cpo/home/Portals/2/Media/post.../PhysicsChpt19. ppt

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2. 5 – 19 Do Now Problems • Define electric current and give the associated units • What is the formula associated with Ohm’s law? • 3. Differentiate between the terms voltage source and potential difference Homework Read & outline 34.5 – 34.11

4. Water vs. Electric Current There are many analogous properties between water and electric current Characteristics such as • Pressure • Volume • Flow

5. Pressure In a water system we measure water pressure in In an electrical system pressure is measured in Volts foot lbs or PSI (pounds per square in) A higher voltage pushes the charges through the wire with greater force

6. Pressure How a water tower works:1. Pump station2. Reservoir3. Water user

7. Voltage • Voltageis a measure of electric potential energy, just like height is a measure of gravitational potential energy. • Voltage is measured in volts (V).Using a voltmeter • A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

8. Voltage • Since 1 joule per second is a watt(power), you can interpret voltage as • measuring the available electrical power per amp of current that flows.

9. Voltage Source A pump is a source of fluid pressure difference A battery is a source of electrical potential difference

10. Voltage & Batteries • The positive end of a 1.5 volt battery is 1.5 volts higher than the negative end. • If you connect batteries positive-to-negative, each battery adds 1.5 volts to the total. • Three batteries make 4.5 volts. • Each unit of current coming out of the positive end of the three-battery stack has 4.5 joules of energy.

11. Voltage source VOLTAGE causes current VOLTAGE causes current Charges do NOT flow unless there is potential difference A voltage source is needed to provide a sustained potential difference i.e. batteries or generators The battery or source is represented by an escalator which raises charges to a higher level of energy.

12. Current and voltage • A battery uses chemical energy to create a voltage difference between its two terminals. • In a battery, chemical reactions provide the energy to pump the current from low voltage to high voltage. • A fully charged battery adds energy proportional to its voltage.

13. Volume In our water system volume is in The Electrical equivalent is a cubic feet (ft3) or liters Coulomb Coulomb is a unit of charge, consists of 6.3 x 1018 electrons (a big number indeed)

14. Electric Charge • A building up at the negative terminal of a cell/battery which then flows from this terminal as negative charges repel one another.

15. Flow • Fluid flowing through a system of pipes can behave in many respects like electricity flowing in a circuit • The main difference is that water will fill any space whereas electricity will 'fill' only a conductor

16. Flow • Charge can only flow when there is a potential difference or a difference in voltage across the ends of a conductor

17. Current is a flow of charge

19. Current • Electric current is measured in units called amperes, or amps (A) for short. • One amp is a flow of a certain quantity of electricity in one second. • The amount of electric current entering a circuit always equals the amount exiting the circuit.

20. Electric Current

21. Electrons have a charge of –1.610-19 Coulombs What is need for current to flow? • one amp = 61018 electrons per second By convention, electrical current flows from high voltage to low voltage In other words, a positive current flows from higher to lower electrical potential NO POTENTIAL DIFFERENCE = NO FLOW OF CURRENT

22. Recap • Relate water pressure and electric pressure. • Units for electric pressure? • Relate water volume to electric volume. • Units for electric volume? • Relate water flow and electric flow. • Units for electric flow? • Condition needed for electric flow.

23. 5 – Do Now Problems • What condition is necessary for the sustained flow of charge in a conductor? • Differentiate between AC and DC. • 3. What is voltage? • HW. Pg. 545 - 546 Review questions

24. How does current move through a circuit?

25. Conventional current • Conventional current was proposed by Ben Franklin in the 1700’s. • Scientists later discovered that the particles that carry electricity in a wire actually travel from negative to positive. • Today, we still use Franklin’s definition.

26. Current Getting it Started A potential difference is establishedbetween two points and some charges are released The positive terminal of a battery will attract electrons charges They will be acted on by the electrical force and start to move

27. Voltage source VOLTAGE causes current VOLTAGE causes current Charges do NOT flow unless there is potential difference A voltage source is needed to provide a sustained potential difference i.e. batteries or generators The battery or source is represented by an escalator which raises charges to a higher level of energy.

28. Current & flow of charges Electrical circuits, consist of current in the wire moving from higher to lower electrical potential

29. Current Electricity Example • The continuous flow of charge in a complete circuit.

30. A battery uses chemical energy to move charges. If you connect a circuit with a battery the charges flow out of the battery carrying energy. What does a battery do?

31. Effects of Electric Current on the Body

32. Shock examples

34. Key Question: How are voltage, current, and resistance related? Electrical Resistance and Ohm’s Law

35. Resistance measures how difficult it is for current to flow. Electrical resistance

36. Electrical Resistance • The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage. • The more resistance the circuit has, the less current that flows.

37. Resistance is measured in (W). One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp. The ohm

38. Ohm’s Law  The formulae

39. The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω). Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for. The resistance of electrical devices

40. Ohm Quiz Ohm plate Ice cream Ohm Ohm on the range Broken Ohm Ohm Alone Ohm sweet Ohm

41. Practice Quiz 1. If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12 volt battery? = 12 V / 100000 = 0.00012 A 2. If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24 volt battery, how much current would you draw? See slide 18 = 0.024 A = 24 V / 1000

42. More Practice Problems I = V/R I = 110 V / 2200 Ώ I = .05 A R = V/I R = 110 V/ .5A R = 220 Ώ V = IR V = 1.2 A (100 Ώ) V = 120 V

43. Electric Resistance 1. Length as the electrons will have more distance to move and will therefore be bumping around more. resistance varies with the length of a conductor Ex: The length of a conductor is similar to the length of a hallway.  A shorter hallway would allow people to move through at a higher rate than a longer one. 

44. 2. Cross-sectional area resistance varies inversely with the cross sectional area If the area doubles then the resistance is halved. The wider the pipe, the easier it is for water to flow through …………………………………………………

45. Type of material 3. Type of material good conductors have low resistance and many free electrons electrical conductance is measured in siemens (S). the inverse of the resistance

46. Temperature 4. Temperature most materials experience an increase in resistance as their temperature increases experience a decrease in resistance as their temperature decreases due to the kinetic theory of matter ………………………………………………… …………………………………………………

47. Electric Power The work done by an electric current moving through a circuit is given by W = V I t Power  = Volts x Amps 1 watt = 1 volt x1 amp

48. Power Equations • Power (P) • Rate at which work is performed • Measured in watts (W) • Power • P = V I P = (I R) I = I 2 R

49. Getting Power to Our Homes Thomas Edison's GE company held the patents on the DC generators being used in the early days • Decided to power homes with DC power • DC means direct current: just like what batteries deliver

50. Getting Power to Our Homes • Do we want power plants close to our home? • Probably NOT…so we need to have • an ability to “ship” electricity across states • So power lines are long • Therefore resistance no longer negligible