Chapter 20: Electricity

1. Chapter 20: Electricity Jennie Borders

2. SPS10. Students will investigate the properties of electricity and magnetism. • a. Investigate static electricity in terms of • Friction • Induction • Conduction • Explain the flow of electrons in terms of • alternating and direct current • the relationship among voltage, resistance and current • simple series and parallel circuits

3. Section 20.1 – Electric Charge and Static Electricity • Electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other. • The SI unit of electric charge is the coulomb (C). • Protons have a positive charge and electrons have a negative charge.

4. The Atom • A cloud of negatively charged electrons surrounds the positively charged nucleus. • The atom is neutral because it has an equal number of positive and negative charges. • If an atom gains electrons, it becomes a negatively charged ion. • If an atom loses electrons, it becomes a positively charged ion.

5. Atoms and Ions

6. Electric Forces • Like charges repel, and opposite charges attract. • The force of attraction or repulsion between electrically charged objects is electric force. • The electric force between two objects is directly proportional to the net charge on each object and inversely proportional to the square of the distance between them.

7. Coulomb’s Law

8. Electric Fields • The effect an electric charge has on other charges in the space around it is the charge’s electric field. • The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charges.

9. Electric Field • The more net charge an object has, the greater is the force on it. • The direction of each field line shows the direction of the force on a positive charge.

10. Static Electricity • Static electricity is the study of the behavior of electric charges, including how charge is transferred between objects. • The law of conservation of charge states that the total charge in an isolated system is constant. • Charge can be transferred by friction, contact, and induction.

11. Charging by Friction • Rubbing a balloon on your hair is an example of charging by friction. • Electrons move from your hair to the balloon because atoms in rubber have a greater attraction for electrons than atoms in your hair.

12. Charging by Contact • When a girl touches the Van de Graaff generator sphere, she acquires a charge large enough to make her hairs stand on end.

13. Charging by Induction • You pick up extra electrons when you walk across a carpet, so your hand is negatively charged. • The net negative charge in your hand repels the electrons in the metal doorknob. • Overall, the doorknob is still neutral, but charge has moved within it.

14. Charging by Induction • A transfer of charge without contact between materials is induction.

15. Static Discharge • The spark you feel when touching a doorknob is static discharge. • Static discharge occurs when a pathway through which charges can move forms suddenly. • Lightning is static discharge that occurs because charge can build up in a storm cloud from friction between moving air masses.

16. Section 20.1 Assessment • How is a net electric charge produced? • What determines whether charges attract of repel? • Name two factors that affect the strength of an electric field. • List three methods of charge transfer. • Explain how static discharge occurs.

17. Section 20.1 Assessment • How does electric force depend on the amount of charge and the distance between charges? • What is the law of conservation of charge? • When a glass rod is rubbed with neutral silk, the glass becomes positively charges. What charge does the silk now have?

18. Section 20.2 – Electric Current and Ohm’s Law • A continuous flow of electric charge is an electric current. • The SI unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. • The two types of current are direct current and alternating current.

19. Direct Current • Charge flows only flows in one direction in direct current (DC). • A flashlight and most battery-operated devices use direct current.

20. Alternating Current • Alternating current (AC) is a flow of electric charge that regularly reverses its direction.

21. Current Flow • Electrons flow from the negative terminal of a battery to the positive terminal of a battery. • The current is in the opposite direction because current is the direction in which positive charges would flow.

22. Conductors and Insulators • An electrical conductor is a material through which charge can flow easily. Examples include copper, silver, and most metals. • A material through which charge cannot flow easily is called an electrical insulator. Examples include wood, plastic, rubber, and air.

23. Metal Conductors • Metals tend to be electrical conductors because they are made up of an ion lattice. • The positive metal ions cannot move, but the electrons can move. • This mobile electron lattice is known as the electron sea model.

24. Resistance • As electrons move through a wire, they collide with other particles which converts some kinetic energy into thermal energy. • Resistance is opposition to the flow of charges in a material. • The SI unit for resistance is the ohm (W).

