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Unformatted text preview: Electrical Interactions & Simple Circuits
Electric Forces and Fields Charges in Motion Batteries and Bulbs Current, Voltage, and Power UCSD: Physics 8; 2 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 like charges experience repulsive force (unlike gravity) opposite charges attracted to each other (like gravity) Electrical current is the flow of charge (electrons) UCSD: Physics 8; 2 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 within trillions one trillion is small compared to 1028: less than one quadrillionth of our total charge is unbalanced! UCSD: Physics 8; 2 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. UCSD: Physics 8; 2 "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 UCSD: Physics 8; 2 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 UCSD: Physics 8; 2 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! + + UCSD: Physics 8; 2 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) Since we know gravity is F = GMm/r2, g = GM/r2 acceleration due to gravity is independent of the mass of the "test body" Electric force is F = kQq/r2 Electric field is just E = kQ/r2 so that F = qE q is the charge analog to mass E is the analog to gravitational acceleration: tells how a "test charge", q, will respond (what's the force on it?) units of E work out to volts per meter (V/m) UCSD: Physics 8; 2 Example Electric Fields Around Charges UCSD: Physics 8; 2 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 UCSD: Physics 8; 2 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 1.6 10-19 Coulombs so (negative) one Coulomb is 6 1018 electrons one amp is 6 1018 electrons per second subtle gotcha: electrons flow in direction opposite to current, since current is implicitly positive charge flow, but electrons are negative UCSD: Physics 8; 2 The Quest for Light Given a battery, a light bulb, and one piece of wire, how would you get the bulb to light? UCSD: Physics 8; 2 Would This Work? UCSD: Physics 8; 2 Would This Work? UCSD: Physics 8; 2 Would This Work? UCSD: Physics 8; 2 The Central Concept: Closed Circuit UCSD: Physics 8; 2 Circuit in Diagram Form
battery _ + current bulb In a closed circuit, current flows around the loop electrons flow opposite the indicated current direction! (repelled by negative terminal) Current flowing through the filament makes it glow. No Loop No Current No Light UCSD: Physics 8; 2 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) UCSD: Physics 8; 2 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? How would the brightness of "A" change if I unscrewed "B"? UCSD: Physics 8; 2 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. 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 (is there current even when the battery is disconnected?) UCSD: Physics 8; 2 What Does a Battery Provide? Batteries do supply current just not a constant current More relavently, batteries supply a constant voltage D-cell is about 1.5 volts What is a voltage? Voltage is much like a potential energy the higher the voltage, the more work can be done it takes one Joule to push one Coulomb through one Volt so a Volt is a Joule per Coulomb (J/C) UCSD: Physics 8; 2 Voltage, Current, and Power One Volt is a Joule per Coulomb (J/C) One Amp of current is one Coulomb per second If I have one volt (J/C) and one amp (C/s), then multiplying gives Joules per 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 the higher the voltage and/or the higher the current UCSD: Physics 8; 2 Announcements/Assignments Next up: a simple model for molecules/lattices waves energy from food and the demands of exercise Assignments: First Q/O due Friday, 4/8 by 5PM via WebCT read chapter 2: pp. 5257, 6566; chapter 6: pp. 190191; chapter 3: pp. 7984; chapter 8: 263271, 277278 on Efield read chapter 3, pp. 7984, chapter 6 pp. 190191 HW2: Chapter 1: E.8, E.13, E.20, E.21, E.23, E.25, P.8, P.10, P.13, P.14, C.5; Chapter 2: E.28, E.30, P.10, P.11: due 4/08 UCSD: Physics 8; 2 Assignments Read pp. 304309, 317318, 324331 to go along with this lecture Read pp. 224231, 332333, 407 for next lecture HW2 due 4/20: 7.E.1, 7.E.4, 7.P.1, 7.P.2, 7.P.3, 3.P.2, 3.P.4, plus eight additional required problems available on assignments page ...
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