- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAL MECHANICS (CLF2650) Unit Title: ELECTRICITY ___________________________________________________________________________ (CLF2651) Topic: Principles of Electricity Time Year(s) 2 hours 1 / 2 / 3 / 4 ___________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the students will be able to: Learning Outcome #: (N-4) - Define ampere, watt, volt, and ohm. (N-8) - Understand the difference between electrical flow of 240 volts and 120 volts in wiring. Special Material and Equipment: VOM meter References: Cooper, E. L. (1987). AGRICULTURAL MECHANICS: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Erpelding, L. H. (1971). AGRIBUSINESS ELECTRICAL LESSON PLANS. Danville, IL: Interstate Printers & Publishers. Gustafson, R. J. (1980). FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE. Westport, CT: AVI Publishing Company. Singer, B. B. (1972). BASIC MATHEMATICS FOR ELECTRICITY AND ELECTRONICS. New York: McGraw-Hill. Evaluation: Quiz by instructor. TOPIC PRESENTATION: Principles of Electricity A. Basic Theory of Electricity 1. What is electricity? a. Electricity is a convenient form of energy that can produce heat, light, magnetism, and chemical changes. b. It can be generated, transmitted, and controlled. 2. Where does electricity come from? a. Electricity is present in all matter. 1) All matter is made of combinations of elements called molecules, which are in turn made up of even smaller units called atoms. 2) An atom is the smallest amount of an element that retains all the properties of the element. 3) An atom may be broken down into smaller pieces whose relationships have been conceived of as a miniature solar system. a) The center of the atom consists of protons, which carry a positive electrical charge, and neutrons, which carry no charge. b) Electrons, which carry a negative electrical charge, orbit around the center of the atom. c) Two kinds of electrons exist: - Planetary electrons cannot be readily moved from their orbits. - Free or valence electrons are loosely held in the atom's outer orbit and may drift into orbits of nearby atoms. d) When an atom has an equal number of protons and electrons, it is said to be electrically neutral. 3. How is electric current produced? a. An atom becomes electrically charged when it has fewer electrons than protons. b. The random wandering of valence electrons from one atom to another does not produce any permanent changes. c. The overall material will remain the same if no outside influence disturbs the balance. d. If an outside force, such as a battery voltage, disturbs the balance, the loosely-held outer electrons will tend to move in one direction. 1) When voltage is applied across the ends of a conductor, the electrons, which up to then had been moving in different directions, are forced to move in the same direction along the wire. 2) The individual electrons all along the path are forced to leave their atom and travel a short distance to another atom that needs an electron. 3) This motion of electrons is transmitted along the path from atom to atom, as the motion of a whip is transmitted from one end to the other. e. This nonrandom flow of electrons is called an electric current. f. When the free electron move randomly, their energy is small, but when they are forced to move in the same direction, their collective energy is large and can be used for work. B. Measuring Electric Current 1. Electron Flow (Amperage) a. An electric current is a flow of electrons along a conductor. b. The speed of this flow is nearly equal to the speed of light, 186,000 miles per second. c. The flow of electricity is measured by the number of electrons that pass a point in a wire in one second. d. An ampere is a measure of electron flow; it represents a flow of 1 coulomb of electricity (6 billion billion electrons) past a point in a wire in one second. e. Compared to a water system, an ampere would be similar to a measure of water flow through a pipe, such as gallons per minute. 2. Electromotive Force (Voltage) a. Electromotive (electron-moving) force or voltage is electrical potential which provides energy for the movement of electrons in a circuit. b. This electrical potential results from a difference in electron energies at two points in a circuit. c. This difference in electron energy levels in a circuit can be compared to the potential energy of water stored in a high water tower and the kinetic energy of water flowing through the pipe. d. Voltage is measured in units called volts (a difference variable) and is abbreviated with the letter symbols E or V. e. Compared to a water system, a volt would be similar to a measure of water pressure in a pipe, such as pounds per square inch. 3. Resistance to Current Flow (Resistance) a. Resistance is the ability of a material to resist electron flow. b. Materials vary in their number of valence electrons and in the ease with which electrons may be transferred between atoms. c. A conductor is a material through which electrons can flow freely. d. An insulator is a material that provides great resistance to electron flow. e. Resistance is measured in units called ohms. f. The symbol for resistance is R, and the symbol for ohms is the Greek letter omega. 4. Energy a. Electrical energy is the amount of work that can be done by voltage and current over a specific period of time. b. The unit for measuring electrical energy is the watt-hour (more commonly specified as kilowatt hours which is 1000 watt hours); it is usually designated by the letters kWh. c. The mathematical relation between voltage, amperes, resistance, and electrical energy is: 1) kWh = P X T Where, P = Power (watts) T = time (hours) 5. Power a. Electrical power is the amount of work that can be done by voltage and current. b. The unit for measuring electrical power is the watt; it is usually designated by the letter W. c. A watt of power is equal to one volt pushing one ampere of current through a conductor with one ohm of resistance. d. The mathematical relation between power and voltage, resistance and amperes is: 1) P = I X V Where, P = power (watts) I = current (amperes) V = electrical potential (volts) e. Example of Usage 1) Most household/farm shop appliances and equipment are rated in watts (see the nameplate or manufacturer's specifications). Knowing the rating in watts and the voltage to be used (normally 120 or 240), the flow of current in the circuit for the appliance/equipment can be calculated. 