The green magnet and lower red magnet attract each other.

The green magnet and lower red magnet attract each other.
The lower red magnet and the yellow magnet repel each other.

Magnetic Forces How do magnetic poles interact? Like magnetic poles repel one another, and opposite magnetic poles attract one another.

Magnetic Forces Magnetic force is the force a magnet exerts on another magnet, on iron or a similar metal, or on moving charges. Magnetic force is one aspect of electromagnetic force. Magnetic forces, like electric forces, act over a distance. Magnetic force, like electric force, varies with distance.

Magnetic Forces All magnets have two magnetic poles, regions where the magnet’s force is strongest. One end of a magnet is its north pole. The other end is its south pole. The direction of the magnetic force between two magnets depends on how the poles face.

Magnetic Fields How can a magnetic field affect a magnet that enters the field? A magnetic field, which is strongest near a magnet’s poles, will either attract or repel another magnet that enters the field.

Magnetic Fields A magnetic field surrounds a magnet and can exert magnetic forces. Magnetic field lines begin near the north pole and extend toward the south pole. The arrows on the field lines indicate what direction a compass needle would point at each point in space. Where lines are close together, the field is strong. Where lines are more spread out, the field is weak.

Magnetic Fields A magnetic field surrounds every magnet. Iron filings reveal the field lines, which start near the north pole and extend toward the south pole.

Magnetic Fields Magnetic Fields Around Magnets You can use iron filings to visualize how magnetic fields of two magnets interact. The strength of the magnetic field in a particular area is indicated by the density of iron filings.

Magnetic Fields When like poles of two magnets come together, the magnets repel each other. When opposite poles of magnets come together, the magnets attract each other.

Magnetic Fields Magnetic Field Around Earth Earth is like a giant magnet surrounded by a magnetic field. The area surrounding Earth that is influenced by this field is the magnetosphere. A compass points north because it aligns with Earth’s magnetic field.

Magnetic Fields Earth is surrounded by magnetic field lines. These lines are densest at the poles.

Magnetic Materials Why are some materials magnetic while others are not? A magnetic domain is a region that has a very large number of atoms with aligned magnetic fields. When a material is magnetized, most of its magnetic domains are aligned.

Magnetic Materials A property of electrons called “spin” causes electrons to act like tiny magnets. In many materials, each electron is paired with another having an opposite spin so magnetic effects mostly cancel each other. Unpaired electrons in some materials produce magnetic fields that don’t combine because of the arrangement of the atoms.

Magnetic Materials In a few materials, such as iron, nickel, and cobalt, the unpaired electrons make a strong magnetic field. The fields combine to form magnetic domains. A ferromagnetic material, such as iron, can be magnetized because it contains magnetic domains.

Magnetic Materials Nonmagnetized Materials The fact that a material is ferromagnetic does not mean it is a magnet. If the domains of a ferromagnetic material are aligned randomly, it is not a magnet.

Magnetic Materials Magnetized Materials
If you place a nonmagnetized ferromagnetic material in a magnetic field, it will become a magnet when the domains are aligned. Magnetization can be temporary. If the material is moved away from the magnet, the magnetic domains become random. In some ferromagnetic materials, the domains stay aligned for a long time. These materials are called permanent magnets.

Magnetic Materials A magnetic field can magnetize ferromagnetic materials. Before magnetization, domains are random. Domains aligned with the field grow during magnetization. Unaligned domains can shrink.

Magnetic Materials If you cut a magnet in half, each half will have its own north pole and south pole because the domains will still be aligned. A magnet can never have just a north pole or just a south pole.

Assessment Questions Where does the magnetic field of a magnet have the strongest effect on another magnet? the north pole the south pole both poles equally midway between the two poles

Assessment Questions Where does the magnetic field of a magnet have the strongest effect on another magnet? the north pole the south pole both poles equally midway between the two poles ANS: C

Assessment Questions How are the magnetic field lines drawn to show the interaction of two bar magnets that are lined up with their north poles near one another? Field lines begin at the north pole of each magnet and extend to the south pole of the other magnet. Field lines begin at each magnet’s north pole and extend toward its south pole. Field lines extend from the north pole of one magnet to the north pole of the other magnet. Field lines cannot be drawn because the magnetic forces cancel one another.

Assessment Questions How are the magnetic field lines drawn to show the interaction of two bar magnets that are lined up with their north poles near one another? Field lines begin at the north pole of each magnet and extend to the south pole of the other magnet. Field lines begin at each magnet’s north pole and extend toward its south pole. Field lines extend from the north pole of one magnet to the north pole of the other magnet. Field lines cannot be drawn because the magnetic forces cancel one another. ANS: B

Assessment Questions Why does a compass not point exactly toward the geographic north pole? Earth’s magnetic field is constantly changing due to effects of the solar wind. The magnetic pole is near but not exactly at the geographic pole. Earth’s magnetic field lines are too broad for a compass point exactly toward the pole. Daily variations in the magnetic field mean that compasses are not very accurate.

