positive plate. </li></ul><ul><li>This shows that an electrostatic charge can demonstrate characteristics of electric current. </li></ul><ul><li>An electrostatic device becomes discharged when it loses its ability to move electrons. </li></ul>Unit 1 – The Nature of Electricity
  10. 10. What is current electricity? <ul><li>Current electricity is a stream of electrons flowing through a conductor. </li></ul><ul><li>There are many different sources of current electricity. Ex. batteries, generators, etc. </li></ul>Unit 1 – The Nature of Electricity
  11. 11. What is current electricity? <ul><li>Current electricity refers to the behavior of electrical charges in motion. In order for charged particles to flow, some pathway must be provided for them. That pathway is called an electric circuit. An electric circuit typically consists of a source of electricity, such as a battery; an appliance that operates on electric energy, and metal wires connecting those parts of the circuit. </li></ul>Unit 1 – The Nature of Electricity
  12. 12. Electric Cells <ul><li>An electric cell has the capacity to produce electric charge for a longer time than an electrostatically charged object. </li></ul><ul><li>The voltaic cell is called a wet cell because a liquid surrounds the two types of metals or electrodes. </li></ul>Unit 1 – The Nature of Electricity
  13. 13. Voltaic Cells Unit 1 – The Nature of Electricity
  14. 14. Voltaic Cells <ul><li>Volta's electric cell produced a continuous supply of electrons when connected to a conductor. The electric cell consisted of two different metals in a salt or acid solution. A chemical reaction takes place resulting in a supply of electrons. Eventually, the chemical reaction slows down and stops. When the chemical reaction stops, the supply of electrons stops and the cell is &quot;dead.' </li></ul>Unit 1 – The Nature of Electricity
  15. 15. Voltaic Cells Unit 1 – The Nature of Electricity
  16. 16. Dry Cells <ul><li>The voltaic cell is a wet cell because the electrodes are surrounded by a water solution containing an acid, base, or salt. </li></ul><ul><li>The dry cell uses a paste which is also an acid, base, or salt solution. While the dry cell is not really dry, the paste is thick enough for easy handling and it is used in many different electrical devices. </li></ul>Unit 1 – The Nature of Electricity
  17. 17. Dry Cells Unit 1 – The Nature of Electricity
  18. 18. Dry Cells Unit 1 – The Nature of Electricity
  19. 19. What Is Electric Potential? <ul><li>Whenever electric charges move through a conductor, work must be done on them. If work is done on the electric charge, there must be some energy applied. The energy applied to any circuit causing movement of electric charge is called electric potential. </li></ul><ul><li>Potential energy is a common and welcome source of energy in many situations. We can describe potential energy as stored energy. Once the stored or potential energy is released it is capable of doing work. </li></ul>Unit 1 – The Nature of Electricity
  20. 20. Brick Analogy <ul><li>One of the easiest ways for us to understand the meaning of potential energy is to look at an example. A bricklayer was building a wall. He bent over and picked up a brick and placed it on the top of the wall. The brick now has some gravitational potential energy. The brick was accidentally knocked off the wall and fell down, landing on the bricklayer's toe. The potential energy of the brick has been changed into work done on the toe, to the bricklayer's discomfort. </li></ul>Unit 1 – The Nature of Electricity
  21. 21. Electric Potential - Voltaic Cell <ul><li>In the open circuit above, the zinc strip causes electrons to build up at point A and a deficiency of electrons at point B. If a switch is inserted in the circuit above, we can cause the electric potential to do work by simply closing the switch and allowing an electron flow or current to take place as the electrons flow from the negative to the positive electrode. </li></ul>Unit 1 – The Nature of Electricity
  22. 22. Electric Potential - Voltaic Cell <ul><li>An open circuit voltaic cell qualifies as a source of potential energy because: </li></ul><ul><ul><li>the potential energy is stored for as long as the switch is open. </li></ul></ul><ul><ul><li>The potential energy is an &quot;on demand&quot; energy source since we can choose to allow the potential energy to do work by closing the switch whenever we want. </li></ul></ul>Unit 1 – The Nature of Electricity
