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Chapter 21 Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields ► Ancient Greeks observed that magnetite, or lodestone, attracts iron. ► By 1150 AD, Chinese navigators used compasses with magnetized iron needles. ► In 1600, Gilbert published an explanation of the properties of magnets. ► (Magnet is derived from Magnesia region of ancient Greece, known for magnetite mines) 21.1 Magnets and Magnetic Fields Magnetism Magnetic Forces ► Magnetic force is the force a magnet exerts on another magnet, on iron or similar metals, or on moving charges. ► Magnetic forces act over a distance. ► Magnetic forces vary with distance. 21.1 Magnets and Magnetic Fields ► All Magnetism magnets have two magnetic poles, regions where the magnet’s force is strongest. ► One end of a magnet is the north pole; the other is the south pole. ► Like magnetic poles repel one other; opposite magnetic poles attract one another. 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism Magnetic Fields ►A magnetic field surrounds a magnet and can exert magnetic forces. ► A magnetic field, which is strongest near the poles, will either attract or repel another magnet that enters the field. ► Where lines are close together, the field is strong. ► Where lines are more spread out, the field is weak. 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism Magnetic Field Around Earth ► Earth is like a giant magnet surrounded by a magnetic field. 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields ► The area surrounding Earth that is influenced by this field is the magnetosphere. Magnetism 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields ► Earth’s Magnetism magnetic poles are not at the geographical poles. ► The magnetic south pole is at about 81o N latitude. ► The angle between true north and magnetic north is called declination. ► Magnetic declination varies with location. 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism Magnetic Materials ► Within the atom, electrons move around the nucleus. ► This movement and electron spin cause electrons to act like tiny magnets. ► In many materials, electron pairs spin in opposite directions and cancel out the magnetic effects. ► Most materials have weak magnetic fields. 21.1 Magnets and Magnetic Fields ► Unpaired Magnetism electrons produce magnetic fields. ► Usually the fields do not combine because of the arrangement of the atoms. ► In a few materials (Fe, Ni, Co) the unpaired electrons make a strong magnetic field. ► The fields combine to form magnetic domains. 21.1 Magnets and Magnetic Fields ► Magnetic Magnetism domains are regions that have a very large number of atoms with aligned magnetic fields. ► A ferromagnetic material can be magnetized because it contains magnetic domains. ► When a material is magnetized, most of its magnetic domains are aligned. 21.1 Magnets and Magnetic Fields Magnetism Nonmagnetized Materials ► The fact that a material is ferromagnetic does not mean it is a magnet. ► If the domains are aligned randomly, the magnetization is cancelled and the material is not a magnet. 21.1 Magnets and Magnetic Fields Magnetism 21.1 Magnets and Magnetic Fields Magnetism Magnetized Materials ►A nonmagnetized ferromagnetic material can be magnetized by placing it in a magnetic field. ► The applied magnetic field causes magnetic domains aligned with the field to grow larger. ► Such magnetization can be temporary. 21.1 Magnets and Magnetic Fields ► Removal Magnetism of the material from the magnetic field causes the domains to become randomly oriented again. ► In some materials, called permanent magnets, the domains stay aligned for a long time. ► They are not permanent because heat or jarring can realign the domains. 21.1 Magnets and Magnetic Fields ► If Magnetism a magnet is cut in half, each half will have its own north pole and south pole because the domains will still be aligned. ► No matter how many times a magnet is cut, each piece will have two different poles. ► A magnet can never have just a north pole or a south pole. 21.1 Magnets and Magnetic Fields Magnetism 21.2 Electromagnetism Magnetism 21.2 Electromagnetism ► The connection between electricity and magnetism was discovered accidentally by Oersted in 1820. ► During a demonstration using electricity, he noticed a nearby compass needle move. ► When he turned off the current, the needle moved back to its original position. 21.2 Electromagnetism Magnetism Electricity and Magnetism ► Electricity and magnetism are different aspects of the electromagnetic force. ► The electric force results from charged particles. ► The magnetic force usually results from the movement of electrons in an atom. ► Both forces are caused by electric charges. 21.2 Electromagnetism Magnetism Magnetic Fields Around Moving Charges ► Moving electric charges create a magnetic field. ► The moving charges may be the vibrating charges that produce an electromagnetic wave. ► They may also be the moving charges in a wire. 21.2 Electromagnetism ► Figure Magnetism 7 shows how to remember the direction of the magnetic field that is produced. ► Point the thumb of your right hand in the direction of the current (positive charge flow), your fingers curve in the direction of the magnetic field. ► The magnetic field lines form circles around a wire carrying a current. 21.2 Electromagnetism Magnetism 21.2 Electromagnetism Magnetism Forces Acting on Moving Charges ► An electric field exerts a force on an electric charge. ► The force is either in the same direction as the electric field or in the opposite direction, depending on whether it is a positive or negative charge. 21.2 Electromagnetism ►A Magnetism charge moving in a magnetic field will be deflected in a direction perpendicular to both the magnetic field and the velocity of the charge. ► If a current-carrying wire is in a magnetic field, the wire will be pushed in a direction perpendicular to both the field and the direction of the current. 21.2 Electromagnetism ► Reversing Magnetism 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. 21.2 Electromagnetism Magnetism Solenoids and Electromagnets ► In Figure 9B, 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 each turn of the wire. ► A coil of current-carrying wire that produces a magnetic field is a solenoid. 