21.1 Magnets and Magnetic Fields Chapter 21 Magnetism 21.1 Magnets and Magnetic Fields

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

One end of a magnet is its north pole. 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.

A magnetic field surrounds a magnet and can exert magnetic forces 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 Field Around Earth 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 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.

The fields combine to form magnetic domains. 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 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.

A magnet can never have just a north pole or just a south pole. 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.

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

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.

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.

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.

Chapter 21 Magnetism 21.2 Electromagnetism

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.

The electric force results from charged particles. Electricity and Magnetism 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 Electricity and Magnetism Magnetic Fields Around Moving Charges Moving charges create a magnetic field. 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.

Electricity and Magnetism 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.

Solenoids and Electromagnets 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.

Solenoids and Electromagnets 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.

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.

Increase the current flowing through the solenoid. Solenoids and Electromagnets 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.

An electric motor uses a rotating electromagnet to turn an axle. Electromagnetic Devices 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.

Electromagnetic Devices Galvanometers A galvanometer is a device that uses a solenoid to measure small amounts of current.

Electromagnetic Devices 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.

Electromagnetic Devices 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.

It will deflect either up or down on the plane. 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.

reversing the direction of current flow 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

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

Assessment Questions 4. The motion of an electric charge creates an electrical field. True False

21.3 Electrical Energy Generation and Transmission Chapter 21 Magnetism 21.3 Electrical Energy Generation and Transmission

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.

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.

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.

The result is the ratio of the output voltage to the input voltage. 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 Types of Transformers A step-down transformer decreases voltage and increases current.

A step-up transformer increases voltage and decreases current. Transformers A step-up transformer increases voltage and decreases current.

Water from a reservoir behind a dam can also turn a turbine. Electrical Energy for Your Home 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 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.

In a DC generator, the commutator generates an electric current. 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.

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