  23. 23. Electric Potential - Voltaic Cell <ul><li>We have been talking about changing potential energy into work. Some examples illustrating electric potential energy changed into work are listed below. </li></ul><ul><ul><li>An electric saw converts electric energy into work done cutting a board </li></ul></ul><ul><ul><li>An electric starter converts electric energy into work done cranking a car engine to start it. </li></ul></ul><ul><ul><li>An elevator converts electric energy into work done in moving people from one level to another in a building </li></ul></ul>Unit 1 – The Nature of Electricity
  24. 24. Electric Potential - Voltaic Cell <ul><li>This is a good time to look at a model of an electric circuit. A circuit consists of a device that has some potential energy, a pathway for charges to travel, and generally something such as a light, oven etc. that requires energy to operate(there can also be a measuring device such as an ammeter). </li></ul><ul><li>In our analogy we will have four strong people at one end of a tube that is filled with particles we will call marbles. The young people are standing around, waiting to go to work. They represent electric potential. </li></ul>Unit 1 – The Nature of Electricity
  25. 25. Electric Potential - Voltaic Cell <ul><li>On a signal, they begin pushing on the marbles with a plunger. As soon as the marbles move one centimeter on their end of the tube, the particles move one centimeter at the other end of the tube. The energy that they have put into the marbles has instantly traveled through the whole system. </li></ul><ul><li>If we want, we can put a marble counter anywhere in the tube and it will count the number of marbles per second passing by to give us a measure of current. It won't be able to count individual marbles but it might be able to count groups of 1 000 marbles to give us a measure of the current in groups of 1 000 marbles per second. </li></ul>Unit 1 – The Nature of Electricity
  26. 26. Electric Potential - Voltaic Cell <ul><li>Once the circuit is closed we use the word voltage to tell us how much energy is needed to do the work of moving electrons through the circuit. If it takes one joule of energy to move one coulomb of electric charge through some distance on a conductor, we can say that one joule per coulomb or one volt of electric energy was needed. </li></ul><ul><li>The joule is a standard unit of energy in the metric system. Work is also measured in joules. Energy needed to do the work, in this case the work in moving electrons, is measured in joules. </li></ul><ul><li>one coulomb of electric charge is 6,250,000,000,000,000,000 (6.25 x 10 18 )positive or negative charges. </li></ul>Unit 1 – The Nature of Electricity
  27. 27. Electric Potential - Voltaic Cell <ul><li>The volt is a common term used to represent 1 joule of energy used to move one coulomb of electric charge. </li></ul><ul><ul><li>V is the potential in volts(joules per coulomb) </li></ul></ul><ul><ul><li>E is the work in joules </li></ul></ul><ul><ul><li>Q is the charge in coulombs </li></ul></ul>Unit 1 – The Nature of Electricity
  28. 28. Electric Potential - Voltaic Cell <ul><li>Whenever work is done there is a greater amount of potential energy before the work is done than there is after the work is done; that is, there has been a change in the potential energy. </li></ul><ul><li>In electric circuits, the potential energy is greatest at the negative strip(electrode) of the cell. Electrons do work as they move from the negative electrode. As a result the electric potential decreases until it is at its lowest value at the positive electrode. </li></ul>Unit 1 – The Nature of Electricity
  29. 29. Electric Potential - Voltaic Cell Unit 1 – The Nature of Electricity
  30. 30. Circuits, Conductors, & Insulators Unit 1 – The Nature of Electricity
  31. 31. Circuits <ul><li>A circuit is a path for electrons to flow around. The path goes from the negative terminal of a power source, through various components and onward to the positive terminal. </li></ul><ul><li>Think of it as a circle. The paths may split off here and there but they always form a line from the negative to positive. </li></ul><ul><li>NOTE: Negatively charged electrons in a conductor are attracted to the positive side of the power source. </li></ul>Unit 1 – The Nature of Electricity