2) You have purchased a 120 volt plug-in electric space heater for use in your shop. It is rated at 400 watts. How much current will the heater draw? P = I X V Or, I = P divided by V Where, P = 400 watts V = 120 volts Therefore, I = 400 divided by 120 = 3.33 amperes f. The mathematical relation between power and voltage, resistance and amperes can be rewritten as: 1) P = I X V And, V = I X R Therefore, 2 P = I X I X R = I X R C. Ohm's Law 1. The physicist, George Simon Ohm, discovered that the flow of electrical current through a conductor is directly proportional to the electromotive force that produces it and inversely proportional to the resistance in the conductor. a. If the resistance to electron flow through an electrical device is cut in half, the current amperage doubles. b. If the resistance remains constant, but the voltage is doubled, the current amperage doubles. 3. This relationship is expressed in Ohm's law as E = IR. a. I equals current in amperes. b. E equals potential energy in volts. c. R equals resistance in ohms. d. Types of Electricity 1) Direct Current (DC) a. Electrons flow constantly in one direction. b. This is the type of electricity produced by all batteries. 2) Alternating Current (AC) a. Electrons flow first in one direction and then in the reverse direction at a certain rate of reversal (cycles per second). - In the U.S., 60 cycles per second (60 Hertz) is the standard. b. AC current has many advantages over DC, i.e., transformers to increase or decrease voltage can be used only with AC current. 3) Single-Phase Current a. This is the typical current supplied to households and businesses where power requirements are not very high. b. Single phase current can be provided by two wires. 4) Three-phase Current a. This type of current is designed especially for large electrical loads. b. It requires at least three wires. c. Three-phase current is actually three single-phase currents combined so that peak voltages are equally spaced. e. Sources of Electricity 1) Friction a. An electrical charge is produced when certain materials are rubbed together. - For example, walking across a carpeted floor or sliding across an automobile seat cover sometimes results in static electricity (buildup of electrical charge without current flow). b. Static electricity has little practical value; in fact, it tends to be a nuisance or a hazard. 2) Heat a. If two dissimilar metals (for example, copper and constantan), are connected, and heat is applied at the junction, electrons will pass from one metal to the other. b. This is called the thermoelectric process. c. The thermoelectric process is used in furnaces to sense the presence of heat to hold open the fuel supply. When the furnace goes out, lack of heat causes the current flow to stop which in turn shuts off the fuel supply. 3. Light a. Some dissimilar materials have the property of producing electrical voltage when the boundary between them is subjected to light (radiant energy). b. These materials are said to be photovoltaic. Examples are cuprous oxide and copper or an electrode and an electrolyte. c. Photovoltaic materials are used in remote areas (communications satellites, etc.) where it would be impractical to run in lines or to provide batteries. 4. Pressure a. Some materials produce electricity when pressure is applied that changes their shape. b. These materials are said to be piezoelectric. Examples are quartz and Rochelle salt. c. Phonographs using a crystal cartridge utilize the piezoelectric principle to convert the movement of the needle to an electric signal which is then amplified and played through the speakers. 5. Chemical Action a. Primary Cells 1) The combination of certain metals in an electrolyte solution will produce electricity, for example, copper and zinc in sulfuric acid. 2) Examples of batteries that produce electricity from primary cells are dry cell (paste-like electrolyte, carbon and zinc electrodes) and mercury batteries. 3) The zinc is used up in the process and when this happens the batteries go dead. b. Storage Batteries 1) These batteries are similar to primary cells except that the process can be reversed and the battery can be recharged. 2) Examples of storage batteries are the lead-sulfuric acid batteries used in automobiles, tractors, etc. and the nickel-cadmium rechargeable batteries used in flashlights, radios, etc. c. Fuel Cells 1) A container in which fuels react in the presence of an electrolyte and electrons are made available at the negative electrode terminal. 2) Oxygen and hydrogen are used as fuels in space vehicles to produce electricity. 6. Magnetic Action a. A flow of electrons is produced in a coil of wire which is moving within a magnetic field. 1) The magnetic field can be provided by a stationary magnet. 2) Movement of the wire can be provided by: a) Falling water turning a turbine shaft b) Atomic power producing steam which turns a turbine shaft c) An internal combustion engine turning a shaft. d) This is the most common method of producing electrical energy in large quantities to serve the home, farm, and business. _________________________________________________________ ACTIVITY: 1. Measure amperage, voltage, and resistance with a volt-ohm-milliampere meter (VOM). 2. Read a kilowatt-hour meter. 3. Work out problems using the formulas of electricity. _________________________________________________________ 6/26/91 OLR/tf #%&C