1. 19.2 Resistance pp. 700 - 707 Mr. Richter

2. Agenda  Review HW  Chapter 19 Quiz Monday (second half)  Notes:  Resistance  Ohm’s Law  Human Resistance  Resistors and Superconductors

3. Objectives: We Will Be Able To…  Understand the relationship between resistance, current and voltage.  Calculate using Ohm’s Law.  Know what factors affect resistance.  Define resistors and superconductors.

4. Warm-Up:  If current is like water flowing in a river, what do you think resistance is?

5. Resistance

6.  What does resistance mean?  Resistance is the measure of how strongly an object inhibits the flow of electric current.  High resistance = low current, few electrons flow  Low resistance = high current, many electrons flow.

7. Resistance  Every device that uses electrical energy adds to the total resistance to a circuit.  The more total resistance the circuit has, the less the current.

8. Resistance  Remember, electron flow is bumpy and indirect.  The resistance of a material largely depends on (p. 701):  length  cross-sectional area  type of material  temperature

9. Ohm’s Law

10.  The resistance (R) of a material is the ratio of the applied voltage to the current that flows through.  This ratio is known as Ohm’s Law:  Resistance is measured in Ohms [ Ω ] A.K.A.: V = IR

11. Ohm’s Law: Relationships  Voltage and current are directly proportional.  With constant resistance  Resistance and current are inversely proportional.  With constant voltage. A.K.A.: V = IR

12. Human Resistance

13.  Why will a 9-volt battery not shock you on your arm, but shock your tongue?  Human skin usually has a resistance of approximately 500,000 Ω.  A 9-volt battery creates a current of about 9/500,000 or ~1.8 x 10 -5 A (0.000018 A)  Humans only feel current at about 0.01 A

14. Human Resistance  The resistance of wet skin can decrease to as little as 1000 Ω or even 100 Ω, or about 1000 times less resistance.  Water, and especially salt water, creates a better conductor.  Lots of free electrons!  This creates as much as 0.09 amps of current.  This is enough for human nerves to sense. And hurt.

15. Resistors and Superconductors

16. Resistors  Resistors can be used to control the flow of current in a circuit.  Current can also be controlled by adjusting voltage, but voltage is usually constant (like a 120-volt outlet)  A resistor is an element that provides a specified resistance.

17. Superconductors  Superconductors have no resistance below a critical temperature, usually near absolute zero.  Weird stuff: once current is established, it can continue even when the potential difference is removed.  This can create semi-permanent electromagnets, and super-efficient electric circuits.

18. Wrap-Up: Did we meet our objectives?  Understand the relationship between resistance, current and voltage.  Calculate using Ohm’s Law.  Know what factors affect resistance.  Define resistors and superconductors.

19. Homework  p707 #1-5, 7