Assessment Questions Why does a compass not point exactly toward the geographic north pole? Earth’s magnetic field is constantly changing due to effects of the solar wind. The magnetic pole is near but not exactly at the geographic pole. Earth’s magnetic field lines are too broad for a compass point exactly toward the pole. Daily variations in the magnetic field mean that compasses are not very accurate. ANS: B

Assessment Questions What happens to a permanent magnet if its magnetic domains lose their alignment? The magnetic field reverses direction. It loses its magnetic field. It has several north poles and several south poles. It is no longer a ferromagnetic material.

Assessment Questions What happens to a permanent magnet if its magnetic domains lose their alignment? The magnetic field reverses direction. It loses its magnetic field. It has several north poles and several south poles. It is no longer a ferromagnetic material. ANS: B

In 1820 Hans Oersted discovered how magnetism and electricity are connected. A unit of measure of magnetic field strength, the oersted, is named after him.

Electricity and Magnetism
How can an electric charge create a magnetic field? Moving electric charges create a magnetic field.

Electricity and magnetism are different aspects of a single force known as the electromagnetic force. The electric force results from charged particles. The magnetic force usually results from the movement of electrons in an atom.

Magnetic Fields Around Moving Charges Moving charges create a magnetic field. Moving charges may be the vibrating charges that produce an electromagnetic wave or the moving charges in a wire. Magnetic field lines form circles around a straight wire carrying a current.

If you point the thumb of your right hand in the direction of the current, your fingers curve in the direction of the magnetic field. Direction of current Current-carrying wire Direction of electron flow Direction of magnetic field

Forces Acting on Moving Charges A magnetic field exerts a force on a moving charge. A charge moving in a magnetic field is deflected in a direction perpendicular to both the field and to the velocity of the charge. A current-carrying wire in a magnetic field will be pushed in a direction perpendicular to both the field and the direction of the current.

Reversing the direction of the current will still cause the wire to be deflected, but in the opposite direction. If the current is parallel to the magnetic field, the force is zero and there is no deflection.

Force deflecting the charge
Electricity and Magnetism A moving positive charge is deflected at a right angle to its motion by a magnetic field. Force deflecting the charge Velocity of charge

Solenoids and Electromagnets
How is an electromagnet controlled? Changing the current in an electromagnet controls the strength and direction of its magnetic field.

If a current-carrying wire has a loop in it, the magnetic field in the center of the loop points right to left through the loop. Multiple loops in the wire make a coil. The magnetic fields of the loops combine so that the coiled wire acts like a bar magnet.

The field through the center of the coil is the sum of the fields from all the turns of the wire. A coil of current-carrying wire that produces a magnetic field is called a solenoid.

The magnetic field lines around a solenoid are like those of a bar magnet.

If a ferromagnetic material, such as an iron rod is placed inside the coil of a solenoid, the strength of the magnetic field increases. The magnetic field produced by the current causes the iron rod to become a magnet. An electromagnet is a solenoid with a ferromagnetic core. The current can be used to turn the magnetic field on and off.

The strength of an electromagnet depends on the current in the solenoid, the number of loops in the coil, and the type of core. The strength of an electromagnet can be increased using the following methods. Increase the current flowing through the solenoid. Increase the number of turns. Use cores that are easily magnetized.

Electromagnetic Devices
How do galvanometers, electric motors, and loudspeakers work? Electromagnetic devices such as galvanometers, electric motors, and loudspeakers change electrical energy into mechanical energy.

Electromagnets can convert electrical energy into motion that can do work. A galvanometer measures current in a wire through the deflection of a solenoid in an external magnetic field. An electric motor uses a rotating electromagnet to turn an axle. A loudspeaker uses a solenoid to convert electrical signals into sound waves.

Galvanometers A galvanometer is a device that uses a solenoid to measure small amounts of current. A solenoid is attached to a spring and is free to rotate about an iron core. The solenoid is placed between the poles of two permanent magnets.

A current in the solenoid’s coils produces a magnetic field that attempts to align with the field of the permanent magnets. The greater the current, the more the solenoid rotates.

A galvanometer uses an electromagnet to move a pointer. One common application is in an automobile gas gauge. The pointer indicates the current in the wire. The wire is connected to a sensor in the gas tank.

Electric Motors An electric motor is a device that uses an electromagnet to turn an axle. A motor has many loops of wire around a central iron core. In the motor of an electric appliance, the wire is connected to an electrical circuit in a building.

