1. Aim: How do we explain resistance?

2. Do Now:
1. What is the difference between conventional current and electron current? Conventional Current is the flow of positive charges while Electron Current is the flow of negative charges. 2. What condition is necessary for a current to be established? Potential Difference

4. Sources of Potential Difference or Voltage

5. Galvanic Cell

6. Electric Circuit
A path for current to flow.

7. Typical Path of an Electron

8. Three Concepts Potential Difference Current Resistance
What is resistance? An opposition to current

9. What are the four factors that the resistance of a material depends on?

10. Factors that affect Resistance
Length
Cross Sectional Area
Resistivity
Temperature

11. Resistance Problems R=ρL/A=(150 x 10-8 Ω m)(1m)/(0.1 m2 ) R=0.000015 Ω
Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the cross sectional area of wire B than wire A?
Wire B must have 4 times the cross sectional area of wire A
2. Calculate the resistance of a 1 m long nichrome wire with a cross sectional area of 0.1 m^2.
R=ρL/A=(150 x 10-8 Ω m)(1m)/(0.1 m2 )
R= Ω

12. Resistance Problems
3. A 6.50 meter long copper wire at 20 degrees Celsius has a cross-sectional area of 3 mm2. What is the resistance of the wire? R=ρL/A ρ=1.72 x 10-8 Ω m L=6.5 m A= 3 mm2 (10-3 m/mm)(10-3 m/mm)=3 x 10-6 m2 R=0.037 Ω

13. Resistance Problems
A metal conductor is used in an electric circuit. The electrical resistance provided by the conductor could be increased by
Decreasing the length of the conductor
Decreasing the applied voltage in the circuit
Increasing the temperature of the conductor
Increasing the cross-sectional area of the conductor