# Princeton COS 436 - Basic Quantities and Units (12 pages)

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## Basic Quantities and Units

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## Basic Quantities and Units

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Pages:
12
School:
Princeton University
Course:
Cos 436 - Human-Computer Interface Technology
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CS 436 HCI Technology Basic Electricity Electronics Review Copyright 1997 2008 Perry R Cook Princeton University August 27 2008 1 Basic Quantities and Units 1 1 Charge Number of electrons or units of electron charge Units Coulombs C 6 242 1018 electrons worth of charge 1 2 Voltage Potential energy per unit charge Common Symbol V Units Volts V Joules Coulomb 1 3 Current Flow rate of charge Common Symbol I Units Amperes or Amps A Coulombs second 1 4 Power Energy flow same as in basic Newtonian mechanics Common Symbol P Units Watts W Joules second P VI Power Voltage across element Current through element 1 1 5 Resistance Converts current into heat Common Symbol the Ohm Ohm s Law V IR or I V R this means P ower V I V 2 R I 2 R 2 Useful Circuit Analysis Tools and Laws 2 1 Power Sources Ideal Sources Deliver rated voltage or current regardless of load Practical Sources All sources have some internal resistance 3 Kirchoff s Laws Voltage Law X Vloop 0 Conservative Potential Energy Field Current Law X Inode 0 Conservation of Matter charge Voltage Example Vsrc IR1 IR2 0 currents must all be same I R1 R2 Vsrc Ohm s Law So Resistors Add in Series VR1 IR1 I VR2 IR2 Vsrc R1 R2 thus VR2 Vsrc R2 R1 R2 This is called Voltage Division Voltage Division gives rise to a very popular analog interface sensor and device The Potentiometer or Pot Pots are nice because they re cheap and relatively low noise but see below Pots are not so nice because there s scraping involved with use and they can wear out Also cheap ones can get dirty or wet and stop working pretty quickly 3 Pot related voltage division circuits Current Example Isrc IR1 IR2 0 sum of currents must 0 Isrc V R1 V R2 0 Isrc V 1 1 R1 R2 or R2 Ohm s Law V Isrc RR11 R 2 Define Conductance G 1 R inverse of Resistance so G1 1 R1 G1 G2 R1 R2 R1 R2 and 1 R1 G2 1 R2 1 R2 meaning Conductances Add In Parallel or currents divide across parallel conductances This is the DUAL of Resistances which add in series with voltage dividing across

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