Unformatted text preview: Electrical Interactions & Simple Circuits
Electric Forces and Fields Charges in Motion Batteries and Bulbs Current, Voltage, and Power UCSD: Physics 8; 2007 Electric Charge Fundamental particles carry something called electric charge protons have exactly one unit of positive charge electrons have exactly one unit of negative charge Electromagnetic force is one of the basic interactions in nature same charges repel repulsive force (unlike gravity) opposite charges attracted to each other (like gravity) Electrical current is the flow of charge (electrons) 2 UCSD: Physics 8; 2007 Charge Balance Neutral atoms are made of equal quantities of positive and negative charges Neutral carbon has 6 protons, 6 electrons, (& neutrons) Electrons can be stripped off of atoms Electrons occupy the vulnerable outskirts of atoms Usually charge flows in such a way as to maintain neutrality Excess positive charge attracts excess negative charge Your body has 5 1028 positive charges and 5 1028 negative charges, balanced The charge on an electron is..
3 UCSD: Physics 8; 2007 Coulomb Law Illustrated Like charges repel Unlike charges attract +
r + + If charges are of same magnitude (and same separation), all the forces will be the same magnitude, with different directions.
4 UCSD: Physics 8; 2007 "Electrostatic" Force: the Coulomb Law Two charges, Q1 and Q2, separated by distance r exert a force on each other: F = (kQ1Q2) / r2 k is a constant (9 109), Q is in Coulombs, r in meters One unit of charge (proton) has Q = 1.6 10-19 Coulombs Looks a lot like Newton's gravitation in form Electron and proton attract each other 1040 times stronger electrically than gravitationally! Good thing charge is usually balanced! A typical finger spark involves the exchange of a trillion electrons, or about 10-7 Coulombs
5 UCSD: Physics 8; 2007 Coulomb Force Law, Qualitatively Double one of the charges force doubles Change sign of one of the charges force changes direction Change sign of both charges force stays the same Double the distance between charges force four times weaker Double both charges force four times stronger 6 UCSD: Physics 8; 2007 Electric Field Can think of electric force as establishing a "field" telling particles which way to move and how fast Electric "field lines" tell a positive charge which way to move. For example, a positive charge itself has field lines pointing away from it, because this is how a positively-charged "test-particle" would respond if placed in the vicinity (repulsive force).
Run Away! + + In the equation...force is
7 UCSD: Physics 8; 2007 Analogy to Gravity field: On the surface of the earth, the force due to gravity is F = mg, where g is the gravitational acceleration g is a vector, pointing down tells masses how to move (how much force on mass, m) Gravitational Force F = GMm/r2 Gravitational acceleration g = GM/r2 g tells a test mass how to move F=ma Electric force is F = kQq/r2 q is the charge analog to mass Electric field E = kQ/r2 so that F = qE (now E is NOT energy) E tells how a "test charge", q, how to move E plays the role g did in gravity units of E work out to volts per meter (V/m)
8 UCSD: Physics 8; 2007 Gravitational and Electric Fields
Gravity Name Force Gravity Equation F=GMm/r2 Newtons Electric Name Electric Force Electric
2 F=kQq/r Newtons Mass m Charge q Acceleration g=GM/r2 due to Gravity (ms-2) Tells mass how to move Force F=mg 2 Electric Field Tells charge E=kQ/r how to move (Volts/meter) F=qE
9 UCSD: Physics 8; 2007 Example Electric Fields Around Charges 10 UCSD: Physics 8; 2007 But Realistic Picture Folds in Strength Previous pictures conveyed direction, but did not account for 1/r2 strength of the E-field The E-field gets weaker as one goes farther away from a charge In essence, there is an electric field vector (strength and direction) at every point in space This picture shows a sampling of the E-field vectors at 24 points in space around a negative charge What is q in the equation..
11 UCSD: Physics 8; 2007 Electric Current Electric current is simply the flow of charge Electrons flowing in a wire constitute a current Measured in Coulombs per second, or Amperes Colloquially, Amp (A) refers to amount of charge crossing through cross-sectional area per unit time Electrons have a charge of (negative) 1.6 10-19 Coulombs (negative) one Coulomb is 6 1018 electrons one amp is 6 1018 electrons per second electrons flow in direction opposite to current, since current is implicitly positive charge flow, but electrons are negative A Coulomb is..
12 UCSD: Physics 8; 2007 The Quest for Light Given a battery, a light bulb, and one piece of wire, how would you get the bulb to light? Demonstrate charged rods and Van der Graff generator What is electrical current?