  32. 32. Circuits <ul><li>In their most basic form, circuits consist of a source of energy , a conducting loop , and a resistance that uses electrical energy. </li></ul><ul><li>A conducting loop consists of a copper wire attached to the battery and the load in such a way that a path exists for the electrons to travel. The load in a circuit can be a light bulb, electric motor, or any other electric device. </li></ul><ul><li>A load uses the electric potential energy contained in the battery to produce other forms of energy. A motor uses electric potential to produce mechanical energy, while an electric stove uses electric energy to produce heat energy. </li></ul>Unit 1 – The Nature of Electricity
  33. 33. Circuits Unit 1 – The Nature of Electricity
  34. 34. Electric Current <ul><li>When a circuit is closed, electrons travel through the circuit, this flow is called electric current </li></ul><ul><li>The rate at which charge passes through any given point in an electric circuit is called current (I) . Current can be expressed as: </li></ul><ul><li>I = Q/t </li></ul><ul><ul><li>I = current, measured in amperes (A) </li></ul></ul><ul><ul><li>Q = charge, measured in coulombs (C) </li></ul></ul><ul><ul><li>T = time, measured in seconds </li></ul></ul>
  35. 35. Electric Current <ul><li>The unit for current is the ampere, which is the same as one coulomb of charge per second. A device used to measure current is an ammeter. In a circuit diagram, it has this symbol: </li></ul>
  36. 36. Electric Circuits
  37. 37. Electric Circuits <ul><li>There are two ways to connect multiple devices to a voltage source </li></ul><ul><li>One is called series </li></ul><ul><li>The other is called parallel </li></ul>
  38. 38. Electric Circuits
  39. 39. Series Circuits
  40. 40. Series Circuits <ul><li>A single pathway through the circuit </li></ul><ul><li>The current is the same everywhere in the circuit </li></ul><ul><li>Each device provides resistance and total resistance is the sum of the devices </li></ul><ul><li>Voltage divides among the devices </li></ul><ul><li>Voltage drop across each device is IR device </li></ul>
  41. 41. Parallel Circuits
  42. 42. Parallel Circuits <ul><li>Each device connects to the voltage source </li></ul><ul><li>Voltage is the same across each device </li></ul><ul><li>Current from source divides into devices </li></ul><ul><li>Total current is the sum of device currents </li></ul><ul><li>Current in each device is just V/R </li></ul><ul><li>Add devices, lower total resistance </li></ul>
  43. 43. Series & Parallel Circuits
  44. 44. Resistance <ul><li>Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current. </li></ul><ul><li>As electrons move through a conductor, they bump into the fixed particles in the conductor. When they bump into the particles, the electrons slow down and give up some of their energy as heat and light. This opposition to the flow of electron is called resistance . </li></ul><ul><li>The SI unit of electrical resistance is the ohm . </li></ul>Unit 1 – The Nature of Electricity
  45. 45. Resistance <ul><li>If a material has high resistance, it will not let current pass through it as easily. If a material has low resistance, it will let current pass more easily. </li></ul><ul><li>For any material, resistance varies with the length and cross-sectional area. For example: </li></ul><ul><ul><li>The longer a piece of wire, the larger the resistance </li></ul></ul><ul><ul><li>The fatter a piece of wire, the smaller the resistance. </li></ul></ul>Unit 1 – The Nature of Electricity
  46. 46. Resistance & Ohms’ Law <ul><li>The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit. </li></ul><ul><ul><li>R is the resistance of the object, usually measured in ohms, equivalent to J·s/C 2 </li></ul></ul><ul><ul><li>ΔV is the potential difference across the object, usually measured in volts </li></ul></ul><ul><ul><li>I is the current passing through the object, usually measured in amperes </li></ul></ul>Unit 1 – The Nature of Electricity
  47. 47. Resistance Unit 1 – The Nature of Electricity
  48. 48. Ohm’s Law Unit 1 – The Nature of Electricity
  49. 49. Ohm’s Law Unit 1 – The Nature of Electricity
  50. 50. Ohm’s Law Unit 1 – The Nature of Electricity
  51. 51. Schematic Diagrams <ul><li>Sometimes drawing pictures of the objects involved in a circuit takes too much time, so a system of symbols called schematic diagrams are used to make the task easier. </li></ul>
  52. 52. Schematic Diagrams
  53. 53. Circuit Activity - Text
  54. 54. Properties of Series & Parallel Circuits
  55. 55. Circuit Activity - Text
  56. 56. Cells in Series <ul><li>The net effect of placing cells in series is to increase the voltage of the battery. </li></ul><ul><ul><li>If the two cells have an voltage of 1.5 volts each, the result of the series connection is a battery with an voltage of 3.0 volts . </li></ul></ul>
