2. Chapter: Electricity Section 1: Electric Charge
Table of Contents
Chapter: Electricity
Section 1: Electric Charge
Section 2: Electric Current
Section 3: Electric Circuits

3. Electric Charge 1
Electricity
All solids, liquids, and gases are made of tiny particles called atoms.
Atoms are made of even smaller particles called protons, neutrons, and electrons.

4. Electric Charge 1
Electricity
Protons and neutrons are held together tightly in the nucleus at the center of an atom, but electrons swarm around the nucleus in all directions.
Protons and electrons have electric charge, but neutrons have no electric charge.

5. Positive and Negative Charge
Electric Charge 1
Positive and Negative Charge
There are two types of electric charge—positive and negative.
Protons have a positive charge, and electrons have a negative charge. The amount of negative charge on an electron is exactly equal to the amount of positive charge on a proton.

6. Positive and Negative Charge
Electric Charge 1
Positive and Negative Charge
Because atoms have equal numbers of protons and electrons, the amount of positive charge on all the protons in the nucleus of an atom is balanced by the negative charge on all the electrons moving around the nucleus.
Therefore, atoms are electrically neutral.

7. Positive and Negative Charge
Electric Charge 1
Positive and Negative Charge
An atom becomes negatively charged when it gains extra electrons.
If an atom loses electrons it becomes positively charged.
A positively or negatively charged atom is called an ion (I ahn).

8. Electrons Move in Solids
Electric Charge 1
Electrons Move in Solids
If you have ever taken clinging clothes from a clothes dryer, you have seen what happens when electrons are transferred from one object to another.
This imbalance of electric charge on an object is called a static charge.

9. Electrons Move in Solids
Electric Charge 1
Electrons Move in Solids
In solids, static charge is due to the transfer of electrons between objects.
Protons cannot be removed easily from the nucleus of an atom.

10. Electric Charge 1
Ions Move in Solutions
Sometimes, the movement of charge can be caused by the movement of ions instead of the movement of electrons.
When table salt (NaCl) dissolves in water, the sodium ions and chloride ions break apart.
These ions now are able to carry electric energy.

11. Electric Charge 1
Electric Forces
All charged objects exert an electric force on each other.
The electric force between two charges can be attractive or repulsive.

12. Electric Charge 1
Electric Forces
The electric force between two charged objects depends on the distance between them and the amount of charge on each object.
The electric force between two electric charges gets stronger as the charges get closer together.
The electric force between two charged objects increases if the amount of charge on at least one of the objects increases.

13. Electric Charge 1
Electric Fields
Charged objects don’t have to be touching to exert an electric force on each other.
Electric charges exert a force on each other at a distance through an electric field that exists around every electric charge.
An electric field gets stronger as you get closer to a charge, just as the electric force between two charges becomes greater as the charges get closer together.

14. Electric Charge 1
Electric Fields
The lines with arrowheads represent the electric field around charges.
The direction of each arrow is the direction a positive charge would move if it were placed in the field.

15. Insulators and Conductors
Electric Charge 1
Insulators and Conductors
A material in which electrons cannot move easily from place to place is called an insulator.
Examples of insulators are plastic, wood, glass, and rubber.
Charges placed on an insulator repel each other but cannot move easily on the surface of the insulator.
As a result, the charges remain in one place.

16. Insulators and Conductors
Electric Charge 1
Insulators and Conductors
Materials that are conductors contain electrons that can move more easily in the material.
The electric wire is made from a conductor coated with an insulator such as plastic.

17. Insulators and Conductors
Electric Charge 1
Insulators and Conductors
Electrons move easily in the conductor but do not move easily through the plastic insulation.

18. Electric Charge 1
Metals and Conductors
The best conductors are metals such as copper, gold, and aluminum.
When metal atoms form a solid, the metal atoms can move only short distances.
However, the electrons that are loosely bound to the atoms can move easily in the solid piece of metal.
In an insulator, the electrons are bound tightly in the atoms that make up the insulator and therefore cannot move easily.

19. Induced Charge 1 Has this ever happened to you?
Electric Charge 1
Induced Charge
Has this ever happened to you?
You walk across a carpet and as you reach for a metal doorknob, you feel an electric shock.
Maybe you even see a spark jump between tour fingertip and the doorknob.

20. Electric Charge 1
Induced Charge
This rapid movement of excess charge from one place to another is an electric discharge.

