# Chapter 21 Magnetism 21.1 Magnets and Magnetic Fields

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• Chapter 21Magnetism21.1 Magnets and Magnetic Fields

• 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.Magnetic Forces

• All magnets have two magnetic poles, regions where the magnets force is strongest. One end of a magnet is its north pole.The other end is its south pole. 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 EarthEarth 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 Earths magnetic field. Magnetic Fields

• 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 dont 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

• Magnetized MaterialsIf 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

• 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.Magnetic Materials

• Assessment QuestionsWhere does the magnetic field of a magnet have the strongest effect on another magnet? the north polethe south poleboth poles equallymidway 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 magnets 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? Earths magnetic field is constantly changing due to effects of the solar wind.The magnetic pole is near but not exactly at the geographic pole.Earths 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 Magnetism21.2 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.

• 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. Electricity and Magnetism

• Magnetic Fields Around Moving ChargesMoving charges create a magnetic field.Magnetic field lines form circles around a straight wire carrying a current.Electricity and Magnetism

• 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 electron flow Direction of current Current-carrying wireDirection of magnetic field

• Forces Acting on Moving ChargesA 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.Electricity and Magnetism

• 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.Solenoids and Electromagnets

• 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.Solenoids and Electromagnets

• 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

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

• Electric MotorsAn 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

• LoudspeakersA 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.Electromagnetic Devices

• Assessment QuestionsA 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 QuestionsWhich change will increase the strength of an electromagnet made by wrapping a conductive wire around an iron nail? reversing the direction of current flowreplacing the nail with a wooden dowelincreasing the number of coils of wire around the nailusing a longer nail

• Assessment QuestionsA loudspeaker uses a magnet to cause which energy conversion?mechanical energy to magnetic energyelectrical energy to mechanical energyelectrical energy to magnetic energymechanical energy to electrical energy

• Assessment Questions4.The motion of an electric charge creates an electrical field. True False

• Chapter 21 Magnetism21.3 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.

• 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 GeneratorsAn 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

• DC GeneratorsA 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 curr

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