* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Exploring the Science of Electricity
Photovoltaics wikipedia , lookup
Eddy current wikipedia , lookup
Alternating current wikipedia , lookup
Hall effect wikipedia , lookup
Electrostatics wikipedia , lookup
Electromigration wikipedia , lookup
Electric charge wikipedia , lookup
Electrical resistance and conductance wikipedia , lookup
Photoelectric effect wikipedia , lookup
Electromagnetism wikipedia , lookup
Electrification wikipedia , lookup
Insulator (electricity) wikipedia , lookup
Static electricity wikipedia , lookup
Electromotive force wikipedia , lookup
Magnetochemistry wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Electrochemistry wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Electricity wikipedia , lookup
Electric current wikipedia , lookup
Lesson Exploring the Science of Electricity Interest Approach What is Electricity? Where does electricity come from? How does it get from its source to where we use it? Interest Approach What happens when you walk across carpet in the winter and receive a “shock” when you touch someone or something metal? Interest Approach Static electricity is actually a transfer of electrons from you to the object you touch Student Learning Objectives Relate electricity to the structure of elements and atoms. Explain conductors, insulators, and semiconductors. Explain the conventional and electron theories of electrical current flow. Student Learning Objectives Describe how electricity can be generated by friction, heat, light, chemical reactions, and magnetism. Describe the difference between and applications of direct current (DC) and alternating current (AC) electricity. Terms o Alternating current o Atoms o Battery o Conductors o Conventional oElectrolyte oElectromagnet theory o Cycle o Direct current o Electricity o Electrodes oFree electrons oFriction oElectron theory oElectrons oElements oFuel cell oHertz oInsulators Terms o Magnetic induction o Neutrons o Photovoltaic effect o Polarity o Primary batteries o Protons o Secondary batteries o Semiconductors oSolar (photo) cell oSynchronous alternator oThermocouple oThermopile oValence Objective 1: How does electricity relate to the structure to of elements and atoms? Elements Elements are substances that cannot be broken down into simpler substances using ordinary chemical methods. All matter is composed of one or more elements. Atoms Atoms are the smallest units of an element. They cannot be divided further without losing physical and chemical properties. Atoms are composed of protons, neutrons, and electrons. Protons and Neutrons Protons have a positive electrical charge, while neutrons have no electrical charge. Protons and Neutrons Protons and neutrons make up the nucleus, or center of each atom. Thus the nucleus of each atom has a positive electrical charge. Electrons Electrons have a negative electrical charge and orbit the nucleus of an atom in rings or shells. Since unlike charges attract each other, the electrons are held in orbit by the positively charged nucleus. Electrons The outer ring of electrons is called the valence ring. Copper As A Conductor The element copper, has 29 protons in its nucleus and 29 electrons which orbit its nucleus. The electrical charges cancel each other. Artwork supplied with permission of Interstate Publishers, Inc. Copper as a Conductor The electrons actually orbit the nucleus in rings. –The first or inner ring can hold a maximum of 2 electrons, the second ring, a maximum of 8 electrons, the third ring, a maximum of 18, the fourth ring, a maximum of 32, the fifth ring, a maximum of 50, and the sixth, a maximum of 72. Copper as a Conductor Generally, the rings closest to the nucleus are filled before additional rings are started. Thus, copper has 1 electron in the fourth ring or valence ring (2 + 8 + 18 + 1). Copper as a Conductor The number of electrons in the valence ring has a key role in determining the electrical characteristics of the element. Free Electrons moving between copper atoms Artwork supplied with permission of Interstate Publishers, Inc. Objective 2: What are conductors, insulators, and semiconductors? Conductors, Insulators, and Semiconductors Conductors, insulators, and semiconductors are important in electricity and electronics. The following will help explain each: Conductors Materials that allow electricity to flow through them easily. Copper, aluminum, silver, and gold are good conductors. Conductors Generally, nearly all metals are good electrical conductors. Any element or material having atoms with three or fewer electrons in its valence ring will be a conductor. Insulators Materials that do not allow electricity to flow through them under normal conditions. Materials such as rubber, plastic, porcelain, and glass are all good electrical insulators. Insulators Any element or material composed of atoms having five or more electrons in the valence ring will be an insulator. Insulators are used to confine the flow of electricity to desired paths. Semiconductors Materials that are neither good conductors nor good insulators. Semiconductors are manufactured from elements having atoms with four electrons in their valence rings. Silicon and germanium are widely used in making semi-conducting materials. Objective 3: What are the conventional and electron theories of electrical current flow? Electricity Electricity is the flow of electrons from atom to atom in a conductor. There