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

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

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

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

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

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

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

Units of Voltage Submultiple units of voltage are:  millivolt (mV) 1-thousandth of a Volt or V  microvolt (  V) 1-millionth of a Volt or 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).

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 A  microvolt (  ) 1-millionth of an Ampere or A

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 (  ).

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.

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.

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

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

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 W  microwatt (  W) 1-millionth of a Watt or W Multiple units of power are:  kilowatt (kW) 1-thousand Watts or 10 3 W  megawatt (MW) 1-million Watts or 10 6 W

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.

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.

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 watts = 4.8 kilowatts energy = 4.8 kW x 12 hours = 57.6 kWh cost = 57.6 x $0.08 = $4.61 per day

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 =

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

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 = = 100 W

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!

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 .