Lect05 - 9/21/11 DC Circuits Review Question? Power...

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DC Circuits • Power dissipation • Circuit Analysis – Kirchoff’s laws • RC Circuits • Resistance vs Temperature September 11 Physics 104, Fal 2011 1 Review Question? Which configuration has the smallest resistance? 1 2 3 September 11 Physics 104, Fal 2011 2 3 Which configuration has the largest resistance? 1 2 3 R 2R R/2 Energy Transfer in the Circuit • Imagine a quantity of positive charge, Q, moving around the circuit from point A back to point A • As the charge moves through the battery from A to B, the potential energy of the system increases by QV • As the charge moves through the resistor, from C to D, it loses energy in collisions with the atoms of the resistor September 11 Physics 104, Fal 2011 Electrical Energy & Power • The rate at which the energy is delivered by the source • This power is dissipated in the resistor • From Ohm’s Law, alternate forms of power are • The SI unit of power is Watt (W) I must be in Amperes, R in ohms and Δ V in Volts • The unit of energy used by electric companies is the kilowatt-hour – This is defined in terms of the unit of power and the amount of time it is supplied 1 kWh = 3.60 x 10 6 J September 11 Physics 104, Fal 2011 4 P = Q t V = IV P = I 2 R = V 2 R Internal Resistance All sources have internal resistance: – Small but not always negligible • Reduces voltage output • Terminal Voltage: V = E - I r • Limits current that can be delivered • Current through a load I = E / (R load + r) September 11 Physics 104, Fal 2011 5 Meters • An ammeter in series measures current – An ideal ammeter has zero resistance – If ammeter resistance is comparable to the circuit’s a correction must be made. • A voltmeter in parallel with a circuit element measures voltage drop across it – An ideal voltmeter should have infinite resistance – If voltmeter resistance is comparable to circuit’s, a correction must be made • Resistance can be measured by measuring current and voltage. September 11 Physics 104, Fal 2011 6
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Why bother with messy circuits? • Reality is messy! There are resistances everywhere – Sources have small resistances – Meters used to measure perturb the circuit – Wires are not necessarily zero resistance – … – These stray resistances make the circuit analysis more complicated September 11 Physics 104, Fal 2009 7 Question September 11 Physics 104, Fal 2009 8 R 1 =10 Ω E 1 = 10 V I B I 1 E 2 = 5 V R 2 =10 Ω I 2 Resistors R 1 and R 2 are 1) in parallel 2) in series 3) neither + - Upper loop contains R 1 and R 2 but also E 2 . Outside loop contains E 1 and R 1 only so they are in parallel. Definition of parallel: Two elements are in parallel if (and only if) you can make a loop that contains only those two elements. Circuit Symantics • Branch – A resistor, a capacitor, … has two ends September 11 Physics 104, Fal 2009 9 R 1 =10 Ω E 1 = 10 V I B I 1 E 2 = 5 V R 2 =10 Ω I 2 - • Junction (Node) – A point where three or more branches meet Loop Kirchhoff’s Rules • There are ways in which resistors can be connected so that the circuits formed cannot be
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Lect05 - 9/21/11 DC Circuits Review Question? Power...

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