Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
III. Voltage Dr. Bill Pezzaglia Updated 2012Aug06 2 A. Voltage 1) Electrostatic Energy 2) Electric Potential (Voltage) 3) Capacitance 3 1. Electrostatic Potential Energy a) Review Gravitational Potential Energy b) Electrostatic Potential a) Review: Gravitational Potential Energy 4 • Near surface of earth, where gravitational field is constant g=9.8 m/s2, then the change of potential energy of lifting a mass “m” up a distance “h” is just: U=mgh • For large distances, gravity follows the inverse square law. A body “m” falling from infinity to the surface of the earth (mass “M”) will have a change of potential energy of: Mm U G R • This would be the amount of energy that a meteor would have hitting the earth and making a big crater! b) Electrostatic Potential Energy • Electric fields also follow the inverse square law. Hence a small test charge “q” pushed from infinity onto a massive ball of charge “Q” of radius “R” will have a change of potential energy of: qQ U k R Note: energy of two positive (or two negative) charges is positive Energy of positive and negative charge is negative 5 6 2. Voltage a) Definition of Voltage b) Sources of Voltage c) Measuring Voltage 2a. Definition of voltage • Potential Energy per unit test charge: (i.e. don’t want test charge to affect field) 7 U V Lim q 0 q • Units: Volt=Joule/Coulomb • Voltage is the “pressure” that makes charges move (current flow). • Even if there is no test charge to experience it, voltage exists 2ai. Cathode Ray Tube • A CRT (Cathode Ray Tube) is a vacuum tube with a large voltage across the electrodes. Electrons are emitted by the Cathode and accelerate towards the anode. • Kinetic energy the electrons gain is hence: U=e V • 1 eV = 1 electron volt is the energy of one electron accelerated through one volt = 1.6x10-19 Joules. http://www.youtube.com/v/XU8nMKkzbT8?f=videos&app=youtube_gdata&autoplay=1 8 2aii. Particle Accelerators SLAC (Stanford Linear Accelerator Center) accelerates electrons to 50 GeV of energy Note: the E=mc2 rest-mass energy of a proton is only 938 MeV 9 10 2b. Sources of Voltage (a) Point Charge Source (b) Superposition of Point Charges (c) Batteries (d) Thermo and Piezoelectrics 2bi. Charge as Source of Voltage • Define the voltage at infinity to be zero • Voltage a distance “r” from the center of a spherical charge Q is: Q V (r ) k r 11 2bii. Batteries are a source of voltage • Volta (1745-1827) “The Newton of electricity” •1800 develops first battery (approximately 30 volts) •By adding batteries together in series, one can make as big as voltage as you want. http://www.corrosion-doctors.org/Biographies/VoltaBio.htm 12 2biii. Piezoelectrics etc Some devices that are useful as detectors • Thermoelectrics: some materials will create a voltage across them due to a temperature difference • Pyroelectrics: heating some materials will create a voltage across them • Piezoelectrics: 1880 Pierre Curie demonstrates effect that some crystals generate a voltage when deformed 13 14 2c. Measuring Voltage Put a “voltmeter” in parallel with element you wish to measure potential difference 2cii. Oscilloscope • Oscilloscopes are used to measure voltage (especially of AC signals). They are essentially a CRT tube with deflection plates. • The amount of deflection of the beam is proportional to the voltage across the plates. 15