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MAGNETISM Magnets and Electromagnets MAGNETS • A special stone first discovered more than 2000 years ago in Greece, in a region called Magnesia; it attracted iron; they called it magnetite hence the magnet name. • About 1000 years ago they noticed that a hanging magnet always pointed to the North Star or Lodestar. So another name for naturally occurring magnets is lodestone. MAGNETISM • Magnetism is the force of attraction or repulsion due to electron motion. MAGNETISM • Magnetic Poles – the ends of the magnet, area where the magnetic effect is the strongest. • If a bar magnet is suspended by a thread or string, it will align itself so that one strong end points north and the other points south, hence the names for the “North” and “South” poles of the magnet. MAGNETISM • Like poles of separate magnets repel – push away from – each other • Unlike poles attract each other. MAGNETISM • The moving electron acts as a mini electrical charge and therefore has a magnetic field associated with it. • Groups of atoms with magnetic poles aligned are called magnetic domains. • Materials with most of the domains lined up are considered magnetized. • The more domains that are aligned, the stronger the magnetic field in the material. MAGNETISM MAGNETISM • When domains are randomly arranged, their forces cancel each other out and there is no net magnetic effect. • When domains have their magnetic effect in alignment, their forces are additive and create a strong magnetic effect. MAGNETS • If you snap a magnet in half, the inside pieces become the opposite poles: DRAW THE DOMAINS MAGNETISM AND ELECTRICITY • A connection between electricity and magnetism was discovered (accidentally) over 100 years ago. • Hans Christian Oersted discovered in 1820 that an electric current near a compass causes the compass needle to be deflected. • Oersted's experiment showed that every electric current has a magnetic field surrounding it. • A magnetic field causes a magnet to align in the direction of the opposite pole. Hans Christian Oersted (1777-1851) MAGNETISM AND ELECTRICITY • See how a magnetic field compares to an electric field. ELECTRIC FIELD MAGNETIC FIELD MAGNETIC FIELD • Field lines were introduced by Michael Faraday , who named them “lines of force.’’ . All magnetic fields originate from moving electric charges. • A magnetic field is the magnetic effect of electric currents and magnetic materials. A magnetic field appears only when relative motion is present between an electric charge and an observer. Electric and magnetic fields are different aspects of a single electromagnetic field. MAGNETIC FIELD • A magnetic field is that region around a magnet that is affected by the magnet. • Strongest at the poles, the Force forms lines that go out of the North Pole and wrap back around to enter in at the South Pole. THE GIANT MAGNET • The Earth’s core is Iron so Earth is a giant magnet. DRAW THE MAGNETIC FIELD LINES MAGNETIC FIELDS LAB INTRODUCTION: A magnetic material is made of small regions called magnetic domains. These magnetic domains can be pictured as small bar magnets. When the domains are aligned, as shown in Figure 1, the magnetic fields of the domain add together. This causes the material to be surrounded by a magnetic field. The magnetic field surrounding a magnet exerts a magnetic force on other magnets and magnetic materials. The direction of the magnetic field around a magnet can be represented by magnetic field lines. Magnetic field lines always begin on the north pole of a magnet and end on the south pole. Magnetic field lines are closer together where the magnetic field is stronger, and farther apart where the field is weaker. ELECTROMAGNETISM • An electromagnet consists of an iron core placed inside a wire coil. • The magnetic field strength of a wire coil carrying an electric current increases in direct proportion to the number of turns of the coil. • Current gives rise to magnetic fields, just as electric charge gave rise to electric fields. ELECTROMAGNETISM • The basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field. • Electricity through the coiled wire creates a magnetic field which induces magnetism in the iron core. The domains in the core become aligned with the magnetic field of the coil. ELECTROMAGNETISM ELECTROMAGNETISM • An electromagnet can be used to move large quantities of metal. • When the current is on, the magnet will pick up the metal. • When you want to drop it, you turn off the power and the electromagnet is disabled and the metal drops. ANIMATION ELECTROMAGNETISM • A rotating electromagnet is used in an electric motor to convert electrical energy to mechanical energy. ELECTRIC MOTOR • Electric motors convert electrical energy to mechanical energy. • An electromagnet turns inside of a permanent magnet. Electromagnet ELECTRIC MOTOR • By changing the direction of the current, the poles will keep switching between N and S, and therefore cause the electromagnet to continually rotate. • Electric motors are used in most household appliances which convert electricity into motion. ELECTROMAGNETIC INDUCTION • When a magnet is moved near a wire, an electric current can be created. • This process is called electromagnetic induction. • The direction of the induced current can be reversed by reversing the motion of the wire or reversing the field direction. • The strength of the current depends on the strength of the magnetic field and the speed of the wire's motion. • This is the basis for electric generators. ELECTRIC GENERATOR • Electromagnetic induction is used in the operation of a generator, a device that converts mechanical energy to electrical energy. • In a generator, an electric current is produced when a coil of wire is mechanically rotated in a magnetic field. ELECTRIC GENERATOR VS ELECTRIC MOTOR Electric Generator Electric Motor Animation mechanical to electrical electrical to mechanical. USES OF ELECTROMAGNETISM • The use of magnetic waves to make pictures of the inside of the body. • Using a large magnet, radio waves, and a computer, a Magnetic Resonance Image, or MRI, produces two-dimensional and threedimensional pictures. USES OF ELECTROMAGNETISM • A transformer is a device composed of two unconnected coils, usually wrapped around a soft iron core, that can increase or decrease the voltage of AC current. USES OF ELECTROMAGNETISM • A transformer is used to step the voltage down and the current up (P=IV) so that we can use it. • Low power is desired for the transport of electricity over long distances to avoid loss of energy to heat loss. USES OF ELECTROMAGNETISM • A moving coil activated by voice vibrations is used as a microphone. • The coil induces a current in the magnet that can be amplified or recorded. USES OF ELECTROMAGNETISM • The experimental Japanese Maglev train uses magnetic forces for both support and propulsion. • It is a magnetic levitation (maglev) railway system based on the principle of magnetic repulsion between the track and the cars. • In April 2015, a manned seven-car Maglev train reached a speed of 603 km/h (375 mph). USES OF ELECTROMAGNETISM • An Aurora is an electric discharge through the atmosphere connected with magnetic disturbance. • Most aurora are green and red emission from atomic oxygen. • Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurora. • Northern Hemisphere: aurora borealis • Southern Hemisphere: aurora australis EOC PRACTICE 1. Which of these correctly identifies both devices illustrated? a. Picture A: electric generator; Picture B:electromagnet b. Picture A: electromagnet; Picture B: electric motor c. Picture A: electric motor; Picture B: electromagnet d. Picture A: electric generator; Picture B: electric motor EOC PRACTICE 2. A magnetic field can be found in which of the following? a. b. c. d. electric motor sound wave plastic cord glass rod