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Chapter 6 - Electricity (& Magnetism)

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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 + + + + e- strongly held e- + + + + conductor loosely held e- move from atom to atom path for e- to travel 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

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

5 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 – touch two charged objects together to transfer charge - - - - - - spark charge transferred charge shared leaves move apart neutral electroscope charge becomes evenly distributed

6 now positively charged But still connected to ground
Charge by INDUCTION – two objects never actually touch charge by using electric forces (induced charge) NO DIRECT CONTACT bring charged rod close- pushes 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 e- (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 + + leaves try to get as far away as possible Separate because likes repel – like hair in Van de Graaf demo Remove the charged rod + redistribute leaves separate for good NET POSITIVE CHARGE

7 Coulomb’s Law - forces on charges
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 (mC, 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 empirical- brute force F=k q1q2 / d 2 stiff wire F from calib simpler model q1 q2 q1 d d q2 F = force (in N) q1, q2 – charges (in C) d - separation between charges (m) k = 9x109 Nm2/C2 Coulomb constant Coulomb actually measured!

8 } } Force is a vector – direction important F=k q1q2 / d 2 + and + or
force acts along a line joining two charges F=k q1q2 / d 2 } + 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? 1 A = m model qp= +1.6x10-19 C proton–positive charge equal to magnitude of e- + - q e = - 1.6x10-19 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?

9 Application: Lightning – electric discharge from clouds
Ben Franklin – first to experiment with lightning F=k q1q2 / d 2 + + + + Large distance but huge charge – big F - - - - - - + + + + + - - - - - water evaporates ionized by high velocity motion + + + 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

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

11 } 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- } 3X voltage of a single cell + - + - + - Connected in series

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) test mass Example: gravity Mass feels force from touching field Field points IN -attractive force -mass follows line mass ELECTRIC FIELD - positive test charge to measure long distance force of charges Positive charge will go: outward repulsive inward attractive 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 Field lines form closed loops! point from N. Pole to S. Pole S N CANNOT SPLIT POLES S N Break apart - get 2 magnets both have N & S SIMILARITIES: Like poles repel, opposite poles attract

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

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

16 Ampere - two currents exert forces on each other
no permanent magnets involved! Magnetism- has to do with moving charges I 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

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 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

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 -receiver - carbon granules compress with diaphragm
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) 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!

21 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- make collisions w/ atoms in wire -does not accelerate -lose energy -move at a very small speed (drift velocity) e- E 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

22 how current flows in conductors
George Simon Ohm - how current flows in conductors + - Current depends on potential difference (V) V A 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 (W) 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
and batteries and wires- connections with no resistance R V - + Two ways to combine resistors: SERIES COMBINATION - same current thru each resistor R1 R R3 Req I V V equivalent circuit Equivalent - Total - Combined Resistance: Req = Rtot = R1 + R2 + R3 total bigger than individual looks like a longer resistor -each will resist current Can analyze I-V characteristics of circuit with Ohm’s Law V = I Req How much I battery life

24 Parallel Combination of resistors
Divided circuit in which the current can travel in multiple paths same potential difference across each component R1 R2 Req R3 V equivalent circuit V Combined Resistance: 1/Req = 1/R1 + 1/R2 +1/ R3 Total smaller than individuals 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

25 Example : light bulbs 1. Three light bulbs with resistances of 5 W, 8 W, and 12 W 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 W. a) What is the equivalent resistance of the combination? b) What is the current flowing thru the circuit?

26 Heat Power of Currents P= I2 R P = I2R = V2/ R = I V (V=IR)
Collision of electrons with atoms - hit atoms - atoms vibrate (gain energy) -heats wire- JOULE HEATING JOULE’S LAW - wires heat up as current flows V P= I2 R ***remember power=(work energy) time A more current - e- make more collisions higher resistance- more energy lost to atoms material impedes flow Joule’s Experiment P = I2R = V2/ R = I V most general Can rewrite with Ohm’s Law (V=IR) 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.

27 Joule heating used in many electrical applications
More examples: A radio uses 0.5 A through a resistance of 6 W During operation. How much power is consumed? A 3 W 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

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 I from plant I to house 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 primary coil Changes voltage by secondary coil


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