Chapter 6 - Electricity (& Magnetism) Electricity - deals with interactions between electric charges * causes forces motion * two types of charges: + positive proton - negative electron Ancient Greeks - rub amber and it attracts small objects electron - from Greek for amber Law of Electric charges - basic law of interaction opposites attract, likes repel Where do charges come from? Atomic Theory - smallest particles of nature Neutral atom + - + + ++ ++ - - nucleus - made of protons - fixed positions electrons - tiny negatives - - move quickly around nucleus - some move between atoms
remove electrons - add electrons transfer charges between objects Charges are transferred between objects ions - charged atoms atom acquires extra electron - negative ion loses an electron (to another atom) - positive ion Rub balloon on your hairelectrons transferred to balloon (friction) balloon acquires negative charge - --- ++ -++ +- no + -++ forces -- force from balloon charge attracts +, repels attracted to balloon Induced charge - uses law of electric charges to separate charge LAW OF CHARGE CONSERVATION - when one body acquires a charge from another, the second acquires an equal and opposite charge from the first -net charge in universe constant -charge neither created nor destroyed charges dont just appear out of nowhere! Electrical properties of materials Two general behaviors of matter regarding electricity: how they act in the presence of charge
Conductors - transmit charge readily + fixed nucleus + + e- + + + + + + strongly held e- conductor loosely held eAlso conduct heat well move from atom to atom path for e- to travel from motion of eExample: wires - transport charge for use in circuits Insulators - charge cannot freely move + + + +
+ + + + no loose eget stuck on surface poor heat conductors Some materials have both properties atmospheric air nitrogen Oxygen Carbon dioxide water (humidity) Polar - act like separated charge + GOOD GOOD INSULATOR CONDUCTOR damp day - charges leak off water molecules form chains to drain e- to ground SEMICONDUCTORS - properties of both normally insulators add energy loosely held states energy from light, heat, electrical used as switches - add energy for charge to flow
Electrostatics - charge is confined to an object - charge assumed not moving - static electricity - accumulated charge at rest like charge on balloon or charge on your body from walking Electroscope - early device used to measure charge add charge here metal leaves (gold) spread apart when charged -likes repel -more charge, spread more Methods to charge objects: conduction and induction (and friction) CONDUCTION -- neutral electroscope -- touch two charged objects together to transfer charge -- -- - -
spark charge transferred - - -- charge shared leaves move apart charge becomes evenly distributed Charge by INDUCTION two objects never actually touch charge by using electric forces (induced charge) NO DIRECT CONTACT -- -- -- -- bring charged rod closepushes e- away leaves separate + +++ -- -- e- try to get as far away as possible
neutral electroscope -- still neutral same number of + as - -- -- -- connection to ground e- can get even further from charged rod leaves fall + +++ now positively charged But still connected to ground -- e- - - (Earth) Ground - reservoir of electrons
- can accept or donate any number of ew/ no resistance -- -- -- + + + + Remove the charged rod + redistribute leaves separate for good NET POSITIVE CHARGE Break connection w/ ground e- cant go back leaves try to get as far away as possible Separate because likes repel like hair in Van de Graaf demo ELECTRIC forces between charges CHARGE physical quantity; described by the Coulomb SI UNIT : for charge (Q,q) Coulomb (C) actually very large charge, 10-6 C on a balloon (C, nC) FUNDAMENTAL CHARGE electron (e-) charge = 1.6 x 10-19 C cannot transfer less than 1 e- to charge objects all charge in multiples of an electron fundamental
charge not continuous Coulombs Law - forces on charges F from calib F=k q1q2 / d 2 stiff wire q1 empiricalbrute force simpler model q2 q1 d q2 d F = force (in N) q1, q2 charges (in C) d - separation between charges (m) k = 9x109 Nm2/C2 Coulomb constant Coulomb actually measured! Force is a vector direction important force acts along a line joining two charges 2
F=k q1q2 / d + and + or _ and _ } positive force charges repel + } negative force charges attract and - or just remember opposites attract, likes repel Example: What is the electric force between an electron and proton in a hydrogen atom, spaced about 0.53 A apart? model 1 A = 10-10 m qp= +1.6x10-19 C protonpositive charge + equal to magnitude of e-
-19 - q e = - 1.6x10 C d=5.3x10-11 m Another example: A balloon charged to 3.4x10-5 C is located 2.6 m from a can charged at -5.6x10-5 C. What is the direction and magnitude of the force between them? Application: Lightning electric discharge from clouds Ben Franklin first to experiment with lightning + + + + - - -- - + + ++ + water evaporates ionized by high velocity motion F=k q1q2 / d 2 Large distance but huge charge big F -- --+ + + Induces charge on objects Puts force on cloud charges greatest force for highest objects (d smaller)