In this motor, a battery supplies current to a loop of wire through the commutator. When current flows through a loop of wire, the field of the permanent magnet pushes one side of the loop. The other side of the loop is pulled. These forces rotate the loop. If there were no commutator ring, the coil would come to rest. As the loop turns, each C-shaped half of the commutator connects with a different brush, reversing the current. The forces now change direction, so the coil continues to rotate. As long as current flows, rotation continues.

Loudspeakers A loudspeaker contains a solenoid placed around one pole of a permanent magnet. The current in the wires entering the loudspeaker changes direction and increases or decreases.

The changing current produces a changing magnetic field in the solenoid coil. The magnetic force exerted by the permanent magnet moves the coil back and forth. As the coil moves, it causes a thin membrane to vibrate, producing sound waves that match the original sound.

Assessment Questions A charged particle is moving across a plane from left to right as it enters a magnetic field that runs from top to bottom. How will the motion of the particle be changed as it enters the magnetic field? It will accelerate. It will deflect either up or down on the plane. It will deflect perpendicular to the plane. Its motion will not be affected.

Assessment Questions A charged particle is moving across a plane from left to right as it enters a magnetic field that runs from top to bottom. How will the motion of the particle be changed as it enters the magnetic field? It will accelerate. It will deflect either up or down on the plane. It will deflect perpendicular to the plane. Its motion will not be affected. ANS: C

Assessment Questions Which change will increase the strength of an electromagnet made by wrapping a conductive wire around an iron nail? reversing the direction of current flow replacing the nail with a wooden dowel increasing the number of coils of wire around the nail using a longer nail

Assessment Questions Which change will increase the strength of an electromagnet made by wrapping a conductive wire around an iron nail? reversing the direction of current flow replacing the nail with a wooden dowel increasing the number of coils of wire around the nail using a longer nail ANS: C

A loudspeaker uses a magnet to cause which energy conversion?
Assessment Questions A loudspeaker uses a magnet to cause which energy conversion? mechanical energy to magnetic energy electrical energy to mechanical energy electrical energy to magnetic energy mechanical energy to electrical energy

A loudspeaker uses a magnet to cause which energy conversion?
Assessment Questions A loudspeaker uses a magnet to cause which energy conversion? mechanical energy to magnetic energy electrical energy to mechanical energy electrical energy to magnetic energy mechanical energy to electrical energy ANS: B

Assessment Questions The motion of an electric charge creates an electrical field. True False

Assessment Questions The motion of an electric charge creates an electrical field. True False ANS: F, a magnetic

Photographs of large cities, such as Seattle, Washington, are visible reminders of how much people rely on electrical energy.

Generating Electric Current
How is voltage induced in a conductor? According to Faraday’s law, a voltage is induced in a conductor by a changing magnetic field.

A magnetic field can be used to produce an electric current. Electromagnetic induction is the process of generating a current by moving an electrical conductor relative to a magnetic field. Changing the magnetic field through a coil of wire induces a voltage in the coil. A current results if the coil is part of a complete circuit.

When a magnet is placed inside a coil of wire attached to a galvanometer, the galvanometer will detect no current if the magnet is not moving. If the magnet is quickly moved out of the coil, the current flows briefly and then immediately drops back to zero. Moving the magnet in and out of the coil causes an electric current first in one direction and then in the other.

According to Faraday’s law, the moving magnetic field induces a current in the coil. Movement of magnet Coil Galvanometer shows that the current is flowing.

Generators Name two types of generators. The two types of generators are AC generators and DC generators.

Generators Most of the electrical energy used in homes and businesses is produced at large power plants using generators. A generator is a device that converts mechanical energy into electrical energy by rotating a coil of wire in a magnetic field. Electric current is generated by the relative motion of a conducting coil in a magnetic field.

Generators AC Generators An AC generator produces alternating current, in which charges flow first in one direction and then in the other direction. The generator looks very similar to an electric motor. While a motor converts electrical energy into mechanical energy, a generator does the opposite.

Generators In a simple AC generator, an external force rotates the loop of wire in the magnetic field. This induces a current in the wire. Wire loop Slip rings Direction in which the loop is turned

Generators A wire coil in the generator is attached to metal bands called slip rings. The slip rings are in contact with metal brushes that are in turn attached to a circuit. As the loop of wire is rotated, the magnetic field induces a current in the wire, which reverses direction every half rotation.

Generators Small generators provide power in areas that are not served by power companies or provide electrical energy during a power outage.