  57. 57. Cells in Parallel <ul><li>Two cells are connected in parallel. The two positive electrodes are connected and the two negative electrodes are also connected. </li></ul><ul><li>Total voltage is equal to the voltage from a single cell </li></ul><ul><li>This set up allows for the </li></ul><ul><li>batteries to last longer. </li></ul>
  58. 58. Voltage and Current in Series and Parallel Circuits <ul><li>The greater the current the brighter the glow of a bulb. </li></ul><ul><li>Current is conserved in all circuits. </li></ul><ul><li>Current is determined by the resistance. Higher resistance means smaller current; lower resistance means larger current. </li></ul><ul><li>Resistors and appliances also can be connected in series and parallel. </li></ul><ul><li>Two resistors in series are a larger resistance than one.Two resistors in parallel are a smaller resistance than one. </li></ul>
  59. 59. Voltage and Current in Series and Parallel Circuits <ul><li>Light bulbs in series all go out (or will not light up) if one bulb is removed or burned out. Bulbs in parallel remain on or will light if one or more bulbs is removed or burned out. </li></ul><ul><li>When resistors are in series, all current must flow through each resistor. (I T = I 1 = I 2 …) When resistors are in parallel, current splits at junctions. In this way, part of the current goes through each resistor. The sum of the current going through each branch in parallel equals the total current in the circuit. (I T = I 1 + I 2 …) </li></ul>
  60. 60. Voltage and Current in Series and Parallel Circuits <ul><li>When resistors are in series, the total potential is equal to the sum of potentials across each resistor. (V T = V 1 + V 2 …) </li></ul><ul><li>When resistors are in parallel, the potential across each resistor is the same. </li></ul><ul><li>(V T = V 1 = V 2 …) </li></ul><ul><li>Circuits can be drawn in schematic or representational form. Schematic diagrams are more common. </li></ul><ul><li>Potential drop across a resistor and the voltage are the same measurement. </li></ul>
  61. 61. Voltage and Current in Series and Parallel Circuits Unit 1 – The Nature of Electricity
  62. 62. Producing Electricity <ul><li>To create electric potential energy we must accumulate and separate charge. As the negative charges move, the energy is transformed into kinetic energy. If we do not replace the negative charge the potential reduces very quickly and no more current will flow. For a continuous flow of charge we must maintain this accumulation of charge. </li></ul><ul><li>Whenever there is an electric current in a conductor, work can be done in some way . </li></ul>
  63. 63. Producing Electricity <ul><li>For work to be done, there must always be a source of energy. The energy source must be able to create a charge separation and accumulation. Five sources of potential energy that can be used to move electric charge through a circuit are described in the following slides. </li></ul>
  64. 64. Producing Electricity
  65. 65. Producing Electricity <ul><li>Chemical Energy </li></ul><ul><li>Thermoelectric Energy </li></ul><ul><li>Photoelectric Energy </li></ul><ul><li>Piezoelectric Energy </li></ul><ul><li>Electromagnetic Energy </li></ul>
  66. 66. Chemical Energy <ul><li>You have seen that a copper wire and an iron nail stuck in a lemon can produce an electric potential. Another example of chemical potential energy is the voltaic cell. Both the voltaic cell and the lemon cell work on the same principle. The chemical action deposits a charge on the electrodes of the cell. </li></ul><ul><li>A charge separation on the positive and negative electrodes provides the potential energy necessary to push electrons through the bulb, causing it to produce heat and light. </li></ul>
  67. 67. Thermoelectric Energy <ul><li>Heat energy can be converted to electrical energy through a process called the thermoelectric effect. The diagram below illustrates the thermoelectric effect . </li></ul>
  68. 68. Thermoelectric Energy <ul><li>The thermoelectric effect occurs when two different metals (copper and iron in this case) are heated at one junction and cooled at another. </li></ul><ul><li>When one set of copper and iron junctions is used, the set up is called a thermocouple . When several combinations are used in series, the result is called a thermopile . </li></ul><ul><li>Thermocouples are used as temperature measuring devices (e.g., the temperature gauge in a car). As the engine heats up, more current travels through the circuit. A galvanometer placed in the dash of the car is calibrated to measure the temperature of the engine coolant. </li></ul>
