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Chapter 6 - Electricity (& Magnetism) Electricity - deals with interactions between electric charges * causes forces motion * two types of charges: + positive.

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Presentation on theme: "Chapter 6 - Electricity (& Magnetism) Electricity - deals with interactions between electric charges * causes forces motion * two types of charges: + positive."— Presentation transcript:

1 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

2 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 hair- electrons 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 don’t just appear out of nowhere!

3 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 loosely held e- move from atom to atom path for e- to travel strongly held e- conductor Also conduct heat well from motion of e- Example: wires - transport charge for use in circuits Insulators - charge cannot freely move no loose e- get stuck on surface poor heat conductors e-

4 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 Some materials have both properties atmospheric air nitrogenwater (humidity) Oxygen Carbon dioxide GOOD INSULATOR GOOD CONDUCTOR Polar - act like separated charge + - 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

5 Electroscope - early device used to measure charge metal leaves (gold) spread apart when charged -likes repel -more charge, spread more add charge here Methods to charge objects: conduction and induction (and friction) CONDUCTION – touch two charged objects together to transfer charge neutral electroscope spark charge transferred charge shared leaves move apart charge becomes evenly distributed

6 leaves try to get as far away as possible Separate because likes repel – like hair in Van de Graaf demo Charge by INDUCTION – two objects never actually touch charge by using electric forces (induced charge) NO DIRECT CONTACT neutral electroscope bring charged rod close- pushes e- away leaves separate e- try to get as far away as possible still neutral same number of + as connection to ground e- can get even further from charged rod leaves fall (Earth) Ground – reservoir of electrons can accept or donate any number of e- w/ no resistance now positively charged But still connected to ground Break connection w/ ground e- can’t go back Remove the charged rod + redistribute leaves separate for good NET POSITIVE CHARGE e-

7 ELECTRIC forces between charges CHARGE – physical quantity; described by the Coulomb SI UNIT : for charge (Q,q) Coulomb (C) actually very large charge, C on a balloon (  C, nC) FUNDAMENTAL CHARGE electron (e-) charge = 1.6 x C cannot transfer less than 1 e- to charge objects all charge in multiples of an electron – fundamental charge not continuous Coulomb’s Law - forces on charges stiff wire d q1q1 q2q2 F from calib F=k q 1 q 2 / d 2 q1q1 q2q2 d simpler model F = force (in N) q 1, q 2 – charges (in C) d - separation between charges (m) k = 9x10 9 Nm 2 /C 2 Coulomb constant Coulomb actually measured! empirical- brute force

8 Force is a vector – direction important F=k q 1 q 2 / d 2 + and + or _ and _ } positive force charges repel + and - } negative force charges attract or just remember “opposites attract, likes repel” force acts along a line joining two charges Example: What is the electric force between an electron and proton in a hydrogen atom, spaced about 0.53 A apart? 1 A = m model + - d=5.3x m proton–positive charge equal to magnitude of e- q p = +1.6x C q e = - 1.6x C 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?

9 Application: Lightning – electric discharge from clouds water evaporates ionized by high velocity motion F=k q 1 q 2 / d 2 Induces charge on objects Puts force on cloud charges greatest force for highest objects (d smaller) Ben Franklin – first to experiment with lightning Large distance but huge charge – big F 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

10 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 (flow of electrons) Zn C Led to idea of galvanic cell - battery Electrolyte- conducting solution Zn produces electric current Zn +2 e- positive terminalnegative terminal stores charge- Hook up to use electrons can flow discharge-dead metals used up

11 Chemical work -energy to move e- from + to - terminal + - 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 } 3X voltage of a single cell Connected in series wires - no energy lost by e- provides energy for electrical work - light bulb heats

12 FORCE FIELDS - visual representation of invisible“action-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 test mass mass Field points IN -attractive force -mass follows line ELECTRIC FIELD - positive test charge to measure long distance force of charges Mass feels force from touching field - + inward attractive outward repulsive Positive charge will go: Force along field lines

