1. Aim: How can we account for resistivity and capacitance?
HW: Problem Set

2. Using Kirchoff’s Rules Solve for the current through each resistor!
Write a full equation without numbers first!
Then solve for the current!

3. Solution
Junction Rule: Single wire with no branches Loop: 0 = -30V + IR1 + 10V + IR2 + IR3 + IR4 I = 0.385A

4. Defining Resistance
The opposition that a device or conductor offers to the flow of electric current.
Measured in Ohms (Ω)
J.s/C2
Symbol: R + 

5. What variables determine resistance in a wire or object?
Length of wire (L) in meters.
Area (A) in meters2.
Resistivity of material () in ohmsmeters.
Temperature (Increase Temp., Increase R)
What is resistivity?
How well a certain material “resist” the flow of electrons.
Does a Copper cable have the same resistance as an aluminum cable?

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 2 mm. Assume a temperature of 20C. a. What is the wire’s area? (Ω.m) 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. Station Activity
Determine the resistance of a 6.00 m length of silver wire having a cross-sectional area of 3.14x10-6 m2. Assume a temperature of 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

9. Summary: How is current, voltage and resistance all related?
Symbol
Name
Units
Equation V
Potential Difference
Volts (V)
PE = qV I
Current
Amps (A)
I=q/t R
Resistance
Ohms (Ω)
R = ρL/A
What is the graph of Voltage vs. Current? From Lab students know Slope = Resistance

10. Summary What decides an objects resistivity?
How will the resistance of a wire change if I double the area, but cut the length in half?
How will current behave in a circuit of parallel resistors? What “rule” does this exemplify?