are two different theories to help describe electricity: Conventional Theory Says that electricity is the flow of positively charged particles through a conductor. This is the older theory and was developed before scientists discovered the existence of electrons. With this theory, it was assumed that current flow in an electrical circuit was from positive to negative. The direction of current flow according to the conventional theory. Artwork supplied with permission of Interstate Publishers, Inc. Electron Theory This is the accepted model of electrical current flow, however, because of tradition, the conventional theory is still widely used. Either theory may be used as long as it is used consistently. Electron Theory The electron theory says that electricity is the flow of electrons through a conductor. Remember conductors have three or fewer electrons in their valence ring. Electron Theory When there are so few electrons in the valence ring they are not held very tightly. These free electrons can be dislodged if sufficient external force is applied. Electron Theory An example of external force is a battery, which has a positive terminal and a negative terminal. The direction of current flow according to the electron theory. Artwork supplied with permission of Interstate Publishers, Inc. Electron Theory Since unlike charges attract each other, if a copper wire were attached to the two terminals, the negatively charged free electrons would be dislodged and pushed by the negative terminal and pulled by the positive terminal of the battery, causing the electrons to flow. This would continue until the battery discharges. Objective 4: How can electricity be generated by friction, heat, light, chemical reactions, and magnetism? How can electricity be generated by friction, heat, light, chemical reactions, and magnetism? An external force must be applied to cause free electrons to flow through a conductor. This force is the production or generation of electricity, which can be generated in the following ways: Friction Friction is caused when two or more materials rub against each other. Friction When this happens, some of the free electrons from one material are transferred to the other material, causing one of the materials to have a positive electrical charge and the other to have a negative electrical charge. Friction When the negatively charged material touches a material with a neutral or positive charge, the excess electrons will flow to the second object. Friction This can be demonstrated when you walk across carpet in the wintertime. As you walk across the carpet, the soles of your shoes rub over the carpet which transfers electrons from the carpet to your body. Friction When you touch someone else or a doorknob, the excess electrons discharge through your hands resulting in a static electricity shock. Friction is not a practical method of generating electricity. Thermocouple Heat or temperature differences can be used to generate electricity using a thermocouple. A thermocouple consists of two dissimilar metals, such as iron and nickel, joined together to form two junctions. Thermocouple When heat is applied to one of the junctions, the difference in temperature between the junctions causes free electrons to flow from the iron wire into the nickel wire and away from the hot junction toward the cold junction. Thermocouple To increase output, several thermocouples are often combined to form a device called a thermopile. These are often used in flame detectors, furnace safety valves, and in precision heat measurement devices. Photovoltaic Effect Light from the sun can also be used to generate electricity. It requires a solar (or photo) cell for converting the light into electricity through a process known as the photovoltaic effect. Photovoltaic Effect Certain materials (such as gallium, silicon, and cadmium sulfide) will convert light energy into electrical energy through the photovoltaic effect. The solar cell is made of a thin disk of silicon to which other chemicals have been added. Photovoltaic Effect When light strikes the disk, electrons move from one side to the other side. The electrons move through the conductors and provide electrical energy to power the circuit load(s). –Solar energy is used to power to such things as calculators and electric fence controllers. Chemical Reactions between certain chemicals can be used to produce electricity. Chemical First, a battery is made of two or more chemical cells connected together. Each cell is composed of two dissimilar metal plates called electrodes. They are separated from each other and immersed in an electrolyte. Chemical An electrolyte is a chemical solution that contains positively and negatively charged atoms called ions. Chemical Using a chemical cell that is composed of a zinc plate, a copper plate, and an electrolyte solution of sulfuric acid and water as an example, the acid/water solution reacts with the zinc plate causing it to lose positively charged ions. Chemical Each positive ion lost leaves behind two electrons. Thus, the zinc plate soon takes on a negative charge. It then becomes the negative terminal of the cell. Chemical As the positive ions move through the electrolyte, they collide with the copper plate. The positive ions attract free electrons from the copper plate. Chemical Thus, the copper plate soon takes on a positive charge due to this loss of electrons and becomes the positive terminal