1. Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine tomburbine@astro.umass.edu

2. Quiz on Thursday Coulomb’s Law Electric Potential Electric current Electrical resistance Ohm’s Law Voltage = current x resistance Power = current x voltage Series Circuit Parallel Circuit

3. http://hilaroad.com/camp/projects/lemon/lemon_b attery.htmlhttp://hilaroad.com/camp/projects/lemon/lemon_b attery.html 1 lemon can generate approximately 0.83 Volts

4. Electric potential (volt) = electric potential energy charge

5. A conductor is a material which contains movable electric charges (electrons) Zinc-iron nails and copper are good conductors of electricity Insulators are non-conducting materials with fewer mobile charges (electrons), which resist the flow of electric current

6. Voltage is like water pressure Voltage is analogous to a water pressure difference A high pressure difference (higher voltage) will result in a faster electron flow

7. Current Electric current, measured in amperes, is the rate of flow of electric charge One ampere is the rate of flow of one coulomb of charge (6.24 x 10 18 electrons) per second

8. Electrical Resistance The electrical resistance is a measure of its opposition to the passage of an electric current Resistance = Voltage Current

9. The resistance of a wire depends on three factors: the length of the wire, the cross-sectional area of the wire, the resistivity of the material composing the wire The resistance would be greater for a longer wire, less for a wire of larger cross sectional area, and depends upon the material out of which the wire is made

10. Electrical resistance of a conductor is a measure of how difficult it is to push the charges along

11. Ohm’s Law Current = voltage resistance Units: 1 Ampere = 1 volt 1 ohm

12. Usually written as V = IR V is Voltage in Volts (V) I is Current in Amperes (A) R is Resistance in Ohms (  )

13. Example How much current flows through a lamp with a resistance of 60  when the voltage across the lamp is 12 V?

14. Example How much current flows through a lamp with a resistance of 60  when the voltage across the lamp is 12 V? V = IR I = V/R = 12 V/ 60  = 0.2 A

15. What is the resistance of a toaster that draws a current of 12 A when connected to a 120-V circuit?

16. V = IR R = V/I = 120 V/ 12 A = 10 

17. Why do you get more of a shock when you touch a battery with wet hands than dry hands?

18. Example Why do you get more of a shock when you touch a battery with wet hands than dry hands? I = V/R Resistance of hands soaked in salt water = 100  Resistance of dry hands = 100,000  Lower resistance equals higher current

20. Series Circuits Electric current passes through a single pathway Total resistance is the sum of the resistances Current is equal to the Voltage divided by the sum of the resistances A break in the path results in an open circuit

21. Series Circuits

22. V = I(R 1 + R 2 + R 3 + …)

23. Series Circuits If one lamp burns out, the current through the other lamps cease. All lamps go out Adding more lamps reduces the current through the circuit Adding more lamps reduces the brightness of each lamp

24. Parallel Circuits The Voltage is the same across each lamp Current divides along the parallel branches

25. Parallel Circuits

26. If one of the lamps in a parallel circuit burns out, the other lamps are unaffected However, the total current in the circuit will decrease

27. Parallel Circuits If you add another lamp, the brightness of the other lamps is unchanged However, the total current will increase

28. Parallel Circuits Overloading is when you add too many devices in parallel The total current increases too much The wire can overheat

29. Series Parallel

30. Power Power = current x voltage Power = charge x energy = energy time charge time Units: Watts = amperes x volts 1 kilowatt = 1,000 Watts

31. power = energy/time energy = power x time 1 kilowatt-hour = amount of energy consumed in 1 hour at the rate of 1kilowatt

32. If electric energy equals 25 cents/kilowatt-hour, how much would it cost to operate a 100-W bulb for 10 hours?

33. cost = rate x power x time cost = (25 cents/kilowatt-hour)(0.1 kW)(10 hours) Cost = 25 cents

34. Example Lamp at 120 Watts is operating on a 120 Volt line, what current does it draw?

35. Example Lamp at 120 Watts is operating on a 120 Volt line, what current does it draw? Power = current x voltage current = power/voltage = 120 Watts/120 Volts current = 1 ampere

36. Any Questions?