  69. 69. Photoelectric Energy
  70. 70. Photoelectric Energy <ul><li>When light strikes certain materials, it has enough energy to knock electrons free. These electrons are able to create a weak electric current. This process is called the photoelectric effect. The photoelectric effect is used in camera light meters to set the exposure for the camera automatically. Automatic doors also use the photoelectric effect to open and close doors. </li></ul>
  71. 71. Piezoelectric Energy <ul><li>Certain types of crystals will produce an electric potential when they are pressed together. Rochelle salts, ceramics and quartz demonstrate this property called the piezoelectric (pie-ee-zoe-electric) effect (piezo comes from the Greek word that means pressure). </li></ul>
  72. 72. Electromagnetic Energy <ul><li>In 1831, Michael Faraday demonstrated that a moving magnetic field could produce a current in a coil of wire . The diagram shown below illustrates how a bar magnet can be pushed into a copper coil to produce an electric current. </li></ul><ul><li>When the bar magnet is pushed into the coil, the galvanometer needle is deflected. When the bar magnet is pulled out, the needle is deflected, in the opposite direction. </li></ul>
  73. 73. Electromagnetic Energy <ul><li>Faraday reasoned that it would be easy to create a continuous moving magnetic field by rotating a wheel with spokes, which were conductors, between the poles of a horseshoe magnet. The result was a continuous supply of reasonably constant current. As more conductor spokes are added the current becomes constant and a cheap, readily available, supply of electricity has been achieved. This supplier of electricity became known as an electric generator. </li></ul><ul><li>A source of energy needed to turn the generator was at first supplied by steam engines but the most common source of potential energy today is the hydroelectric dam. </li></ul>
  74. 74. Hydroelectric Dam
  75. 75. Hydroelectric Dam
  76. 76. Electromagnetic Energy
  77. 77. Household Circuits <ul><li>Household circuits use alternating current instead of direct current. Alternating current travels in one direction and then reverses to travel in the opposite direction. This complete cycle takes place 60 times every second. We can say the frequency of the current in our system is 60 cycles per second or 60 hertz . </li></ul><ul><li>A hertz is a unit used to express how many times a repetitive action takes place in a second. </li></ul>
  78. 78. Household Circuits <ul><li>Household circuits operate at higher voltages. There are three wires that bring electricity into a home. A neutral wire is attached to the ground. Two hot wires carry 120 volts each. The hot wires form a potential difference of 120 volts each with the neutral wire. When there are two hot wires connected to the dwelling, it allows the household circuits to operate at either 120 volts or 240 volts. </li></ul><ul><li>Household circuits usually use 120 volts of energy and 15 amps of current: a circuit such as this one supplies current to several bedrooms. Larger appliances, such as electric stoves and clothes dryers, require more energy. These appliances use 240 volts of energy and up to 50 amps of current. </li></ul>
  79. 79. Household Circuits <ul><li>All circuits are connected to a service panel in the home that supplies electricity to them. You may have noticed the service panel in your home has many wires entering it. The wires attached to the service panel form the circuits. </li></ul><ul><li>The outlets in a house circuit are connected in parallel. A group of parallel resistors always receive the same amount of voltage. By connecting outlets in parallel, all the appliances receive 120 volts on a typical circuit. Each appliance requires current to operate. In a typical household parallel connection, the current requirements of each appliance on one circuit add up to 15 amps. </li></ul>
  80. 80. Household Circuits
  81. 81. Household Circuits <ul><li>The outlets in a house circuit are connected in parallel. A group of parallel resistors always receive the same amount of voltage. By connecting outlets in parallel, all the appliances receive 120 volts on a typical circuit. Each appliance requires current to operate. In a typical household parallel connection, the current requirements of each appliance on one circuit add up to 15 amps. </li></ul>These wires carry 15 amps 5 A Appliance 5 A Appliance 5 A Appliance
