chapter: electricity table of contents section 3: electric circuitselectric circuits section 1:...

Chapter: Electricity

Table of ContentsTable of Contents

Section 3: Electric Circuits

Section 1: Electric Charge

Section 2: Electric Current

• All solids, liquids, and gases are made of tiny particles called atoms.

• Atoms are made of even smaller particles called protons, neutrons, and electrons.

Electricity

Electric ChargeElectric Charge

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• 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.

Electricity


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• 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.

Positive and Negative Charge


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• 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.



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• An atom becomes negatively charged when it gains extra electrons.

• If an atom loses electrons it becomes positively charged.



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• A positively or negatively charged atom is called an ion (I ahn).

• 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.

Electrons Move in Solids


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Electrons Move in Solids


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• In solids, static charge is due to the transfer of electrons between objects.

• Protons cannot be removed easily from the nucleus of an atom.

• 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.

Ions Move in Solutions


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• These ions now are able to carry electric energy.

• All charged objects exert an electric force on each other.

• The electric force between two charges can be attractive or repulsive.

Electric Forces


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• 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.

Electric Forces


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• The electric force between two charged objects increases if the amount of charge on at least one of the objects increases.

• 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.

Electric Fields


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• 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.

• 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.

Electric Fields


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• 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.

Insulators and Conductors


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• 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.

• 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.



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• Electrons move easily in the conductor but do not move easily through the plastic insulation.

• 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.

Metals and Conductors


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• 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.

• Has this ever happened to you?

• You walk across a carpet and as you reach for a metal doorknob, you feel an electric shock.

Induced Charge


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• Maybe you even see a spark jump between tour fingertip and the doorknob.

Induced Charge


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• This rapid movement of excess charge from one place to another is an electric discharge.

• 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.

Induced Charge


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• 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.

• 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.

Grounding


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• One way to avoid the damage caused by electric discharges is to make the excess charges flow harmlessly into Earth’s surface.

• 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.

Grounding


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11Section CheckSection Check

Question 1

The two types of electric charge are _______ and _______.

Answer

The answer is positive and negative.


Question 2

An imbalanced of electric charge on an object is called a(n) _______.

A. electric dischargeB. electric currentC. insulatorD. static charge


Answer

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


Question 3

Like charges _______ while unlike charges _______.


Answer

Like charges repel and unlike charges attract.

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.

Electric CurrentElectric Current

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Flow of Charge

• In liquids, the flowing charges are ions.


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• Electric current is measured in units of amperes (A).

• A model for electric current is flowing water.

A Model for a Simple Circuit

• The gravitational potential energy of water is increased when a pump raises the water above Earth.


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• Electric charges will flow continuously onlythrough a closed conducting loop called a circuit.

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.


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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.


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Electric Circuit• They move away from the negative battery

terminal and toward the positive terminal.


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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.


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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).


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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.


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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.


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Batteries


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• As these electrons move toward the positive battery terminal, this electric potential energy is transformed into other forms of energy.

Batteries• For the alkaline battery, the two terminals are

separated by a moist paste.


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• Chemical reactions in the moist paste cause the negative terminal to become negatively charged and the positive terminal to become positively charged.

Batteries


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• This produces the electric field in the circuit that causes electrons to move away from the negative terminal and toward the positive terminal.

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.


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• Once the original chemicals are used up, the chemical reactions stop and the battery is “dead.”

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.


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Resistance• As electrons flow through a circuit, they

collide with the atoms and other electric charges in the materials that make up the circuit.


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Resistance

• These collisions cause some of the electrons’ electrical energy to be converted into thermal energy—heat—and sometimes into light.


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• The amount of electrical energy that is converted into heat and light depends on the resistance of the materials in the circuit.

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.


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• As a result, copper wire is used in household wiring because the wires usually don’t become hot enough to cause fires.

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.


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Lightbulb Filaments

• In a lightbulb, the filament is made of wire so narrow that it has a high resistance.


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Lightbulb Filaments


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• 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.


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.


Question 2

Electric charges will flow continuously only through a closed conducting loop called a _______.

A. circuitB. batteryC. conductorD. filament


Answer

The answer is A. However, if the path is not closed the circuit is broken and the electrons do not flow.


Question 3

The amount of electrical energy converted to thermal energy in a wire increases as the _______ of the wire increases.

A. thicknessB. static chargeC. resistanceD. force


AnswerThe 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.

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.

Electric CircuitsElectric Circuits

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Controlling the Current• The amount of

current is determined by the voltage supplied by the battery and the resistance of the conductor.


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Controlling the Current• To help understand this relationship, imagine

a bucket with a hose at the bottom.


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• If the bucket is raised, water will flow out of the hose faster than before.

• Increasing the height will increase the current.

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.


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• 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.

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.


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• He found a simple relationship among voltage, current, and resistance in a circuit that is now known as Ohm’s law.

Ohm’s Law

• According to Ohm’s law, when the voltage in a circuit increases the current increases.


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• However, if the voltage in the circuit doesn’t change, then the current in the circuit decreases when the resistance is increased.

Series and Parallel Circuits

• For current to flow, the circuit must provide an unbroken path for current to follow.


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• There are two kinds of basic circuits—series and parallel.

Wired in a Line

• A series circuit is a circuit that has only one path for the electric current to follow.


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• 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.

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.


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Branched Wiring

• Houses, schools, and other buildings are wired using parallel circuits.


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• 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.

Branched Wiring


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• 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.

Protecting Electric Circuits

• In a parallel circuit, the current that flows out of the battery or electric outlet increases as more devices are added.


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• 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.



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• When the current becomes larger than 15 A or 20 A, a piece of metal in the fuse melts ora switch in the circuit breaker opens, stopping the current.



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• The cause of the overload can then be removed, and the circuit can be used again byreplacing the fuse or resetting the circuit breaker.

Electric Power• The rate at which electrical energy is

converted into other forms of energy is electric power.


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• In an electric appliance or in any electric circuit, the electric power that is used can be calculated from the electric power equation:

Electric Power• The electric power is

equal to the voltage provided to the appliance times the current that flows into the appliance.


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• In the electric power equation, the SI unit of power is the watt.

Cost of Electric Energy

• Power is the rate at which energy is used, or the amount of energy that is used per second.


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• Using electrical energy costs money.

Cost of Electric Energy


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• 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.

Electrical Safety • Electricity can have dangerous effects.


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• In 1997, electric shocks killed an estimated 490 people in the United States.

Electric Shock• You experience an electric shock when an

electric current enters your body.


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• 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.

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.


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Lightning Safety

• On average, more people are killed every year by lightning in the United States than by hurricanes or tornadoes.


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• If you are outside and can see lightning or hear thunder, take shelter indoors immediately.

Lightning Safety


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• 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.

Lightning Safety


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∙ Avoid object that conduct current such as bodies of water, metal fences, picnic shelters, and bleachers.

Section CheckSection Check

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If the computer listed in the table was plugged into a 110-V outlet, how much current would flow in the computer?

Question 1


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The current into the computer would be 3.2 A.

Answer


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What is Ohm’s Law?

Question 2

Ohm’s law can be stated as V = IR, or voltage equals current (in amperes) times resistance (in ohms).

Answer


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A circuit with more than a single path for the wiring to follow is called a _______.

Question 3

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.

Answer

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.

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.

chapter: electricity table of contents section 3: electric circuitselectric circuits section 1:...

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