21.2 Electromagnetism Magnetism 21.2 Electromagnetism ► Placing Magnetism a ferromagnetic material inside the coil of the solenoid, increases the strength of the magnetic field. ► The magnetic field produced by the current causes the material inside the coil to become an electromagnet. ► Changing the current in an electromagnet controls the strength and direction of its magnetic field. 21.2 Electromagnetism ► The Magnetism strength of an electromagnet depends on the current in the solenoid, the number of loops in the coil in the solenoid, and the type of ferromagnetic core. 21.2 Electromagnetism Magnetism Electromagnetic Devices ► Electromagnetic devices can convert electrical energy into motion that can do work. ► Electromagnetic devices such as galvanometers, electric motors, and loudspeakers change electric energy into mechanical energy. 21.2 Electromagnetism Magnetism Galvanometers ►A galvanometer 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. 21.2 Electromagnetism ► When Magnetism there is a current in the solenoid’s coils, the resulting magnetic field attempts to align with the field of the permanent magnets. ► The greater the current, the more the solenoid rotates as shown by a pointer on the scale. 21.2 Electromagnetism Magnetism 21.2 Electromagnetism Magnetism Galvanometer 21.2 Electromagnetism Magnetism Electric Motors ► An electric motor is a device that uses an electromagnet to turn an axle. ► When current flows through a loop of wire, one side of the loop is pushed by the field of the permanent magnet. ► The other side of the loop is pulled. ► These forces rotate the loop. 21.2 Electromagnetism ► As Magnetism the loop rotates, the commutator connects with a different brush, reversing the current. ► As long as current flows, rotation continues. 21.2 Electromagnetism An Electric Motor Magnetism 21.2 Electromagnetism Magnetism Loudspeakers ►A loudspeaker contains a solenoid placed around one pole of a permanent magnet. ► 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. 21.2 Electromagnetism ► As Magnetism the coil moves, it causes a thin membrane to vibrate, producing sound waves that match the original sound. 21.2 Electromagnetism Loudspeaker Magnetism Microphone 21.3 Electrical Energy Generation and Transmission Magnetism 21.3 Electrical Energy Generation and Transmission ► All of the electrical energy used comes from the two aspects of the EM force. ► 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. 21.3 Electrical Energy Generation and Transmission ► Faraday Magnetism discovered EM induction in 1831. ► According to Faraday’s law, a voltage is induced in a conductor by a changing magnetic field. ► Changing the magnetic field through a coil of wire induces a voltage in the coil. ► A current is only produced if the coil is part of a complete circuit. 21.3 Electrical Energy Generation and Transmission ► Moving Magnetism the magnet in and out of the coil causes an electric current first in one direction and then in the other. ► Similar alternating current occurs if you move the coil and keep the magnet still. 21.3 Electrical Energy Generation and Transmission Magnetism Generators ► Most of the electrical energy used 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. 21.3 Electrical Energy Generation and Transmission Magnetism AC Generators ► AC generators produce alternating current, in which charges flow in one direction and then in the other. ► While a motor converts electrical energy into mechanical energy, a generator does the opposite. See figure 14. ► A wire coil in the generator is attached to slip rings. 21.3 Electrical Energy Generation and Transmission AC Generator Magnetism 21.3 Electrical Energy Generation and Transmission ► Slip Magnetism rings are in contact with brushes that are attached to a circuit. ► As a loop of wire is rotated, the magnetic field induces a current in the wire. ► The current flows in one direction, and then when the loop turns halfway, the current reverses direction. 21.3 Electrical Energy Generation and Transmission Magnetism DC Generators ►A DC generator produces a direct current. ► A commutator replaces the slip rings. ► As the loop rotates, one side of the commutator contacts a brush. ► When the loop rotates, current is induced in the other direction, but the other side of the commutator contacts that brush, so current only flows in one direction. 21.3 Electrical Energy Generation and Transmission Magnetism Transformers ►A transformer is a device that increases or decreases voltage and current of two linked AC circuits. ► A transformer works only with AC because the alternating current induces a constantly changing magnetic field. 21.3 Electrical Energy Generation and Transmission ►A Magnetism 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. 21.3 Electrical Energy Generation and Transmission Magnetism Why are Transformers Needed? ► Over long distance, the resistance of the wire causes large losses of power due to heat. ► Power losses can be reduced by using lower current transmitted at a higher voltage. 21.3 Electrical Energy Generation and Transmission Magnetism Changing Voltage and Current ► 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. 21.3 Electrical Energy Generation and Transmission ► The Magnetism 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. 21.3 Electrical Energy Generation and Transmission Magnetism Types of Transformers ►A step-down transformer decreases voltage and increases current. ► A step-up transformer increases voltage and decreases current. 21.3 Electrical Energy Generation and TransmissionFigure 16 Magnetism 21.3 Electrical Energy Generation and TransmissionFigure 16 Magnetism 21.3 Electrical Energy Generation and Transmission Magnetism Electrical Energy for Your Home ► Most of the electrical energy (52%) generated in the US is produced using coal as an energy source. ► Other sources include nuclear energy (20%), natural gas (16%), hydroelectric (7%), wind (2%), and petroleum (3%). 21.3 Electrical Energy Generation and Transmission Magnetism 21.3 Electrical Energy Generation and Transmission ►A Magnetism turbine is a device with fanlike blades that turn when pushed. ► Burning fossil fuels or nuclear reactions are used to heat water to produce steam that spins a turbine. ► To produce electrical energy, the turbine turns the coils of a generator or spins magnets around the coils of wire. 21.3 Electrical Energy Generation and Transmission Magnetism