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Electricity

Electricity. Chapter 16. Review. Smallest particles of matter are called atoms Electrons Protons Neutrons. …. Protons Positive charge. Electrons Negative charge. **If atom has equal number of protons & electrons there is no net charge***. Charge & Force.

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Electricity

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  1. Electricity Chapter 16

  2. Review • Smallest particles of matter are called atoms • Electrons • Protons • Neutrons

  3. … Protons • Positive charge Electrons • Negative charge **If atom has equal number of protons & electrons there is no net charge***

  4. Charge & Force • Charges produce a force between objects • Opposite charges attract • Like charges repel

  5. Moving Electrons • Electrons can be moved • Rubbing hair against a balloon will move some electrons from the hair to the balloon • Both the hair and the balloon will have a charge. • What will the charge on the balloon be? • What will the charge on the hair be?

  6. SI Unit for electric charge is the Coulomb, C

  7. Electric Charge • If there is an imbalance with the number of electrons and protons, there is a net electric charge • If there is more electrons than protons, object is negative • Electron’s charge = -1.6 x 10-19C • If there is more protons than electrons, the object is positive • Proton’s charge = 1.6 x 10-19C

  8. Two types of materials • Conductor • allows electrons to move through it easily • e- are loosely held • ex: metals like copper and silver

  9. …. • Insulator • material that doesn’t allow electrons to move through it easily • e- are tightly held • ex: plastic, wood, rubber, glass

  10. Electric Field • An electric field surrounds all charged objects. • Electric forces act at a distance because of this field.

  11. Static Electricity • Static means not moving • Static electricity is electricity at rest • Friction can cause it • Objects rub together and electrons move from one object another.

  12. Static discharge • Eventually static electric charge will move. • Slowly the electrons may move into moisture in the air • Or quickly in a spark.

  13. Lightning • Wind rubs particles in cloud together • Cloud gains charge • Induce charge in ground • Eventually a big charge jumps • Lightning rod protects buildings

  14. Transfer of Electrons • Charging by • Induction • charging an object without actually touching the object to any other charged object Charging by friction • Electrons are transferred when different materials are rubbed together • Depends on materials Charging by contact (Conduction) • When negative object touches neutral object, electrons move objects

  15. Electroscope • Consists of: • Flask • Metal bar (conductor) through rubber stopper (insulator) • 2 pieces of thin foil on the bottom • Charge on the metal will push the foil apart because they have the same charge

  16. … No Charge- leaves hang straight down

  17. Induction Rod with negative charge

  18. … • Rod with negative charge: • Pushes negative charges away - down electroscope

  19. … • Extra negative charges cause leaves to move apart

  20. … Remove rod everything returns

  21. Conduction Rod with negative charge

  22. … Rod with negative charge touches electroscope

  23. Electrons are transferred from rod to electroscope

  24. Extra negative charges cause the leaves to move apart.

  25. Remove rod leaves stay apart.

  26. Potential Difference • Often called voltage • Change in the electrical potential energy of a charged particle divided by its charge • Occurs when charge moves from one place to another • SI Unit for potential difference is the Volt (V) • 1 V = 1 J/C

  27. Current • Flow of electrons • Number of electrons move through a conductor • Measured in Amperes of Amps (A)

  28. Resistance • Internal friction, which slows the movement of charges through a conducting material • A dim 40W bulb has a higher resistance than the filament of a bright 100W bulb • SI Unit is ohm (Ω) • Ω = Volts/Amps • Found by dividing the voltage across the conductor by the current

  29. Tungsten - high resistance Copper - low resistance • electrical energy is converted to thermal energy & light • Resistance depends on… • the conductor (low) • Wire thickness • Less resistance in thicker wires • Wire length • Less resistance in shorter wires • Temperature • Less resistance at low temps

  30. V R I Ohm’s Law • The relationship among current, voltage, and resistance. • Ohm’s law states that the current in a circuit is equal to the voltage divided by the resistance • I = V R

  31. Ohm’s Law V: potential difference (V) I: current (A) R: resistance () V = I × R • Voltage ↑ when current increases. • Voltage ↓ when resistance increases.

  32. V R I Ohm’s Law • A light bulb with a resistance of 160  is plugged into a 120-V outlet. What is the current flowing through the bulb? GIVEN: R = 160  V = 120 V I = ? WORK: I = V ÷ R I = (120 V) ÷ (160 ) I = 0.75 A

  33. Do the Math • A car has a 12 volt system. The headlights are on a 10 amp circuit. How much resistance do they have? • Your house uses 120 volts. What amount of current would flow through a 20 ohm resistor?

  34. Circuit • Circuit • Path through which electrons can flow

  35. Circuit Components A - battery C - light bulb B - switch D - resistor

  36. Circuits • For current to flow there must be a complete loop • Electrons flow from negative to positive terminal • Work is done if there is a resistance in the wire.

  37. 2 Types of Circuits • Series Circuit • current travels in a single path • 1 break stops the flow of current • current is the same throughout circuit • lights are equal brightness • each device receives a fraction of the total voltage • get dimmer as lights are added

  38. Series Circuits

  39. Series Circuits

  40. Series Circuits Break in the wire turns off all the lights

  41. Series Circuits

  42. Parallel Circuits • Parallel Circuits • current travels in multiple paths • one break doesn’t stop flow • current varies in different branches • takes path of least resistance • “bigger” light would be dimmer • each device receives the total voltage • no change when lights are added

  43. Parallel Circuit

  44. Parallel Circuit

  45. Electrical Power • rate at which electrical energy is • converted to another form of energy P: power (W) I: current (A) V: potential difference (V) P = I × V

  46. Electrical Power • A calculator has a 0.01-A current flowing through it. It operates with a potential difference of 9 V. How much power does it use? GIVEN: I = 0.01 A V = 9 V P = ? WORK: P = I · V P = (0.01 A) (9 V) P = 0.09 W P V I

  47. Electrical Energy • energy use of an appliance depends on power required and time used E = P × t E: energy (kWh) P: power (kW) t: time (h)

  48. The SI unit for energy is a joule. • Kilowatt-hour meters measure the electricity used in your home (kWh)

  49. Electrical Energy • A refrigerator is a major user of electrical power. If it uses 700 W and runs 10 hrs each day, how much energy (in kWh) is used in 1 day? GIVEN: P = 700 W = 0.7 kW t = 10 h E = ? WORK: E = P · t E = (0.7 kW) (10 h) E = 7 kWh E t P

  50. Types of current • Direct current: electrons that flow in the same direction in a wire (DC) • From batteries • Alternating current: electrons that flow in different directions in a wire (AC) • From Genrators • Used in your home • Transformers change AC to DC

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