21. Electric Charge 1
Induced Charge
Lightning is another example of an electric discharge.
In a storm cloud, air currents sometimes cause the bottom of the cloud to become negatively charged.
This negative charge induces a positive charge in the ground below the cloud.
A cloud-to-ground lightning stroke occurs when electric charge moves between the cloud and the ground.

22. Electric Charge 1
Grounding
Lightning is an electric discharge that can cause damage and injury.
Even electric discharges that release small amounts of energy can damage delicate circuitry in devices such as computers.
One way to avoid the damage caused by electric discharges is to make the excess charges flow harmlessly into Earth’s surface.

23. Electric Charge 1
Grounding
The process of providing a pathway to drain excess charge into Earth is called grounding.
The pathway is usually a conductor such as a wire or a pipe. Lightning rods at the top of buildings and towers are made of metal and are connected to metal cables that conduct electric charge into the ground if the rod is struck by lightning.

24. Section Check 1
Question 1
The two types of electric charge are _______ and _______.
Answer
The answer is positive and negative.

25. Section Check 1
Question 2
An imbalanced of electric charge on an object is called a(n) _______.
A. electric discharge
B. electric current
C. insulator
D. static charge

26. Section Check 1
Answer
The answer is D. Any time your hair rises up toward a comb you are witnessing the effect of static charge.

27. Question 3 1 Like charges _______ while unlike charges _______.
Section Check 1
Question 3
Like charges _______ while unlike charges _______.

28. Section Check 1
Answer
Like charges repel and unlike charges attract.

29. Electric Current 2
Flow of Charge
Electrical energy in our homes comes from an electric current, which is the flow of electric charge.
In solids, the flowing charges are electrons.

30. Flow of Charge 2 In liquids, the flowing charges are ions.
Electric Current 2
Flow of Charge
In liquids, the flowing charges are ions.
Electric current is measured in units of amperes (A).
A model for electric current is flowing water.

31. A Model for a Simple Circuit
Electric Current 2
A Model for a Simple Circuit
The gravitational potential energy of water is increased when a pump raises the water above Earth.

32. A Model for a Simple Circuit
Electric Current 2
A Model for a Simple Circuit
As the water falls and does work on the waterwheel, the water loses potential energy and the waterwheel gains kinetic energy.
Electric charges will flow continuously only
through a closed conducting loop called a circuit.

33. Electric Current 2
Electric Circuit
The simplest electric circuit contains a source of electrical energy, such as a battery, and an electric conductor, such as a wire, connected to the battery.
As long as there is a closed path for electrons to follow, electrons can flow in a circuit.

34. Electric Current 2
Electric Circuit
They move away from the negative battery terminal and toward the positive terminal.

35. Electric Current 2
Voltage
In a water circuit, a pump increases the gravitational potential energy of the water by raising the water from a lower level to a higher level.
In an electric circuit, a battery increases the electrical potential energy of electrons.
This electrical potential energy can be transformed into other forms of energy.

36. Electric Current 2
Voltage
The voltage of a battery is a measure of how much electrical potential energy each electron can gain.
As voltage increases, more electrical potential energy is available to be transformed into other forms of energy.
Voltage is measured in volts (V).

37. Electric Current 2
How a Current Flows
When the ends of a wire are connected to a battery, the battery produces an electric field in the wire.
The electric field forces electrons to move toward the positive battery terminal. As an electron moves, it collides with other electric charges in the wire. After each collision, the electron again starts moving toward the positive terminal.

38. Batteries 2 A battery supplies energy to an electric circuit.
Electric Current 2
Batteries
A battery supplies energy to an electric circuit.
When the positive and negative terminals in a battery are connected in a circuit, the electric potential energy of the electrons in the circuit is increased.

39. Electric Current 2
Batteries
As these electrons move toward the positive battery terminal, this electric potential energy is transformed into other forms of energy.

40. Electric Current 2
Batteries
For the alkaline battery, the two terminals are separated by a moist paste.
Chemical reactions in the moist paste cause the negative terminal to become negatively charged and the positive terminal to become positively charged.

41. Electric Current 2
Batteries
This produces the electric field in the circuit that causes electrons to move away from the negative terminal and toward the positive terminal.

42. Electric Current 2
Battery Life
Batteries contain only a limited amount of the chemicals that react to produce chemical energy.
These reactions go on as the battery is used and the chemicals are changed into other compounds.
Once the original chemicals are used up, the chemical reactions stop and the battery is “dead.”

