Chapter 8: Electrical Relationships

Georg Ohm (1787-1854) determined the relationship between current, voltage and resistance.

Using Ohm’s law to analyze a circuit

Ohm’s law V=IR Not a “law” of nature!

Ohm’s law is only true in conductors.

Not true in all circumstances!

Thus, the name is a misnomer.

Should be called “Ohm’s rule for conductors.”

Real laws of nature concerning electricity are Maxwell’s laws

Ohm’s law is a special case of Maxwell’s laws.

Ohm’s law (“rule”) is necessary for designing electric circuits

Audio circuit used in radioshttp://www.mitedu.freeserve.co.uk/Prac/readdiag.htm

Infrared remote control circuithttp://www.electronicsteacher.com/circuits-and-diagrams/infrared-based-schematics/

If a device has the same resistance under different conditions,

then it obeys Ohm’s law.

Not all electrical devices obey Ohm’s law!

If resistance changes, then the device does not obey Ohm’s law.

Example: light bulb’s resistance when the voltage and current

Showing Ohm’s law on a graph

Resistors obey Ohm’s law I vs. V graph = straight line

R is same, for all values of I and V

R = slope of the line

Diodes don’t obey Ohm’s law I vs. V is not a straight line. because resistance changes.

Charge is only allowed to flow in one direction

I is zero when V is negative. Diodes are made from semiconductors.

What is the resistance when V = 2, 5 and 5.5 volts?

Read off values: V=2, I=0

R = V÷I = 2÷0 = infinite

V=5, I=0.1

R = V÷I = 5÷0.1 = 50 ohm

V=5.5, I=0.5

R = V÷I = 5.5÷0.5 = 11 ohm

http://www.gcse.com/IV_diode3.htm

What is a resistor? Device that resists the flow of charges.

Used to control current and voltage.

Many kinds of resistors: Fixed and variable.

Fixed resistors have the same resistance for a wide range of temperatures, and electrical currents.

Color code scheme allows us to figure out how much resistance it provides.

http://www.arar93.dsl.pipex.com/mds975/Images/

radios/resistors01.jpg

http://itll.colorado.edu/ITLL/index.cfm?fuseaction=ResistorChart

Potentiometers: Variable resistors (Also called “pots”)

Used to adjust the level of analog signals (e.g. volume controls on audio equipment)

A light dimmer uses a potentiometer to controlthe brightness of lamps.

http://www.answers.com/topic/potentiometer

How a potentiometer works Pots have three wires

Resistance between A and C always stays the same.

As you turn the knob, resistance between A and B changes.

Resistance between B and C also changes.

With wiper set as shown above, resist. between A and B is 2

Resistance between B and C is 8 (10 - 2 )

You can choose how to connect a potentiometer, to change r from zero to its max value.

For the potentiometer here, r can vary between zero and 10 ohms.