Gigantic discharge great amount of charge in cloud causes destructive damage because of energy stored ground to cloud, or cloud to ground (depends on charge) lightning rod sticks above buildings to attract charge thick wire connects to ground bypasses building to ground destructive energy goes directly to ground Heat lightning lightning between clouds from a distance Electric Batteries - galvanic cells History - Galvani and Volta observed frog leg twitch in presence of dissimilar metals Galvani: animal electricity stored electricity released when tissue touches metal Volta: dissimilar metals in contact through a solution produce a current Led to idea of (flow of electrons) galvanic cell - battery produces electric current Zn C positive terminal negative terminal electrons can flow Z n +2 stores chargeHook up to use
+ + + + + + +2 Zn e- - discharge-dead metals used up +2 Zn Zn + 2 Electrolyte- conducting solution Chemical work-energy to move e- from + to - terminal provides energy for electrical work - light bulb heats e- - + e- uses energy as it goes from - terminal to + terminal
battery used up when metal used up RECHARGABLE - able to reverse chemical process lithium ion, NiCad, wet/dry cell, fuel cells, solar POTENTIAL DIFFERENCE - voltage Describes amount of chemical energy available to charge V = Work/q work per charge J/C SI Volt (V) how much work a charge is able to do related to chemical work (potential) PE or Work W=qV Increase battery :voltage (potential) add more galvanic cells wires - no energy lost by e- -+ -+ -+ Connected in series } 3X voltage of a single cell FORCE FIELDS - visual representation of invisibleaction-at-a-distance interactions -shows lines of force - extends all thru space - force on object in direction of lines - measure with test particle (field map)
Example: gravity Field points IN -attractive force -mass follows line test mass mass Mass feels force from touching field ELECTRIC FIELD - positive test charge to measure long distance force of charges Positive charge will go: outward repulsive Force along field lines + - inward attractive Magnetism - acts between moving charges - current ANCIENT GREEKS lodestone-natural magnet like magnetite attracts small pieces of iron Magnetic fields different from other forces 1. Field not in direction of force
force perpendicular to field 2. NO MAGNETIC MONOPOLES -cannot isolate poles North and South poles always paired N S Field lines form closed loops! point from N. Pole to S. Pole CANNOT SPLIT POLES N S N Break apart get 2 magnets both have N & S S SIMILARITIES: Like poles repel, opposite poles attract EARTHS Magnetic Field Motion of molten iron core N S EARTH N S Earth North Pole
N S Compass S. Pole of compass magnet points to N. Pole of Earth for navigation Deflects solar wind - high energy particles ejected from Sun Magnetism from electricity What causes magnetism? Oersted A current (electron flow) causes a force on a compass needle SI UNIT Current I = Q / t (C/s=Ampere = 1 A) how fast electrons are flowing in a wire N S I (current) N S S N S
N Force perpendicular to both magnet and current Compass needle points around in circle surrounding wire magnetic field forms circle around wire A current exerts a force on a permanent magnet! Ampere - two currents exert forces on each other no permanent magnets involved! I Magnetismhas to do with moving charges 2 two wires are attracted I 1 If currents opposite repel Also invented solenoid electromagnet (wire coiled on bolt) loop of wire produces field through center Coil intensifies
the magnetic field at the center: Looks like bar magnet magnetic domains Permanent magnets: Electrons in atoms move electric currents produce field Atomic magnets line up in magnetic materials: iron, nickel, cobalt, etc. domain boundaries Electricity from magnetism Faraday : can magnetism produce electricity? -built on Oersteds & Amperes results Coil and galvanometer magnetic sitting in field - no current take out - current flows put in - current flows Faradays Law of Induction induced voltage and current produced by changing magnetic field or circuit motion in field electromagnetic induction Dynamo - electric generator uses mechanical energy to produce electricity turbine turns circuit in magnet water wheel, steam. Nuclear Produces current- electricity force electrons through a circuit
Applications of Electromagnetism Electric meter - detects flowing currents galvanometer -coil wound on on pointer needle -force when current flows in magnet -force bigger when current larger use to measure I, V, and R Electromagnetic Switch (Relay) -small switch closes to produce small current in solenoid -solenoid produces magnetic field to pull in metal contact so larger current can flow Telephone -receiver - carbon granules compress with diaphragm changing resistance -changes current which is transmitted Speaker -current changes in magnetic field -force on coil moves cone Electric Motor -converts electrical energy to mechanical energy -rotating electromagnet spins in stationary magnetic field -electromagnet current changes
