BE Lesson 2: Resistance What is conductance? What are resistors? What is Ohms law? What is a Watt? © 2012 C. Rightmyer, Licensed under The MIT OSI License,

BE Lesson 2: Resistance

• What is conductance?

• What are resistors?

• What is Ohms law?

• What is a Watt?

© 2012 C. Rightmyer, Licensed under The MIT OSI License, 20 July 2012

Pure silicon atoms are good insulators

Adapted from Getting Started in Electronics, Master Publishing, Inc., Forrest M. Mims III, 2010

Copper atoms are good conductors

Copper, a metallic material, happens to be an excellent electricalconductor. It has 29 protonsand 29 electrons.

The electrons are in 4 distinctorbital orbit locations called “shells”.

Note that the highest shell contains a single electron. This electron is located far away from the nucleus, and is therefore weakly attached to the atom’s charged particle structure.

BE project 2-1. Take apart a shop light and see how it worksES proj 4-3

BE project 2-2. Use a multimeter to measure resistance

Test Item Ohms Value Conductor or Insulator?PlywoodGlassAirDry cottonDry silkDry rayonCeramicPaperPlasticRubberDistilled waterSalt wateryour skinAluminumCopperSilver

ES proj 4-4

What are insulators?

Insulators are materials that greatly resist the flow of electrons. Here are some examples:

glass rubber oil asphalt fiberglass porcelain ceramic quartz (dry) cotton (dry) paper (dry) wood plastic air diamond pure water

Is air really a poor conductor?

What are the electrical components called resistors?

Resistors are electrical components very often used in electrical circuits to control the amount of current. Resistor are available in a large range of values and are typically made using these types of materials:

Resistive metal filmResistive wireCarbon filmCarbon compositionMetal oxidesCement

This is the electrical symbol for a resistor. Its resistance value, R, is measured in ohms.

Smaller resistors are 1/8 watt; larger is 1/4 watt

Brown = 1%Red = 2%Gold = 5%Silver = 10%

Examples:

1 = Brown-Black-Gold10 = Brown-Black-Black100 = Brown-Black-Brown1000 = Brown-Black-Red10,000 = Brown-Black-Orange

Note exeception

How do we read the ohm value () of older resistors?

Wide gap

Black 0 Black 0Brown 1 Brown 1Red 2 Red 2Orange 3 Orange 3Yellow 4 Yellow 4Greeen 5 Greeen 5Blue 6 Blue 6Violet 7 Violet 7Gray 8 Gray 8White 9 White 9

Read first 2 or 3 * color bands

Add number zeros to first two

* Note: some resistors have a third digit before the multiplier.

How do we read the ohm value () newer resistors?

Adapted from Vellerman K4001 kit assembly manual. www.vellerman-kit.com

BE project 2-3. Measure resistor Ohm values ()

Ohms Ohms Color codeOhms with

Digital Meter

100 100 brown-black-brown330 330 orange-orange-black

1000 1K brown-black-red100,000 100K brown-black-yellow

ES demo 4-5

How to use a wire stripper.

Electric Circuits for Grades 3-6, Lawrence Hall of Science, University of California Berkeley, LHS GEMS, 2004

BE project 2-4. Make a battery connectorES proj 4-1

Typical breadboard with an X-ray vision of the copper strips on the back of the board

Adapted from Make: Electronics, Oreily Media Inc, Charles Platt, Dec 2009

Breadboard orientation

BE ckt 2-5. Circuit to learn about the plug in breadboard

+

9.0

volts

330

+

Long lead of LED oriented towards top

LED

ES ckt 4-6

(1827) George Simon Ohm develops Ohm’s Law

Voltage = (current) x (resistance)

http://www-history.mcs.st-and.ac.uk/Biographies/Ohm.html

Ohm’s Law

Ohm’s law states: V = I R

where

V is voltage (measured in volts),

I is current (measured in Amps)

R is resistance (measured in Ohms)

This equation can be restated as I = V/R, or R = V/I

Use Ohm’s law to calculate current in this circuit

+

-

V = 9.0 v

I = ?

R = 1000 ohms

• To calculate the current, divide the voltage drop across the resistor by the

resistor’s ohm value. For instance I = V/R = (9.0/1000) = (0.009 amps) = 9 ma.

• To calculate the voltage drop across the resistor, multiply the current by the

ohms. For instance V = IR = (0.009 x 1000) = (9.0 v).

• To calculate resistance, divide the voltage across the resistor by the

current. For instance R = V/I = (9.0/0.009) = 1000 ohms

BE ckt 2-6. Learn about Ohm’s law (step 1)

+

9.0

volts

ES ckt 4-7

BE ckt 2-6. Ohm’s law hookup (step 2)

+

ma

9.0

volts

Digital multimeter setto measure 200 mA

BE ckt 2-6. Ohm’s law hookup (step 3)

1000 ohm = Brown-Black-Red = 1K ohm

+

ma

9.0

volts

1K

Digital multimeter setto measure 200 mA

BE ckt 2-6. Ohm’s law hookup (step 4)

+

ma

9.0

volts

vdc

Second digital multimeterset to measure 20 volts DC

1000 ohm = Brown-Black-Red = 1K ohm

1K

Measure the voltage drop across one of the two series resistors (BE ckt 2-7)

I = 9.0/(1000+1000) = 9.0/(2000) = 0.0045 amps=4.5 milliamps = 4.5 ma.

VR = I x R = (4.5 ma) x (1000 ohms) = 4.5 volts

+

-

V = 9.0 v

I = ?

R = 1000 ohms

R = 1000 ohms

DC Amps

VOM: ma

4.5 v

+

ma

9.0

volts

vdc Second digital multimeterset to measure 20 volts DC

1000 ohm = Brown-Black-Red = 1K ohm

1K

Same as circuit 2-2 except for addition of a second 1K resistor.

BE ckt 2-7. Learn about resistors in series

1K

ES ckt 4-8

What happens if we add a second resistor in parallel?(BE ckt 2-8)

I = (9.0/1000) + (9.0/1000)= (0.009 + 0.009) = 0.018 amps= 18 milliamps = 18 ma

+

-

V = 9.0 v

I = ?DC Amps

VOM: ma

1000 1000 9.0 v

BE ckt 2-8. Learn about resistors in parallel

Expected current through two parallel 1000 ohms resistors = ~ 18 ma.

+

vdc

ma

9.0

volts

1K1K

ES ckt 4-9

BE project 2-9. Investigate characteristics potentiometer (pot) -- a variable resistor

[Walch Hands-on Science Series: Electricity and Magnetism, Weston Walch Publisher, 2000]

ES demo 4-10

Light Emitting Diode (LED)

Adapted from http://www.kpsec.freeuk.com/components/diode.htm

Symbol for LED

BE ckt 2-10. Measure the voltage and current required to turn on an LED.

LED

ma

Notes: (1) LED’s require about 1.5 to 1.7 volts to begin operation. (2) Brightly lit LEDs consume about 12 milliamps .

+ -

50k pot

9 v+

vdc

Hookup diagram for BE ckt 2-10

Longer lead of LEDgoes to the top (+).

Expected about 1.8 volts and 5 to 8 ma for bright LED operation.

50K pot

+

9.0

volts

ES demo 3-5

ma

vdc

LED

Watts = Volts * Amps

+

-

VOMDC Amps

+ -

VOMDC Volts

+

-

Energy consumption examples measured in Watts

Oscilloscope(50 watts)

HiFi Amplifier(200 watts)

Space heater(1500 watts)

A “smart” meter reading kWh power consumption