Chapter21[1]

# Chapter21[1] - Chapter 21 Definition of electric current I...

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Chapter 21 Definition of electric current : Amp 1 s C t Q I Current is simply charges in motion! (Mostly, but not ALWAYS, electrons). Effects of current on the human body: 1 mA – you can just “feel” it. 5 mA – ouch! 10 mA – causes involuntary muscle spasms 15 mA – loss of muscle control 70 mA – fatal The current that kills you comes from your own body’s electrons. To receive a “shock” (current flows), there must be a Δ V between two different parts of the body (discuss bird on a wire, lizard on the A/C contacts, James Bond on a high- tension wire). Discuss 3-prong plugs: (1 “hot”, 1 neutral, and 3 rd prong connects chassis to ground). Biological effects: 1) tissue damage due to burning 2) muscle contraction 3) disruption of cardiac rhythm 4) shuts down nerve centers that control breathing Definition of direct-current circuit: electrons flow around a closed path back to the starting point (often a battery). What causes charges to flow? There must be a potential difference! Do hose-segment filled with water analogy.

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We must do WORK to raise one end of the hose so that water flows. A battery does this work. The battery uses chemical reactions to produce a potential difference between its two ends ( terminals). Definition of electromotive force (emf, E : ). When the battery is disconnected, measure the potential difference (V) between its terminals. = W/ Δ Q Example: A battery has an emf of = 1.5 V, and delivers a current of I = 0.44 A, to a flashlight bulb for Δ t = 64 s. Find Δ Q and W. Q I t Q (0.44 A)(64s) 28C W Q (1.5V)(28C) 42J   E Ben Franklin’s mistake. Direction of current flow around a circuit assumes a positive charge. Draw simple circuit with arrow.
Drunkard’s Walk: The electrons push on all their neighbors and that’s why the flashlight shines immediately. Resistance and Ohm’s Law: Resistance: frictional forces impeding electron flow. Discuss “garden hose analogy”: V, I, R. Ohm’s Law V = I R (Only for “Ohmic” materials). R (Volts/Amp) = Ohms ( ) Example: Consider a 12-V battery. Dry skin has a resistance of 100 k . If you touch each battery terminal with your index fingers, I = 0.12 mA. Wet skin has a resistance of 1 k . I = 12 mA Resistivity: ρ ( · m ) Resistance depends on size and shape of material (extrinsic). Resistivity depends only on the microscopic structure of the material (intrinsic).

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A L R Do football stadium analogy: Few people vs. lots of people, still people vs. dancing people, crates lining the walls, tunnel gets longer. Example: 21-2 A current I = 1.82 A, flows through a copper wire Cu ( ρ = 1.68 x 10 -8 · m), of length, L=1.75 m, and dia. = 1.10 mm. Find Δ V V = I R R =  0309 . 0 L A L 2 2 d V=0.0562 V Discuss superconductivity. ρ depends on T and can be used to measure T (temperature sensing devices, i.e.
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## This note was uploaded on 10/13/2011 for the course PHY 102 taught by Professor Alexandrakis during the Fall '06 term at FIU.

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Chapter21[1] - Chapter 21 Definition of electric current I...

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