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