of the cell. Chemical If we connect a conductor to the two terminals, electrons will flow through the conductor from the zinc plate to the copper plate. Chemical This flow will continue until the difference in charge between the two plates has dissipated. Once this happens, the cell will be discharged. Chemical Batteries may be classified as primary batteries (carbon-zinc, alkaline, and mercury) which cannot be recharged or as secondary batteries (lead-acid and nickel-cadmium) which can be recharged. Chemical A second chemical reaction to produce electricity is a fuel cell, which is similar to a battery, but different in the way the chemicals are supplied. Chemical In a battery, the chemicals are built in. In a fuel cell, the chemicals are pumped into the cell from an external source. Currently, fuel cells are limited to military and space applications. Magnetism The final method of generating electricity discussed here will be that of magnetism. Magnetism If a bar magnet is suspended freely from a string, the magnet will turn until one end points north and the other end points south. The end pointing north is said to be the north pole of the magnet and the end pointing south the south pole. Magnetism An electromagnet may also be created using electricity. If electricity flows through an insulated conductor that is wound around a metal object, the metal object will become a magnet. Magnetism Any time a conductor cuts across a magnetic field or a magnetic field cuts across a conductor, electricity will flow in the conductor. Electricity generated in this manner is produced through the process of magnetic induction. Generation of Electricity Through Magnetism Magnetism This would be a very impractical method of generating electricity. Electricity could also be generated by rotating a magnetic field around a stationary conductor or by rotating a conductor inside stationary magnetic field. Magnetism As the rotating conductor cuts across the lines of magnetism in the stationary magnetic field, electrons will be forced to flow through the conductor. Magnetism As the conductor continues to rotate, the conductor will travel parallel to the lines of magnetism and no electrons will flow. Magnetism As the conductor continues in its rotation, it will cut across the lines of magnetism in the opposite direction, causing the electrons in the conductor to flow in the opposite direction. The cycle continues as the rotation continues. Magnetism In a commercial electricity generation power plant, a synchronous alternator is used to produce electricity. This device has a rotating field winding and a stationary winding. Magnetism A small amount of electricity is supplied to the alternator’s field windings, which produces a magnetic field around the windings (the field windings become an electromagnet). Magnetism As the field windings are turned (by a turbine driven by an external power source), the magnetic field also turns, cutting across the alternator’s stationary windings. Magnetism Since the conductors are cutting across a magnetic field, electricity is induced into the stationary conductors. Magnetism Power plants may burn coal or use nuclear energy to make steam from heated water in order to turn the turbine, or they may use the kinetic energy of falling water to turn the turbine. Objective 5: What is the difference between direct current and alternating current and what are some applications of each? AC/DC Electricity may be classified as direct current or alternating current depending on the pattern of flow of the electrons in the circuit. Direct Current In direct current or DC electricity, the electrons flow in only one direction. Direct Current Since electrons flow from the negative terminal to the positive terminal in an electrical circuit, sources of DC electricity must have a fixed polarity. Direct Current This means one specific terminal is always negative while the other is always positive. Thermocouples and thermopiles, solar cells, batteries and fuel cells all produce DC electricity. Direct Current A DC generator may also be used to produce DC electricity through magnetism. Alternating Current With alternating current or AC, electricity flows first in one direction, stops, reverses and flows in the opposite direction. Alternating Current Once this occurs, the electricity is said to have completed one cycle. –AC electricity is the type generated by electric power plants and is what is used in homes, businesses, and other locations. Alternating Current One cycle of electrical flow is produced with each revolution of the plant’s synchronous alternator. –In the U.S., they turn at a speed of 60 revolutions per second. –Therefore, current generated in the U.S. completes 60 cycles per second. Alternating Current The term hertz (Hz) represents one cycle per second, so in the U.S. our electricity is generated and delivered at 60 Hz. In several other countries, 50 Hz AC electricity is the standard. Review / Summary 1. Relate electricity to the structure of elements and atoms. Review / Summary 2. Explain conductors, insulators, and semiconductors. Review / Summary 3. Explain the conventional and electron theories of electrical current flow. Review / Summary 4. Describe how electricity can be generated by friction, heat, light, chemical reactions, and magnetism. Review / Summary 5. Describe the difference between and applications of direct current (DC) and alternating current (AC) electricity.