LHWHS Physics Unit 6 - Electricity NOTES

Transcription

1 LHWHS Physics Unit 6 - Electricity NOTES NAME I. History Lightening - people have observed (negative) charges moving from the bottom of clouds in an attempt to 'balance' the net charge of the cloud - flash of lightening is accompanied by loud sound wave ~1730's - Ben Franklin - accurately described the phenomena of lightening in terms of discharging static electricity Static Electricity - 'non-moving' charges that build up and are stored on the surface of materials ~ Leyden jar- developed at University of Leyden in Netherlands, early type of capacitor ~1760's James Watt - Scottish engineer who made important improvements to steam engines, unit of power named after him watts - joules / second ~1780's Charles Coulomb - measured how electric charges behave by observing the attracting and repelling forces the charges exerted Coulomb's law - Newton's law - F e = k q 1 q 2 / r 2 F g = G m 1 m 2 / r 2 k = coulomb's constant G = newton's constant q = amount of charge m = mass r = distance between mass r = distance between charges (mass and distance determine force due to gravity) (charge and distance determine force due to electricity) coulombs (C) - standard unit of charge electrons have the charge of 1 coulomb

2 ~1799 Alessandro Volta - designed the 'voltaic pile' which was the first practical battery using metals and salt solution volts (V) - unit for electric potential difference joules / coulomb ~1820's Andre'-Marie Ampere - demostrated the relationship between moving charges (current) and the strength of the magnetic field created amps (A)- unit of current coulombs / sec ~1826 Georg Ohm - developed the mathematical model for the relationship between current, voltage, and resistance ohms (Ω) - unit for resistance ~1850 James Joule - studied how energy can be transferred between different systems......heat, mechanical, and electrical systems II. Electrostatics joules (J) - unit of energy A. Definition - study of electric charges, forces, and fields static electricity-electricity at rest, 'non-moving' electrons can 'sit' on a surface B. Materials- glass, rubber, clothing C. Electrostatics of metals electrons spread out on metals, metal atoms have 'loose' hold on electrons 1. explains conductivity 2. explains shiny appearance and malleability 3. explains ability to reflect electromagnetic radiation (light)

3 D. Like charges - repel E. Opposite charges - attract F. Insulator - high resistance, does not allow charge to move easily (rubber, plastic, etc...) G. Conductor - low resistance, allow charge to move easily (metal) III. Electric Field...similiar to _Gravitational field 1. introduce electrons into an electric field - the electrons begin to move (accelerate) 2. introduce a ball into a gravitational field - the ball begins to move (accelerate) 3. Potential energy - energy based on position in the field electric = determined by the distance between charged objects (Coulomb's law) gravity = determined by the distance (height) from Earth (Newton's law) 4. When accelerating ===> FORCE ===> WORK (ENERGY) ==> POWER IV. Electric Potential -represents a difference in potential energy of each charge in the electric field (Joules / Coulomb = Voltage) A. Volt - defined as electric potential difference,

4 -a measure of the work done to move a charge from one location to another, - electromotive force (EMF) - measure electric field strength, - named for Alessandro Volta JOULES / COULOMB V. Capacitance - measure of the amount of charge stored on conductors for a given potential difference (voltage) Capacitors - device that stores charge on conductors separated by an insulator, ex: Leyden jar, clouds in lightening storm, nerve membranes, flash chargers in camara, on virtually all circuit boards in electronic devices A. Farad (F)- unit for capacitance, named for Michael Faraday - typically use µf ( 10-6 F) COULOMB / VOLT VI. Circuits - closed loop or pathway that allows electric charge to flow A. Current- amount of charge that flows through an area in a given amount of time 1. How to produce a current...how to move electricity Direct Current (DC) a. static charge will move to metal b. BATTERY - chemical reaction between metal and salt solution (acid) c. using a magnet and coil of wire - produces an alternating current (AC) 2. amps (A) - unit for current, named for Ampere COULOMBS / SECOND B. Resistance- an opposition to the flow of charge

5 - property that determines how much current will flow - equal to voltage divided by current - resistance of a wire depends on material used, length and cross-sectional area of the wire small diameter==> high resistance large diameter===> low resistance temperature...hotter wire==> more resistance 1. ohms (Ω) - unit for resistance, named for Georg Ohm C. Ohm's law - V = I R V = volts I = current R = resistance D. Power - rate that energy is transferred 1. Watt - joules / sec mechanical systems electrical systems P = Force distance P = Current Voltage time = Newton meters sec = amps volts = coul. / sec joules/coul. = joules / sec = joules / sec = watts = watts E. Series circuits

6 1. example 2. resistance in series 3. voltage in series 4. current in series F. Parallel circuits 1. example

7 2. resistance in parallel 3. voltage in parallel 4. current in parallel G. Direct current vs. Alternating current (DC vs AC) electrons move in the same direction electrons change direction (back and forth) 1. Electromagnetism Moving electricity generates a magnetic field Moving a magnet causes electricity to move (induces a current)

8 2. Moving Charges History (1700's - early 1800's) DIRECT CURRENT Lightening - static charges are discharged or moved toward metal (and ultimately the ground) Chemical Reactions - metals and salt solutions (acid) can move electrons from one metal to another Coil of Wire with Magnet (~1820's) produces an alternating current (AC) VII. Power usage- most outlets are 120 volts in home, amps vary with appliance 1. power bill - electric meter@ home measures power usage kw hrs = total kilowatts total hours (1000 watts = 1 kilowatt) 2. high wattage appliances - typically 'lose' a lot of energy to heat (ex: toaster, microwave, refridgerator, space heater, some types of lights) voltage - relatively high current - relatively high resistance - relatively low 3. efficiency - ratio of power in / power out