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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2002 The McGraw-Hill Companies Grob Schultz

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Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2003 The McGraw-Hill Companies 3 CHAPTER Ohm’s Law

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Topics Covered in Chapter 3 Ohm’s Law Formulas Practical Units of Resistance Multiple Units of Resistance Linear Proportion Between V and I Electric Power

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Topics Covered in Chapter 3 (continued) Power Formulas Choosing Resistors Electric Shock Open- and Short-Circuits

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Increasing V causes more I to flow in the bulb. I = V R R I V flow = pressure opposition amperes = volts ohms

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There are three forms of Ohm’s Law: I = V/R V = IR R = V/I Where: I = Current V = Voltage R = Resistance V I R

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20 V 4 ? I = 20 V 4 = 5 A ? 12 1 A V = 1A x 12 = 12 V 6 V? 3 A R = 6 V 3 A = 2 Applying Ohm’s Law V IR

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Units of Voltage Submultiple units of voltage are: millivolt (mV) 1-thousandth of a Volt or 10 -3 V microvolt ( V) 1-millionth of a Volt or 10 -6 V Multiple units of voltage are: kilovolt (kV) 1-thousand Volts or 10 3 V megavolt (MV) 1-million Volts or 10 6 V The basic unit of voltage is the Volt (V).

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Units of Current The basic unit of current is the Ampere (A). Submultiple units of current are: milliampere (mA) 1-thousandth of an Ampere or 10 -3 A microvolt ( ) 1-millionth of an Ampere or 10 -6 A

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Units of Resistance Multiple units of resistance are: kilohm (k ) 1-thousand Ohms or 10 3 Megohm (M ) 1-million Ohms or 10 6 The basic unit of resistance is the Ohm ( ).

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When V is constant: I decreases as R increases I increases as R decreases Examples: 16 V 4 4 A 16 V 8 2 A If R doubles, I is reduced to half. 16 A 16 V 1 If R is reduced to ¼, I increases by 4. This is known as an inverse relationship.

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0 1 2 3 4 5 6 7 8 9 1 2 3 4 Volts Amperes 2 + _ 0 to 9 Volts Fixed resistors have linear volt-ampere relationships. 2 1 4 The smaller the resistor, the steeper the slope.

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Volts Amperes Example of a Non-Linear Volt-Ampere Relationship As the tungsten filament gets hot, its resistance increases.

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Volts Amperes 2 nd Example of a Non-Linear Volt-Ampere Relationship As the thermistor gets hot, its resistance decreases. Thermistor

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Power is the time rate of doing work. The basic unit of power is the Watt (W). Submultiple units of power are: milliwatt (mW) 1-thousandth of a Watt or 10 -3 W microwatt ( W) 1-millionth of a Watt or 10 -6 W Multiple units of power are: kilowatt (kW) 1-thousand Watts or 10 3 W megawatt (MW) 1-million Watts or 10 6 W

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The rate of work can be found by multiplying potential difference times flow. 1 Volt = 1 Joule 1 coulomb 1 Ampere = 1 coulomb 1 second Power = Volts x Amps 1 Joule 1 coulomb 1 second x 1 Joule 1 second = Power (1 Watt) = and First, recall that: The base unit of power is the Watt or Joule/second.

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The amount of work (energy) can be found by multiplying power times time. The amount of work (energy) is used for calculating electric bills. The kilowatt-hour is the billing unit.

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To calculate electric cost, start with the power: Convert to kilowatts: Multiply by hours: (Assume it runs half the day) Multiply by rate: (Assume a rate of $0.08) An air conditioner operates at 240 volts and 20 amperes. The power is P = V x I = 240 x 20 = 4800 watts. 4800 watts = 4.8 kilowatts energy = 4.8 kW x 12 hours = 57.6 kWh cost = 57.6 x $0.08 = $4.61 per day

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Combining Ohm’s Law and the power formula V = IR P = VI P = (IR)I = I 2 R Substitute IR for V to obtain: Substitute V/R for I to obtain: P = V x V R V2V2 R = V R I =

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Power Formulas Where: I = Current R = Resistance P = Power V = Voltage

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Applying Power Formulas 20 V 4 5 A P = VI = 20 x 5 = 100 W P = I 2 R = 25 x 4 = 100 W P = V2V2 R = 400 4 = 100 W

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Electrical Shock Hazard When possible, work only on circuits that have the power shut off. If the power must be on, use only one hand. Hand-to-hand shocks can be very dangerous!

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Open and Short Circuits An open circuit has zero current flow. 16 V 0 A 16 V 0 excessive I A short circuit has excessive current flow. I = 16 V 0 = undefined As R approaches 0, I approaches .

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