1. Electrical Resistance and Resistivity Syllabus Statements 5.1.6-5.1.7, 5.2.3 We are in chapter 17 of your textbook!

2. What IS Resistance? On what will resistance depend?

3. On what will a wire’s resistance depend?  There are 4 primary factors when determining a wire’s resistance:  Material composition  Length of the wire  Cross-sectional Area of the wire  Temperature  HOW?  Turn and talk!

4. Question 17.3a Wires I Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? d A = 4d B a) d A = 4d B d A = 2d B b) d A = 2d B d A = d B c) d A = d B d) d A = 1/2d B e) d A = 1/4d B

5. Resistivity (  ) of a wire  A physical property of the wire that depends on the material of which the wire is constructed; determines how easily current can flow through the material  When the temperature is CONSTANT:  Units =  ·m  Conductivity (  ) is the inverse of resistivity

6. Question 17.3a Wires I—try again Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? d A = 4d B a) d A = 4d B d A = 2d B b) d A = 2d B d A = d B c) d A = d B d) d A = 1/2d B e) d A = 1/4d B

7. area is less arearadiussquared diameter of A must be two times less than the diameter of B The resistance of wire A is greater because its area is less than wire B. Since area is related to radius (or diameter) squared, the diameter of A must be two times less than the diameter of B. Question 17.3a Wires I Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? d A = 4d B a) d A = 4d B d A = 2d B b) d A = 2d B d A = d B c) d A = d B d) d A = 1/2d B e) d A = 1/4d B

8. Question 17.3b Wires II A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? it decreasesby a factor of 4 a) it decreases by a factor of 4 it decreasesby a factor of 2 b) it decreases by a factor of 2 it stays the same c) it stays the same d) it increasesby a factor of 2 d) it increases by a factor of 2 e) it increasesby a factor of 4 e) it increases by a factor of 4

9. doubledhalved Keeping the volume (= area x length) constant means that if the length is doubled, the area is halved. Since, this increases the resistance by a factor of 4. Question 17.3b Wires II A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? it decreasesby a factor of 4 a) it decreases by a factor of 4 it decreasesby a factor of 2 b) it decreases by a factor of 2 it stays the same c) it stays the same d) it increasesby a factor of 2 d) it increases by a factor of 2 e) it increasesby a factor of 4 e) it increases by a factor of 4

10. Resistivities for Common Conductive Materials (table 17.1)

11. Practice Problem!  What is the resistance of a piece of copper wire that is 10.0 m long and 1.2 mm in diameter?

12. Temperature effect on R  Would you expect a higher temperature to cause an INCREASE or a DECREASE in the resistance of a wire? Why?

13. Temperature effect on R  Temperature increases causes resistance to increase:  R 0 = resistance at some reference temperature  R f = resistance at some temperature t (in °C) above the reference temperature   = temperature coefficient for material

14. Effective Resistance: Series

15. Resistors in Series: Example  Given the following information, what is the effective resistance of this series of resistors:

16. Effective Resistance: Parallel  The effective resistance will be smaller than the lowest resistance in the parallel combination

17. Resistors in Parallel: Example  Determine the effective resistance of this parallel configuration of resistors:

18. Daily Wrap-up!  Covered today:  Resistivity  Resistors in Series and Parallel  Due Tomorrow:  Lab: Resistors and Resistance