direction to maintain rotation (always repels in magnet) -armature and commutator change current -generator in reverse Electric currents provide electrical work - + Electric current - flow of charge from induced current (generator) or battery I = charge passing a certain point = Q / t = J/s (Ampere) time Historically: Ben Franklin(first to experiment with electricity) Wrongly assumed + charges move conventional current -still used today Actually - charges move in typical circuits - + fixed current is flow of electrons in wire Electric field in wires forces e- to go from - to + . Does work on electrons - gives them energy POTENTIAL DIFFERENCE - energy/charge available to electrons - voltage V=work/charge = (Work Energy) / q SI: J/C = Volt (V) provides energy to circuits! Example : Car battery A 12 V car battery is used to start a car. If 1x109 electrons go from the negative terminal to the positive terminal, then how much work is done? charge equivalent: 1 e- = 1.6x10-19 C V = W/q W = qV
current flow in wires e- e- make collisions w/ atoms in wire -does not accelerate -lose energy -move at a very small E speed (drift velocity) Electric field moves at speed of light electrons move very slowly (hours to from switch to light socket) Large number of charges (1015) produce current - drip out like full water hose George Simon Ohm how current flows in conductors -+ V A Current depends on potential difference (V) OHMS LAW I=V/ R R - resistance to a flow of current how difficult it is to pass a current Resistance (R) SI: Volt/Amp = ohms () how energy is lost - flow of electrons impeded
depends on: - type of material (copper, gold, graphite) - length of wire - longer, more resistance - cross-sectional area thinner wire, more resistive less charge can flow - temperature superconducting @ low T - no R! How current flows determines how circuits work! Combinations of resistances most circuits are combinations R of resistances V and batteries + and wires- connections with no resistance Two ways to combine resistors: SERIES COMBINATION - same current thru each resistor R1 R2 R3 Req I V equivalent
circuit Equivalent - Total - Combined total bigger Resistance: than individual looks like a longer resistor V Req = Rtot = R1 + R2 + R3 -each will resist current Can analyze I-V characteristics of circuit with Ohms Law V = I Req How much I battery life Parallel Combination of resistors Divided circuit in which the current can travel in multiple paths same potential difference R1 across each component R2 Req R3 V equivalent circuit V Combined Resistance: Total smaller
than individuals 1/Req = 1/R1 + 1/R2 +1/ R3 must take reciprocal for Req path of least resistance - most of the divided current will go through resistor with the smallest resistance For parallel, current can bypass broken circuit (burned out) elements Christmas lights - will stay lit even if one light burns out Home outlets wired in parallel Example : light bulbs 1. Three light bulbs with resistances of 5 , 8 , and 12 are connected in parallel across a 5 V battery. a) What is the total (combined, equivalent) resistance of the combination? b) How much current is drawn from the battery? REMEMBER for parallel : flip for resistance 2. Three light bulbs are connected in series across a 20 V battery. The resistance values of the light bulbs are all 5 a)What is the equivalent resistance of the combination? b) What is the current flowing thru the circuit? Heat Power of Currents Collision of electrons with atoms - hit atoms - atoms vibrate (gain energy) -heats wire- JOULE
HEATING JOULES LAW - wires heat up as current V flows A Joules Experiment P= I2 R ***remember power=(work energy) time more current e- make more collisions higher resistancemore energy lost to atoms material impedes flow Can rewrite with Ohms Law (V=IR) P = I2R = V2/ R =IV most general Example: car revisited How much energy is used to start a car? The car uses 10 A for 4 second with a 12 V car battery. More examples: A radio uses 0.5 A through a resistance of 6 During operation. How much power is consumed?
A 3 lightbulb is connected is connected to a 120 V Source of potential difference. How much power is used? Joule heating used in many electrical applications -hair dryer -space-heater -toaster -stove -lightbulb - filament heated to > 2500oC Heat generated also a problem Broken cord: loose connection high resistance heat Short circuit: bypasses load large current heat P = I2R I=V/R Power Stations provide current to homes Called power station because it provides current and voltage Dont pay for power Pay for energy! kilowatt-hour meter E=Pt Safety device to limit dangerous current fuse- filament heats up too much and will melt I from I to
plant house -connection to current source broken -circuit breaker similar Low melting point conductor Voltage lost as current travels along power lines Joule heating TRANSFORMER steps up the voltage But at the expense of the current Constant power device P=IV increase V, decrease I Changes voltage by primary coil secondary coil
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