13 UCSD: Physics 8; 2007 Would This Work? 14 UCSD: Physics 8; 2007 Would This Work? 15 UCSD: Physics 8; 2007 Would This Work? 16 UCSD: Physics 8; 2007 The Central Concept: Closed Circuit 17 UCSD: Physics 8; 2007 Circuit in Diagram Form
battery _ + current bulb In a closed circuit, current flows around the loop electrons flow opposite the indicated current direction! (e are repelled by negative terminal) Current flowing through the filament makes it glow. No Loop No Current No Light 18 UCSD: Physics 8; 2007 Current is the Central Concept It sometimes helps to think of current as flow of water, which is more familiar to us. High current means lots of water flow per unit time. Low current is more like a trickle. In electronics, it is the flow of charge, not water, that is described by the word current. And it's always electrons doing the flowing (thus electronics)
19 UCSD: Physics 8; 2007 Currents Divide and Merge at Junctions
- + A B How much would the current through the battery change if I unscrewed one of the 2 bulbs? 20 UCSD: Physics 8; 2007 Answer The battery is supplying an equal amount of current to each of the two bulbs. If one of the bulbs is disconnected, the current through the battery will be halved. 21 UCSD: Physics 8; 2007 Currents Divide and Merge at Junctions
- + A B How would the brightness of "A" change if I unscrewed "B"? 22 UCSD: Physics 8; 2007 Answer Unscrewing "B" would not affect the current through "A" so it will stay the same brightness. Why wouldn't more current flow through A? The battery does not supply constant current (there is no current when the battery is disconnected) 23 UCSD: Physics 8; 2007 What Does a Battery Provide? Batteries do supply current but not a constant current More relevantly, batteries supply a constant voltage D-cell is about 1.5 volts What is a voltage? Voltage is much like (g x height) for gravity the larger the voltage, the more work can be done it takes one Joule to push one Coulomb through one Volt Similar to: Potential Energy = mgh 1 Volt = 1 Joule per Coulomb (J/C) 24 UCSD: Physics 8; 2007 Voltage, Current, and Power One Volt is one Joule per Coulomb (J/C) One Amp of current is one Coulomb per second one volt (J/C) x one Amp (C/s) = Joules/second (J/s) this is power: J/s = Watts So the formula for electrical power is just: P = VI power = voltage current More work is done per unit time when the voltage and/or the current is larger 25 UCSD: Physics 8; 2007 Batteries Many types with different metals (eg lead (Pb) for negative and lead dioxide (PbO2) for positive) and electrolytes (eg dilute sulfuric acid). http://www.cnet.com.au/laptops/laptops/0,239035649,239145143-6,00.htm Often joined together to increase voltage car battery gets 12 volts from 6 cells each 2 V When a car batter discharges (gives out a current) Pb on negative plate combines with SO4 in electrolyte giving PbSO4 which is more like the +ve plate, lowers voltage When recharge the car battery In car: disconnect Negative (- black not red) first, since it is (usually) connected to car body. car uses gasoline to power engine which turns an alternator which makes electric current to reverse the chemical reactions in battery. If you disconnect positive first, and wrench touches car metal, will short the battery
26 UCSD: Physics 8; 2007 NiMH Re-chargeable Batteries rated in approx current I they could provide times the time t in hours:
I.t = milliAmpHours (mAh) AA 1.2V typical, It = 2500 mAh Are more efficient at low currents NiMH hold constant Voltage, then suddenly end No memory Never recharge normal alkali: may leak Current I Drawn by Device 100 mA 250 mA 2500 mA Expected Life t = 2500 mAh/I 25h 10h 1h Actual Life 27h 10h 50 min 1700 mAh can recharge 1000 times (overnight), a 2500 mAh only 500x (=735 recharges of the 1700) Can store 2-3yr but need several slow charge/ recharge to redistribute the electrolyte.
27 UCSD: Physics 8; 2007 Fuel Cells
Releases energy from:
O2 + 2H2 = H2O + energy Zero CO2 emission from the cell itself Ordinary battery used chemical energy stored in a (reversible) reaction nothing is consumed Fuel cells consume a fuel, hydrogen Need energy to make Hydrogen molecules
electricity from a power station + losses in transmission is more efficient (less CO2 emission) that a car engine Seen as a way to power cars
Replacing gasoline or led acid batteries (dirty, polluting, heavy) Or longer lasting laptop batteries Video: http://www.fuelcells.org/
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