  82. 82. Household Circuits <ul><li>Since a typical circuit has a 15 amp circuit breaker, users must be careful that the total current of the appliances does not exceed 15amps. Electrical contractors are careful to set up circuits that when used in an ordinary way would not have current requirements of more than 30 amps. If you used a power bar, for example, and loaded appliances on it, then probably the 15 amp limit would be exceeded. A power overload would result and the circuit breaker would flip off. </li></ul>
  83. 83. Household Circuits <ul><li>Parallel circuits also mean you are able to turn off a single appliance without affecting any other appliances. You will recall that in series circuits all the current travels through each of the resistors. If the circuit is opened anywhere, none of the resistors receive any current and all the lights stop working. As you can see, series circuits do not function well as household circuits. </li></ul>
  84. 84. Electric Power <ul><li>Power is a description of the rate at which work is produced, absorbed, or transferred . </li></ul><ul><li>As an example, two students are traveling up the same hill. They both reach the top of the hill, so they have the same increase in energy. The second student, however, reached the top in less time than the other. This person did the same amount of work or gained the same energy in less time and, as a result, he or she used more power. </li></ul>
  85. 85. Electric Power <ul><li>The formula shown below can be used to calculate electric power. </li></ul><ul><li>When energy is measured in joules and time in seconds , power is measured in joules per second or watts (W) . </li></ul><ul><li>1 kilowatt = 1000 watts . </li></ul>
  86. 86. Electric Power <ul><li>Example problem </li></ul><ul><li>A computer CPU uses 240 000 joules of electric energy over a time of 1200 seconds. What power does the CPU consume? </li></ul>
  87. 87. Electric Power <ul><li>Example problem </li></ul><ul><li>A clothes dryer uses 4 500 000 joules of energy over a period of 15 minutes. What electric power does the dryer consume? </li></ul>
  88. 88. Electric Power <ul><li>Power can be calculated using a different formula than the one we just looked at. Sometimes we don't know values for energy and time, but we do know values for voltage and current. In household circits we usually know, or can easily determine voltage and current. The following notes show how a power formula using energy and time can be changed to a power formula using voltage and current. </li></ul>
  89. 89. Electric Power
  90. 90. Electric Power <ul><li>Example problem </li></ul><ul><li>A 10 ampere car heater runs on a 120 volt circuit. What power does the heater use in </li></ul><ul><ul><li>watts? </li></ul></ul><ul><ul><li>kilowatts? </li></ul></ul>
  91. 91. Electric Energy <ul><li>In order to operate an electrical appliance, electric charge must pass through the appliance. If one joule of energy is required to push one coulomb of charge through the appliance, one volt is needed. If 120 joules of energy are required to move one coulomb of charge through an appliance, 120 volts are needed. </li></ul><ul><li>The formula for electric energy is shown below. </li></ul>
  92. 92. Electric Energy <ul><li>Sometimes it is necessary to determine the total amount of energy used. This is true in our homes. We need to pay the electric company a fee based on our total consumption of electric energy over some period of time, for example over one month. In the case of home consumption of energy, we know the voltage and want to determine the energy used. The formula below can be rearranged to solve for joules of energy. </li></ul>
  93. 93. Electric Energy <ul><li>The formula below can be rearranged to solve for joules of energy. </li></ul>
  94. 94. Electric Energy <ul><li>Example problem </li></ul><ul><li>A 120 volt electric circuit in a house moved 10 coulombs of electric charge. How much energy is used by the circuit? </li></ul>
  95. 95. Purchasing Electrical Energy <ul><li>Electric energy is available for our use on demand. We simply plug an appliance into a wall receptacle and the energy flows into it. While this is an efficient method for transporting and using energy, we must pay for it! How does the electric supplier determine how much energy you have used and the cost for that energy? </li></ul><ul><li>You have most of the calculation tools at your disposal already, we will simply build on them. </li></ul>
  96. 96. Purchasing Electrical Energy <ul><li>The power formula can be modified to calculate electrical energy. </li></ul>
