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Current, Voltage and Resistance ENTC 210: Circuit Analysis I Rohit Singhal Lecturer Texas A&M University

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Administrative Tasks Fix Exam Schedule Lab details TA – Mr. Pankaj Bhagawat Sections Merge

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Atoms and their structure

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Atomic Structure Mass of an Electron = 9.11 x 10 -28 gm. Mass of a Proton = 1.672 x 10 -24 gm. Proton is ~1836 times heavier than the electron

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Atomic Structure Unit of Charge = Coulombs Charge on electron = charge on a proton = 1.6 x 10 -19 C 1 Coulomb = Charge on 6.242 x 10 18 electrons

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Coulomb’s Law Like charges repel, opposites attract F = k Q 1 Q 2 / r 2 k = 9 x 10 9 (units?)

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Coulomb’s Law Like charges repel, opposites attract F = k Q 1 Q 2 / r 2 K = 9 x 10 9 N m 2 /C 2

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Conduction In metals, the electrons are “more free” than the insulators. Whenever there is a charge present at one end, the electrons flow to (or away) from that charge.

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Current Rate of flow of charge 1 Amp = 1 Coulomb / 1 Second.

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Question If a laptop needs constantly needs 2 Amps current from a battery, how many electrons are drained from the battery in one hour? 1 Amp = 6.242 x 10 18 electrons/second 2 Amp = 12.484 x 10 18 electrons/second In one hour - > 3600 x 12.484 x 10 18 electrons Answer is 4.49 x 10 22 electrons

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Question What’s the weight of all those electrons? 4.49 x 10 22 x 9.11 x 10 -28 gm 4.09 x 10 -5 gm

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Equations I = Q/t Q = I x t t = Q/I

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Potential Every particle of mass m raised to a height h above the earth’s surface has a potential energy m.g.h This potential energy can be raised by raising the particle a little higher When the particle is set free, it travels to the point of least potential.

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Electric Potential Similarly, a charge wants to travel to a lower “electric” potential. A negative charge on the other hand, wants to travel to a higher potential. Each point in a circuit has a potential.

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Voltage Voltage is always measured between two points. It is defined as the difference of potential between the two points. Measured in volts

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Volts 1 volt is defined as the potential difference, which results in an energy exchange of 1 Joule due to the movement of 1 Coulomb across it.

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DC Voltage Supply

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Conductivity Copper is the most popular conductor. MetalConductivity (%) Silver105 Copper100 Gold70.5 Aluminum61 Tungsten31.2 Nickel22.1 Iron14 Constantan3.52 Nichrome1.73 Calorite1.44

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Resistance Resistance is proportional to length length direction of current flow

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Resistance Resistance is inversely proportional to the cross sectional area direction of current flow

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Resistance R = ρ L/A ρ is the resistivity of the material (units?)

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Material ρ (10 -8 Ohm-Metres) Silver1.645 Copper1.723 Gold2.443 Aluminum2.825 Tungsten5.485 Nickel7.811 Iron12.299 Tantalum15.54 Nichrome99.72 Tin Oxide250 Carbon3500

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American Wire Gage (AWG) sizes AWG #Diameter (in)Ω /1000ft. 00000.460.0490 0000.4090.0618 00.3250.0983 10.2890.1240 20.2570.1563 40.2040.2485 100.1020.9989 140.06402.525 280.012664.90

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Color Coding 5 Bands of code (3 are mandatory) Bands 1 - 3 the value of the resistor Band 4 the range (tolerance) Band 5 the reliability

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Color Code (Band 1-3) ColorValue Black0 Brown1 Red2 Orange3 Yellow4 Green5 Blue6 Violet7 Gray8 White9

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Example 26x10 3 = 26 K Ohms

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Band 3 (special cases) Gold = 0.1 Red Blue Gold = 2.6 Ohm Silver = 0.01 Red Blue Silver = 0.26 Ohm

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More Bands Band 4Tolerance Gold5% Silver10% None20% Band 5Reliability (after 1000 Hrs of use) Brown1% Red0.1% Orange0.01% Yellow0.001%

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Example = 26 K Ohms ± 5%, 1 in 100,000 fails after 1000 hrs of use

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