43. Electric Current 2
Resistance
The measure of how difficult it is for electrons to flow through a material is called resistance.
The unit of resistance is the ohm (Ω). Insulators generally have much higher resistance than conductors.

44. Electric Current 2
Resistance
As electrons flow through a circuit, they collide with the atoms and other electric charges in the materials that make up the circuit.

45. Electric Current 2
Resistance
These collisions cause some of the electrons’ electrical energy to be converted into thermal energy—heat—and sometimes into light.
The amount of electrical energy that is converted into heat and light depends on the resistance of the materials in the circuit.

46. Buildings Use Copper Wires
Electric Current 2
Buildings Use Copper Wires
Copper has low resistance and is one of the best electric conductors.
Less heat is produced as electric current flows in copper wires, compared to wires made of other materials.
As a result, copper wire is used in household wiring because the wires usually don’t become hot enough to cause fires.

47. Electric Current 2
Resistance of Wires
The electric resistance of a wire also depends on the length and thickness of the wire, as well as the material it is made from. The electric resistance of a wire increases as the wire becomes longer or as it becomes narrower.

48. Electric Current 2
Lightbulb Filaments
In a lightbulb, the filament is made of wire so narrow that it has a high resistance.

49. Electric Current 2
Lightbulb Filaments
When electric current flows in the filament, it becomes hot enough to emit light.
The filament is made of tungsten metal, which has a much higher melting point than most other metals.
This keeps the filament from melting at the high temperatures needed to produce light.

50. Section Check 2
Question 1
The flow of an electric charge is known as _______.
Answer
The answer is electric current. In solids, this is the flow of electrons.

51. Section Check 2
Question 2
Electric charges will flow continuously only through a closed conducting loop called a _______.
A. circuit
B. battery
C. conductor
D. filament

52. Section Check 2
Answer
The answer is A. However, if the path is not closed the circuit is broken and the electrons do not flow.

53. Section Check 2
Question 3
The amount of electrical energy converted to thermal energy in a wire increases as the _______ of the wire increases.
A. thickness
B. static charge
C. resistance
D. force

54. Section Check 2
Answer
The answer is C. Because the electric resistance of copper is low compared to other conductors, less electrical energy is converted into thermal energy as current flows in copper wires.

55. Controlling the Current
Electric Circuits 3
Controlling the Current
When you connect a conductor, such as a wire or a lightbulb, between the positive and negative terminals of a battery, electrons flow in the circuit.

56. Controlling the Current
Electric Circuits 3
Controlling the Current
The amount of current is determined by the voltage supplied by the battery and the resistance of the conductor.

57. Controlling the Current
Electric Circuits 3
Controlling the Current
To help understand this relationship, imagine a bucket with a hose at the bottom.
If the bucket is raised, water will flow out of the hose faster than before.
Increasing the height will increase the current.

58. Voltage and Resistance
Electric Circuits 3
Voltage and Resistance
Just as the water current increases when the height of the water increases, the electric current in a circuit increases as voltage increases.
If the diameter of the tube is decreased, resistance is greater and the flow of the water decreases.
In the same way, as the resistance in an electric circuit increases, the current in the circuit decreases.

59. Electric Circuits 3
Ohm’s Law
A nineteenth-century German physicist, Georg Simon Ohm, carried out experiments that measured how changing the voltage in a circuit affected the current.
He found a simple relationship among voltage, current, and resistance in a circuit that is now known as Ohm’s law.

60. Electric Circuits 3
Ohm’s Law
According to Ohm’s law, when the voltage in a circuit increases the current increases.
However, if the voltage in the circuit doesn’t change, then the current in the circuit decreases when the resistance is increased.

61. Series and Parallel Circuits
Electric Circuits 3
Series and Parallel Circuits
For current to flow, the circuit must provide an unbroken path for current to follow.
There are two kinds of basic circuits—series and parallel.

62. Electric Circuits 3
Wired in a Line
A series circuit is a circuit that has only one path for the electric current to follow.
If this path is broken, then the current no longer will flow and all the devices in the circuit stop working.
In a series circuit, electrical devices are connected along the same current path.

63. Electric Circuits 3
Wired in a Line
However, each new device that is added to the circuit decreases the current because each device has electrical resistance. The total resistance to the flow of electrons increases as each additional device is added to the circuit. By Ohm’s law, if the voltage doesn’t change, the current decreases as the resistance increases.