A DC generator produces a direct current.
Generators DC Generators A DC generator produces a direct current. Its design is very much like the design of an AC generator except that a commutator replaces the slip rings. As opposite sides of the commutator touch the brush, the current that leaves the generator flows in only one direction.

Transformers How can a transformer change voltage and current? A transformer is a device that increases or decreases the voltage and current of two linked AC circuits. A transformer changes voltage and current by inducing a changing magnetic field in one coil. This changing field then induces an alternating current in a nearby coil with a different number of turns.

Transformers Electrical energy from power plants is transmitted through power lines at voltages too high to be used safely in homes. The voltage must be changed, or transformed. A series of transformers changes high-voltage current in power lines into 240-volt current.

Transformers Why Transformers Are Needed Early power plants used DC generators because the power plants were close to the customers. Over long distances, the resistance of transmission wires causes large losses of power, which can be reduced by using lower current and higher voltage. Only AC voltage and current can be transformed.

Changing Voltage and Current
Transformers Changing Voltage and Current A transformer has two sets of coils wrapped around a ring-shaped iron core. When there is an alternating current in the primary coil, the current creates a changing magnetic field in the iron core. Because the iron core is also inside the secondary coil, the changing field induces an alternating current in the secondary coil.

Transformers The number of turns in the primary and secondary coils determines the voltage and current. To calculate the voltage, divide the number of turns in the secondary coil by the number of turns in the primary coil. The result is the ratio of the output voltage to the input voltage.

Transformers Transformers are very efficient because very little energy is lost as heat. Assuming 100% efficiency, the power (I × V) must be the same in the primary and secondary coils. If voltage increases in the secondary coil, the current must decrease in the same ratio.

Transformers Types of Transformers A step-down transformer decreases voltage and increases current. If the primary coil has 400 turns, the secondary coil has 100 turns, and the input voltage in the primary coil is 120 volts, then the output voltage is reduced to 30 volts.

Transformers A step-up transformer increases voltage and decreases current. If the primary coil has 100 turns, the secondary coil has 400 turns, and the input voltage is 20 volts, the output voltage is 80 volts.

Transformers Transformers, such as those at substations of power plants, change voltage. Step-up Transformer High voltage Low voltage 100 turns 400 turns AC Source Secondary coil 100 turns Primary coil Step-down Transformer Low voltage Soft iron core High voltage 400 turns AC Source

Electrical Energy for Your Home
What are some sources of electrical energy in the United States? Most of the electrical energy generated in the United States is produced using coal as an energy source. Some other sources are water (hydroelectric), nuclear energy, wind, natural gas, and petroleum.

A turbine is a device with fanlike blades that turn when pushed, for example, by water or steam. Burning fossil fuels or nuclear reactions can heat water to produce steam that spins a turbine. Water from a reservoir behind a dam can also turn a turbine. To produce electrical energy, the turbine may turn the coils of a generator, or it may spin magnets around the coils of wire.

A turbine turns the magnet inside the coil of a generator.

A power plant transmits electrical energy at hundreds of thousands of volts. After the current passes travels through high-voltage transmission lines, the voltage is stepped down at a substation, to a few thousand volts. The electrical energy is then distributed and stepped down to between 220 and 240 volts. Appliances like an electric stove use 240-volt circuits. Most other appliances in the home use 120 volts.

Voltage is increased for long-distance transmission and then decreased near homes, schools, and businesses.

In a DC generator, the commutator
Assessment Questions In a DC generator, the commutator generates an electric current. converts an alternating current to a direct current. reduces the voltage. reverses the direction of the direct current.

In a DC generator, the commutator
Assessment Questions In a DC generator, the commutator generates an electric current. converts an alternating current to a direct current. reduces the voltage. reverses the direction of the direct current. ANS: B

Assessment Questions A transformer has 400 turns on the primary coil and 1600 turns on the secondary coil. What is the output voltage if the input is 1,000 volts? 250 V 500 V 2,000 V 4,000 V

Assessment Questions A transformer has 400 turns on the primary coil and 1600 turns on the secondary coil. What is the output voltage if the input is 1,000 volts? 250 V 500 V 2,000 V 4,000 V ANS: D

Assessment Questions Which property would you want to increase in transmitting electrical energy as efficiently as possible over long distances? current voltage resistance insulation

Assessment Questions Which property would you want to increase in transmitting electrical energy as efficiently as possible over long distances? current voltage resistance insulation ANS: B

Assessment Questions In electromagnetic induction, an electric current is induced by the motion of a magnet relative to a magnetic field. True False

Assessment Questions In electromagnetic induction, an electric current is induced by the motion of a magnet relative to a magnetic field. True False ANS: F, conductor

The green magnet and lower red magnet attract each other.

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The green magnet and lower red magnet attract each other.

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