25. Resistance • A material’s thickness, length, and temperature affect its resistance. • Resistance is greater in a longer wire because the charges move farther. • As temperature increases, a metal’s resistance increases because electrons collide more often.

26. Superconductor • A superconductor is a material that has almost zero resistance when it is cooled to low temperatures.

27. Voltage • In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.

28. Potential Difference • Potential difference is the difference in electrical potential energy between two places in an electric field. • Potential difference is measured in joules per coulomb, or volts (V). • Potential difference is also called voltage. • Charges flow from higher to lower potential energy.

29. Voltage Sources • Three common voltage sources are batteries, solar cells, and generators. • A battery is a device that converts chemical energy to electrical energy.

30. Ohm’s Law • According to Ohm’s law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R). V = I x R V = voltage (volts) I = current (amps) R = resistance (ohms)

31. Section 20.2 Assessment • List the two types of current. • Name two good electrical conductors and two good electrical insulators. • What variables affect the resistance of a material? • What causes charge to flow? • According to Ohm’s law, how is voltage related to resistance and current?

32. Section 20.2 Assessment • Suppose you have two wires of equal length made from the same material. How is it possible for the wires to have different resistances? • Use Ohm’s law to explain how two circuits could have the same current but different resistances.

33. Section 20.3 – Electric Circuits • An electric circuit is a complete path through which charge can flow. • Circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that are run by the electrical energy.

34. Circuit Diagram

35. Circuit Diagrams • Switches show places where the circuit can be opened. • If the switch is open, the circuit is not a complete loop, and the current stops. This is called an open circuit. • When the switch is closed, the circuit is complete and charge can flow. This is called a closed circuit.

36. Open and Closed Circuit

37. Circuit Diagrams • The + and – on the battery symbol indicate the positive and negative terminals.

38. Series Circuit • In a series circuit, charge has only one path through which it can flow. • If one element stops functioning in a series circuit, none of the elements can operate. • The more bulbs you have, the less brightly they shine.

39. Series Circuit

40. Parallel Circuit • A parallel circuit is an electric circuit with two or more paths through which charges can flow. • If one element stops functioning in a parallel circuit, the rest of the elements can still operate.

41. Parallel Circuit

42. Electric Power • The rate at which electrical energy is converted to another form of energy is electric power. • The unit of electric power is the joule per second, or watt (W). • Electric power can be calculated by multiplying voltage by current.

43. Electric Power P = I x V P = power (W) I = current (A) V = voltage (V)

44. Sample Problem • An electric oven is connected to a 240 volt line, and it uses 34 amps of current. What is the power used by the oven? P = I x V P = I x V P = ? P = 34A x 240V I = 34A P = 8200W V = 240V

45. Practice Problems • A clothes dryer uses about 27 amps of current from a 240 volt line. How much power does it use? • A camcorder has a power rating of 2.3 watts. If the output voltage from its battery is 7.2 volts, what current does it use? P = I x V P = 27A x 240V P = 6500W P = I x V I = P/V I = 2.3W/7.2V I = 0.32A

46. Practice Problems • A power tool uses about 12 amps of current and has a power rating of 1440 watts. What voltage does the tool require? P = I x V V = P/I V = 1440W/12A V = 120V

47. Electrical Safety • Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use. • A fuse prevents current overload in a circuit. • A circuit breaker is a switch that opens when current in a circuit is too high.

48. Electrical Safety • The transfer of excess charge through a conductor to Earth is called grounding.

49. Section 20.3 Assessment • Name two elements included in a circuit diagram. • What is the difference between a series circuit and a parallel circuit? • Write the equation for calculating electric power. • Name five safety devices used with electric current. • You plug in a string of holiday lights and notice that the entire string turns off when you remove one bulb. Explain why this happens.

50. Section 20.3 Assessment • A stereo receiver uses a current of 2.2 amps from a 120 volt line. What is its power? • A television connected to a 120 volt line uses 102 watts of power. How much current flows through it? P = I x V P = 2.2A x 120V P = 260W P = I x V I = P/V I = 102W/120V I = 0.85A