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19.2 Resistance

19.2 Resistance. pp. 700 - 707 Mr. Richter. Agenda. Review HW Chapter 19 Quiz Monday (second half) Notes: Resistance Ohm’s Law Human Resistance Resistors and Superconductors. Objectives: We Will Be Able To…. Understand the relationship between resistance, current and voltage.

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19.2 Resistance

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  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. Resistance • 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. Ohm’s Law • 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. Human Resistance • 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 1000Ω 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

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