Transcript Ohm`s Law
2 EC Polikar Lecture 3 Ohm: The Man, The Law, The Field Ohm: The Man Ohm, Georg Simon (1787-1854), German physicist, best known for his research on electrical currents. He was born in Erlangen and educated at the University of Erlangen. From 1833 to 1849 he was director of the Polytechnic Institute of Nürnberg, and from 1852 until his death he was professor of experimental physics at the University of Munich. His formulation of the relationship between current, electromotive force, and resistance, known as Ohm's law, is the basic law of current flow. The unit of electrical resistance was named the ohm in his honor. Microsoft® Encarta® Reference Library 2002. © 1993-2001 Microsoft Corporation. All rights reserved. Ohm: The Man Erlangen: Gottstadt: Cologne: Berlin: 1787 1825 1817 1806 - 1805 Nurenberg: Nauchatel: Erlangen: Munich: 1849 1811 1809 1833 Nurenberg Ohm: The LawResistance A conductor allows an electric current to flow through it, but it does not permit the current to flow with perfect freedom. Collisions between the electrons and the atoms of the conductor interfere with the flow of electrons. This phenomenon is known as resistance. Resistance “resists” to flow of electricity. W=2r L L R 2 A r How about the effect of temperature? Resistivity L The term resistance is also used when the flow of a fluid or heat is impeded. The forces of friction provide the resistance to the flow of a fluid in a pipe, and insulation provides thermal resistance that reduces the flow of heat from a higher to a lower temperature. ? ? ? Ohm: The LawVoltage and Current Voltage is the electrical force, or "pressure", that causes current to flow in a circuit. It is measured in VOLTS. Current is the movement of electrical charge - the flow of electrons through the electronic circuit. Current is measured in AMPERES. Similar concepts also appear in other disciplines: For example, in hydraulics: Pictures are from: http://webhome.idirect.com/~jadams/electronics/v_c_r.htm Ohm: The law The amount of current passing through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. V I R V IR _ I V + R V: The voltage difference between two locations, aka potential difference. It is analogous to pressure. I: The amount of current flowing between these two points. It is analogous to flow. R: Resistance between the two points. So how did Ohm come with this law…? Many resistances…? Electric circuits with only a single path for electricity to flow along are called series circuits. Electric circuits with multiple pathways for electricity to flow along are called parallel circuits. A circuit’s total resistance to the flow of electricity is calculated differently for series circuits than it is for parallel circuits. Constant current I Constant voltage How about connecting power sources ? (that is power supplies, voltages…?) Resistor Color Code http://www.elexp.com/t_resist.htm Measuring Voltage & Current A device that measures the potential between two points is a voltmeter, whereas a device that measures current along a path is an ammeter. Therefore, a voltmeter is always connected in PARALLEL between the two points of interest, whereas an ammeter is connected in series along the path of the current. Note that no measurement device is perfect and causes some error on measurement. Hence, both ammeter and voltmeter have internal resistances. If you were the manufacturer, what would you make the internal resistances of these devices…? Serves as the resistance aka, “the load” Ohm’s Law In recognition of his work, the unit of resistance is called Ohm, and denoted by . Many heroes in the study of electricity and magnetism emerged between the late 1700s and the early 1800s, many of whom left their names on various electrical units. Charles Augustin de Coulomb (the unit of charge), André Ampère (current), Georg Ohm (resistance), James Watt (electrical power), James Joule (energy), Luigi Galvani gave us the galvanometer, (early form of ammeter) Alessandro Volta (volt), a unit of potential, or electromotive force. C. F. Gauss, Hans Christian Oersted, and W. E. Weber, Faraday, Maxwell all made their mark and left their names on electrical engineering. Only one, despite his outstanding contributions, was unable to get his name on any electrical unit or device: ??? MS Encarta Reference Suite 2002. Power A conductor’s resistance to electric current produces heat. The greater the current passing through the conductor, the greater the heat. Also, the greater the resistance, the greater the heat. A current of I amps passing through a resistance of R ohms for t seconds generates an amount of heat equal to I 2 Rt joules (a joule is a unit of energy equal to 0.239 calorie). E 1 iv T Energy is required to drive an electric current through a resistance. This energy is supplied by the source of the current, such as a battery or an electric generator. The rate at which energy is supplied to a device is called power, that is power is energy supplied per unit time. Power is often measured in units called watts. The power P supplied by a current of I amp passing through a resistance of R ohms is given by V2 PI R VI R 2