1. Aim: How does resistivity affect resistance?
Do Now: The current flowing through a wire is 4 amperes. How many electrons pass a point inside the wire in 2 seconds?

2. Electrical Stopping Power Introduction to Resistance

3. Resistance Resistance: is a measurement of how strongly
an object will oppose current
An object’s resistance depends on FOUR factors:
Resistivity
Length
Is specific to a material – the
higher it is, the more natural
resistance the material has.
How long is the object?
The longer it is, the more it will
oppose current flow.
Cross-sectional Area
Temperature
How big is the object across?
The wider it is, the more current
it will allow to pass.
If the object is warm, the
molecules inside will be bouncing
around more – opposing current.

4. Equation Unit for Resistance is the: OHM (Ω) R = resistance J.s/C2
Symbol: R
R = resistance + 

5. Resistance in Conductors
We would like to have conductors that allow
CURRENT to flow from one place to
another EASILY
“IDEAL” conductor  minimum RESISTANCE
WIDE or NARROW?
LONG or SHORT?
HOT or COLD?
The “IDEAL” conductor has a
LOW RESISTIVITY and is
SHORT, WIDE, and COLD
COLDER conductors have lower resistance because:
atoms are not as energetic – don’t interfere with electrons
WIDER conductors have lower resistance because:
electrons have more room to pass!
SHORTER conductors have lower resistance because:
electrons do not have to travel as far!

6. Factors that affect resistance
Length: Length 
Area: Area 
Temperature: Temp 
Resistance 
Graph relationship of resistance vs. cross sectional-area…. Inverse Relationship
R vs. L… Direct
Resistance 
Resistance 

7. Practice
A copper wire has a length of 4.00 m and a diameter of 2x10-3 m. Assume a temperature of 20C.
(Ω.m)
a. What is the wire’s area?
b. What is the resistivity?
c. What is the resistance
of the wire?
d. Why specify 20C?
Material
Resistivity
Aluminum
2.82 x 10-8
Copper
1.72 x 10-8
Gold
2.44 x 10-8
Nichrome
150 x 10-8
Silver
1.59 x 10-8
Tungsten
5.60 x 10-8

8. Envelope Activity
Answer the following two questions regarding resistivity with a partner.

9. Example #1
Calculate the electrical resistance provided by a piece of copper wire with a length of 1 meter and a cross sectional area of 0.01 m2.

10. Example #2
Determine the composition of a wire with a resistance of 31.8 Ohms if it is 5 x 107 meters long and has a cross-sectional area of m2.
SILVER

11. Graph relationships
Predict & SKETCH what he following graphs would look like.
1. R vs. L
2. R vs. A
Students sketch on smartboard

12. SUMMARY Which factors affect resistance?
How does each factor affect resistance?
How can you calculate resistance?
What are the units for resistance?
Describe an ideal wire with the LEAST amount of resistance