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Quiz on Thursday

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

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Quiz on Thursday

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  1. Introduction to Physical ScienceMonday, Wednesday, ThursdayTom Burbinetomburbine@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_battery.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 1018 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. What is the resistance of a toaster that draws a current of 12 A when connected to a 120-V circuit? • 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

  19. 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

  20. Series Circuits

  21. Series Circuits • V = I(R1 + R2 + R3 + …)

  22. 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

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

  24. Parallel Circuits

  25. Parallel Circuits • 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

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

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

  28. Series Parallel

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

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

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

  32. If electric energy equals 25 cents/kilowatt-hour, how much would it cost to operate a 100-W bulb for 10 hours? • cost = rate x power x time • cost = (25 cents/kilowatt-hour)(0.1 kW)(10 hours) • Cost = 25 cents

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

  34. 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

  35. Any Questions?

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