  97. 97. Purchasing Electric Energy <ul><li>Example problem </li></ul><ul><li>You know that your computer uses 250 watts of electrical power. You used it an average of 1.5 hours per day for all of September(30 days). How much electrical energy did your computer use in September? </li></ul>
  98. 98. Purchasing Electric Energy <ul><li>Example problem </li></ul><ul><li>What would the cost of operating the computer be for the month of September if the energy was 11 cents per kilowatt hour? </li></ul>Cost = Power(watts) x time(hours) x unit price 1000 Cost = 250 x 45 x 0.11 1000 Cost = $1.24
  99. 99. Purchasing Electric Energy <ul><li>Example problem </li></ul><ul><li>What would the cost of operating the computer be for the month of September if the energy was 11 cents per kilowatt hour? </li></ul>Cost = Power(watts) x time(hours) x unit price 1000 Cost = 250 x 45 x 0.11 1000 Cost = $1.24
  100. 100. Efficiency <ul><li>All electrical devices convert electrical energy into other forms of energy. An ordinary incandescent light bulb, for example, converts electrical energy to light energy. In the process of converting electrical energy to light energy; however, much of the electrical energy is also used in producing heat energy. In fact, only about 5 per cent of the electrical energy is actually changed to light energy in the light bulb. In this case the light bulb is 5 per cent efficient. </li></ul>
  101. 101. Efficiency <ul><li>How to calculate the efficiency of any electrical device </li></ul><ul><li>Efficiency is calculated by determining the electrical energy (or power) put into the appliance and dividing it into the converted energy (or power) from the appliance. If efficiency is to be expressed as a percentage, multiply the fraction by 100. </li></ul>
  102. 102. Efficiency <ul><li>Example </li></ul><ul><li>An electric car heater uses 1200 watts of power at 120 volts. The heater is able to produce 360 watts of heat power. What is the efficiency of the heater? </li></ul><ul><li>For every watt of electric power put into the heater, 0.3 watts of heat power are produced. </li></ul><ul><li>To determine the efficiency as a percentage, multiply by 100. </li></ul><ul><li>efficiency = 0.3 x 100% = 30% </li></ul>
  103. 103. Energuide Labels <ul><li>The electrical appliance industry uses a system called Energuide labels that give the consumer some idea of the efficiency of an appliance. The Energuide label shows the amount of energy used by the appliance in typical use over a period of a month. This means no calculations are needed, all you do is compare numbers on the label for the same type of appliance and the lowest number is the most efficient. </li></ul>
  104. 104. Personal Energy Consumption <ul><li>Reading a Hydro Meter </li></ul><ul><li>A hydro meter has five dials which give us a five digit number. The farthest right dial gives the ones digit, the next dial to the right gives us the tens digit, and so on. </li></ul>
  105. 105. Personal Energy Consumption <ul><li>The top set of dials gives a reading of 23 930 units at the end of the current month and the bottom meter is the reading of 20 769 units for the previous month. Notice that the lowest number on the side of the arrow is read. A different reading is taken every one or two months. The two readings below could have been taken on separate months. </li></ul>
  106. 106. Personal Energy Consumption <ul><li>When we purchase electric energy, the difference between the two numbers tells us the amount of energy used. The consumer or the meter reader records the new meter reading and the electric utility company subtracts the previous reading from the new one and determines the amount of electric energy used. The consumer in this example would have used </li></ul><ul><li>-23 930 units (kWh) </li></ul><ul><li>20 769units (kWh)   </li></ul><ul><li>3 161 units of electric energy (kWh) </li></ul><ul><li>Some Hydro meters only have four dials and the reading is multiplied by 10. Look at the meter on your home to see which type you have. </li></ul>
  107. 107. Wattage & Amperage Video
  108. 108. Interpreting a Hydro Bill <ul><li>Every household receives a hydro bill based on energy consumption. Part of being a knowledgeable citizen involves interpreting your hydro bill so you know your resource consumption. Interpreting a hydro bill can also help determine the cause of increased consumption and perhaps even contribute to better resource use by helping reduce energy consumption. </li></ul>
  109. 109. Interpreting a Hydro Bill