1. Plan for Today (AP Physics I) Short Lecture on Ohm’s Law Exploring Temperature Variation Lab Lecture on Temperature Variation and R = p L/A

2. Ohm’s Law

3. From our lab, we saw R = V/I Normally, we write V = IR

4. Example Problem R = 6 ohms V = 6 V V = IR I = V/R = 6/6 = 1 A

5. Example Problem R = 60 ohms V = 6 V V = IR I = V/R = 6/60 =.1 A

6. Example Problem R = 600 ohms V = 6 V V = IR I = V/R = 6/600 =.01 A

7. What’s the pattern? Relationship between R and I – As one increases the other decreases – More R means less I

8. What kind of numbers do we expect for conductors in terms of R and I? Low R High I What kind of numbers do we expect for insulators? Low I High R

9. Ohm’s “Law” Ohm’s Law isn’t really a law More like Ohm’s “sometimes it’s a law” It isn’t like gravity – With gravity – we know gravity will always work – We know stuff will always fall – If we walk out a window, we know what will happen

10. Ohm’s Law Only works for Ohmic materials Graph Example

11. Non-ohmic Materials Semiconductors are a very important type of non-ohmic material Will work on a lab exploring non-ohmic materials

12. Lab Groups

13. Temperature Variation and Resistance

14. Lab Tie-In What did we see in lab? Resistance changed with higher voltage So is the light bulb non-ohmic?

15. Lab Something else is going on As we keep the light bulb plugged in what happens? Change in temperature Causes R to increase Why is this the case?

16. Higher Temperatures and Resistance Higher temperatures mean bigger vibrations of atoms So it is harder for electrons to get by This means more resistance

17. Equation = temperature resistivity on pg 598 = final resistance = initial resistance = change in temperature Units for alpha? (1/degrees C)

18. Example Tungsten wire is used in light bulbs. Its resistance changes from 20 to 100 ohms. What is the temperature change?

19. Example Tungsten wire is used in light bulbs. Its resistance changes from 20 to 100 ohms. What is the temperature change? – 100 = 20 (1 + a(Tf – Ti) – P. 598 – a is 4.5 * 10^-3

20. What this means The light bulb gets very hot Inert gases inside of the bulb keep it from burning up

21. What else can effect the resistance of a wire? Material it is made from – Copper is a good choice – not very resistive

22. What else can effect the resistance of a wire? Length Is this a direct or indirect relationship? What’s harder– to walk to the cafeteria or walk to the arch? Walk to the arch So this is a direct relationship

23. What else can effect the resistance of a wire? Cross sectional area Is this a direct or indirect relationship? What’s harder– to walk down the halls with 1000 of your closest friends or walk down the football field Walk down the football field So this is an indirect relationship

24. Equation for Resistance in a Wire P is the resistivity In what units? Ohms/m P at various temps – p. 598 as well

25. What this equation means This equation explains why extension cords get thicker The longer the cord, electric code requires a maximum resistance for an extension cord so it doesn’t overheat How to get this? As length increases, A must increase too to compensate

26. Example Find the resistance of a 100 m long copper wire with a diameter of 1 mm at 100 degrees Celsius

27. Example Find the resistance of a 100 m long copper wire with a diameter of 1 mm at 100 degrees Celsius – L = 100 – A =.003 – P = 1.7 * 10^-8 – R = pL/A

28. Example A copper wire 1mm in diameter is 200 m long. What is its resistance? How does the resistance change if the temperature changes from 20 to 120 degrees C?

29. Example A copper wire 1mm in diameter is 200 m long. What is its resistance? – Find this first – L = 200 – P = 1.7 * 10^-8 – A =.0005^2 * pi – R = pL/A How does the resistance change if the temperature changes from 20 to 120 degrees C?

30. Example A copper wire 1mm in diameter is 200 m long. What is its resistance? How does the resistance change if the temperature changes from 20 to 120 degrees C?

31. Example A copper wire 1mm in diameter is 200 m long. What is its resistance? How does the resistance change if the temperature changes from 20 to 120 degrees C?

32. Example A copper wire 1mm in diameter is 200 m long. What is its resistance? How does the resistance change if the temperature changes from 20 to 120 degrees C?