13 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 S N Field lines form closed loops! point from N. Pole to S. Pole CANNOT SPLIT POLES S N S N Break apart - get 2 magnets both have N & S SIMILARITIES: Like poles repel, opposite poles attract

14 EARTH’S Magnetic Field EARTH N S N S S N Compass S. Pole of compass magnet points to N. Pole of Earth Motion of molten iron core Deflects solar wind - high energy particles ejected from Sun for navigation Earth North Pole

15 N S N S 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 Compass needle points around in circle surrounding wire magnetic field forms circle around wire N S N S I (current) A current exerts a force on a permanent magnet! Force perpendicular to both magnet and current

16 Ampere - two currents exert forces on each other I 1 I 2 two wires are attracted If currents opposite repel Also invented solenoid – electromagnet (wire coiled on bolt) Magnetism- has to do with moving charges no permanent magnets involved! loop of wire produces field through center Coil intensifies the magnetic field at the center: Looks like bar magnet Permanent magnets: Electrons in atoms move – electric currents produce field Atomic magnets line up in magnetic materials: iron, nickel, cobalt, etc. magnetic domains domain boundaries

17 Electricity from magnetism Faraday : can magnetism produce electricity? -built on Oersted’s & Ampere’s results Coil and galvanometer magnetic sitting in field - no current take out - current flows put in - current flows Faraday’s Law of Induction induced voltage and current produced by changing magnetic field or circuit motion in fieldelectromagnetic 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

18 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

19 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

20 + - Electric currents provide electrical work Electric current - flow of charge from induced current (generator) or battery I = charge passing a certain point time = Q / t = J/s (Ampere) 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! 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

21 Example : Car battery A 12 V car battery is used to start a car. If 1x10 9 electrons go from the negative terminal to the positive terminal, then how much work is done? charge equivalent: 1 e- = 1.6x C V = W/qW = qV current flow in wires E e- e- make collisions w/ atoms in wire -does not accelerate -lose energy -move at a very small 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 (10 15 ) produce current - drip out like full water hose

22 George Simon Ohm - how current flows in conductors V + - A Current depends on potential difference (V) OHM’S 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 low T - no R! How current flows determines how circuits work!

23 Combinations of resistances most circuits are combinations of resistances and batteries and wires- connections with no resistance R V Two ways to combine resistors: SERIES COMBINATION - same current thru each resistor V I Equivalent - Total - Combined Resistance: R eq = R tot = R 1 + R 2 + R 3 V R 1 R 2 R 3 equivalent circuit R eq looks like a longer resistor -each will resist current Can analyze I-V characteristics of circuit with Ohm’s Law V = I R eq How much I battery life total bigger than individual + -

24 Parallel Combination of resistors Divided circuit in which the current can travel in multiple paths same potential difference across each component R1R1 R3R3 R2R2 V V equivalent circuit R eq Combined Resistance: 1/R eq = 1/R 1 + 1/R 2 +1/ R 3 Total smaller than individuals must take reciprocal for R eq “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

25 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?

26 Heat Power of Currents Collision of electrons with atoms - hit atoms - atoms vibrate (gain energy) -heats wire- JOULE HEATING JOULE’S LAW - wires heat up as current flows A V P= I 2 R ***remember power=(work energy) time Joule’s Experiment Can rewrite with Ohm’s Law (V=IR) P = I 2 R = V 2 / R = I V 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 current - e- make more collisions higher resistance- more energy lost to atoms material impedes flow

27 Joule heating used in many electrical applications -hair dryer -space-heater -toaster -stove -lightbulb - filament heated to > 2500 o C 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? Heat generated also a problem Broken cord: loose connection high resistance heat Short circuit: bypasses load large current heat P = I 2 R I=V/R

28 Power Stations provide current to homes Called power station because it provides current and voltage Don’t 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 -connection to current source broken -circuit breaker similar Low melting point conductor I from plant I to house 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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