Transcript Lesson 2 Resistance

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 electrical
conductor. It has 29 protons
and 29 electrons.
The electrons are in 4 distinct
orbital 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 works
ES proj 4-3
BE project 2-2. Use a multimeter to measure resistance
ES proj 4-4
Test Item
Plywood
Glass
Air
Dry cotton
Dry silk
Dry rayon
Ceramic
Paper
Plastic
Rubber
Distilled water
Salt water
your skin
Aluminum
Copper
Silver
Ohms Value
Conductor or Insulator?
What are insulators?
Insulators are materials that greatly resist the flow of
electrons. Here are some examples:
glass
oil
fiberglass
ceramic
(dry) cotton
(dry) wood
air
pure water
rubber
asphalt
porcelain
quartz
(dry) paper
plastic
diamond
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 film
Resistive wire
Carbon film
Carbon composition
Metal oxides
Cement
This is the electrical symbol for a resistor. Its
resistance value, R, is measured in ohms.
How do we read the ohm value (W) of older resistors?
Read first 2 or 3 *
color bands
Black
Brown
Red
Orange
Yellow
Greeen
Blue
Violet
Gray
White
0
1
2
3
4
5
6
7
8
9
Add number zeros
to first two
Black
Brown
Red
Orange
Yellow
Greeen
Blue
Violet
Gray
White
Wide gap
0
1
2
3
4
5
6
7
8
9
Brown = 1%
Red = 2%
Gold = 5%
Silver = 10%
Examples:
1
= Brown-Black-Gold
10
= Brown-Black-Black
100
= Brown-Black-Brown
1000 = Brown-Black-Red
10,000 = Brown-Black-Orange
Note exeception
* Note: some resistors have a third digit before the multiplier.
Smaller resistors are 1/8 watt; larger is 1/4 watt
How do we read the ohm value (W) newer resistors?
Adapted from Vellerman K4001 kit assembly manual. www.vellerman-kit.com
BE project 2-3. Measure resistor Ohm values (W)
ES demo 4-5
Ohms
Ohms
Color code
100
330
1000
100,000
100
330
1K
100K
brown-black-brown
orange-orange-black
brown-black-red
brown-black-yellow
Ohms with
Digital Meter
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 connector
ES 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
ES ckt 4-6
Long lead of LED oriented towards top
330 W
+
9.0
volts
+
LED
(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=IR
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
I=?
+
V = 9.0 v
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)
ES ckt 4-7
+
9.0
volts
BE ckt 2-6. Ohm’s law hookup (step 2)
ma
+
9.0
volts
Digital multimeter set
to measure 200 mA
BE ckt 2-6. Ohm’s law hookup (step 3)
Digital multimeter set
to measure 200 mA
1K
ma
+
9.0
volts
1000 ohm = Brown-Black-Red = 1K ohm
BE ckt 2-6. Ohm’s law hookup (step 4)
vdc
1K
ma
Second digital multimeter
set to measure 20 volts DC
+
9.0
volts
1000 ohm = Brown-Black-Red = 1K ohm
Measure the voltage drop across one of the two series resistors
(BE ckt 2-7)
I=?
DC Amps
VOM: ma
R = 1000 ohms
+
V = 9.0 v
-
4.5 v
R = 1000 ohms
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
BE ckt 2-7. Learn about resistors in series
ES ckt 4-8
vdc
Second digital multimeter
set to measure 20 volts DC
1K
ma
+
9.0
1K
volts
1000 ohm = Brown-Black-Red = 1K ohm
Same as circuit 2-2 except for addition of a second 1K resistor.
What happens if we add a second resistor in parallel?
(BE ckt 2-8)
I=?
DC Amps
VOM: ma
+
V = 9.0 v
1000
1000
9.0 v
-
I = (9.0/1000) + (9.0/1000)= (0.009 + 0.009) = 0.018 amps
= 18 milliamps = 18 ma
BE ckt 2-8. Learn about resistors in parallel
ES ckt 4-9
ma
1K
1K
vdc
+
9.0
volts
Expected current through two parallel 1000 ohms resistors = ~ 18 ma.
BE project 2-9. Investigate characteristics potentiometer (pot) -- a
variable resistor
ES demo 4-10
[Walch Hands-on Science Series: Electricity and Magnetism, Weston Walch Publisher, 2000]
Light Emitting Diode (LED)
Adapted from http://www.kpsec.freeuk.com/components/diode.htm
BE ckt 2-10. Measure the voltage and current
required to turn on an LED.
ma
LED
vdc
+
9v
+
Symbol for LED
50k pot
Notes:
(1) LED’s require about 1.5 to 1.7 volts to begin operation.
(2) Brightly lit LEDs consume about 12 milliamps .
Hookup diagram for BE ckt 2-10
ES demo 3-5
ma
Longer lead of LED
goes to the top (+).
vdc
+
9.0
LED
volts
50K W pot
Expected about 1.8 volts and 5 to 8 ma for bright LED operation.
Watts = Volts * Amps
+
+
VOM
DC Amps
-
+
VOM
DC 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