64. Electric Circuits 3
Branched Wiring
Houses, schools, and other buildings are wired using parallel circuits.
A parallel circuit is a circuit that has more than one path for the electric current to follow.
If one path is broken, electrons continue to flow through the other paths.

65. Electric Circuits 3
Branched Wiring
In a parallel circuit, the resistance in each branch can be different, depending on the devices in the branch.
The lower the resistance is in a branch, the more current flows in the branch.

66. Protecting Electric Circuits3
Protecting Electric Circuits
In a parallel circuit, the current that flows out of the battery or electric outlet increases as more devices are added.
As the current through the circuit increases, the wire heats up.
To keep the wire from becoming hot enough to cause a fire, the circuits in houses and other buildings have fuses or circuit breakers.

67. Protecting Electric Circuits3
Protecting Electric Circuits
When the current becomes larger than 15 A or 20 A, a piece of metal in the fuse melts or
a switch in the circuit breaker opens, stopping the current.

68. Protecting Electric Circuits3
Protecting Electric Circuits
The cause of the overload can then be removed, and the circuit can be used again by
replacing the fuse or resetting the circuit breaker.

69. Electric Circuits 3
Electric Power
The rate at which electrical energy is converted into other forms of energy is electric power.
In an electric appliance or in any electric circuit, the electric power that is used can be calculated from the electric power equation:

70. Electric Circuits 3
Electric Power
The electric power is equal to the voltage provided to the appliance times the current that flows into the appliance.
In the electric power equation, the SI unit of power is the watt.

71. Cost of Electric Energy
Electric Circuits 3
Cost of Electric Energy
Power is the rate at which energy is used, or the amount of energy that is used per second.
Using electrical energy costs money.

72. Cost of Electric Energy
Electric Circuits 3
Cost of Electric Energy
Electric companies generate electrical energy and sell it in units of kilowatt-hours to homes, schools, and businesses.
One kilowatt-hour,
kWh, is an amount of electrical energy equal to using 1 kW of power continuously for 1 h.

73. Electrical Safety 3 Electricity can have dangerous effects.
Electric Circuits 3
Electrical Safety
Electricity can have dangerous effects.
In 1997, electric shocks killed an estimated 490 people in the United States.

74. Electric Circuits 3
Electric Shock
You experience an electric shock when an electric current enters your body.
In some ways your body is like a piece of insulated wire.
The fluids inside your body are good conductors of current.
The electrical resistance of dry skin is much higher.
Skin insulates the body like the plastic insulation around a copper wire.

75. Electric Circuits 3
Electric Shock
A current can enter your body when you accidentally become part of an electric circuit. Whether you receive a deadly shock depends on the amount of current that flows into your body.

76. Electric Circuits 3
Lightning Safety
On average, more people are killed every year by lightning in the United States than by hurricanes or tornadoes.
If you are outside and can see lightning or hear thunder, take shelter indoors immediately.

77. Electric Circuits 3
Lightning Safety
If you cannot go indoors, you should take these precautions:
∙ Avoid high places and open fields.
∙ Stay away from tall objects such as
trees, flag poles, or light towers.

78. Electric Circuits 3
Lightning Safety
∙ Avoid object that conduct current such as bodies of water, metal fences, picnic shelters, and bleachers.

79. Section Check 3
Question 1
If the computer listed in the table was plugged into a 110-V outlet, how much current would flow in the computer?

80. Section Check 3
Answer
The current into the computer would be 3.2 A.

81. Question 2 Answer 3 What is Ohm’s Law?
Section Check 3
Question 2
What is Ohm’s Law?
Answer
Ohm’s law can be stated as V = IR, or voltage equals current (in amperes) times resistance (in ohms).

82. Section Check 3
Question 3
A circuit with more than a single path for the wiring to follow is called a _______.
Answer
It is a parallel circuit. The electrical outlets in your house are on a parallel circuit. If they weren’t, no appliance would work if any one appliance was turned off.

83. Chapter 22 Review 1 of 2
Two types of electric charges: positive and negative.
Like charges repel and unlike charges attract.
Electrons do not move freely through an insulator.
The human body is a good conductor of electricity.
Electric discharge: rapid movement of excess charge from one place to another.

84. Chapter 22 Review 2 of 2
Materials have electric resistance because electrons collide with atoms in the materials, releasing thermal energy and light.
Increase in resistance = increase in energy lost as heat and light.
Voltage increase = current increase.
Series circuit: electricity only has one path to follow.
Parallel circuits: are used in houses, schools, and other buildings.