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Magnetism and its uses Physical Science Chapter 7 Section 1: Magnetics • Discovered over 2000 years ago • Magnetite-Naturally magnetic material • Magnetism-Properties and interactions of magnetics. Magnetic Force • Magnets exert a force on one another and on certain objects (mostly metals) • The strength depends on distance and the strength of the magnetic field • As magnets move closer, the force increases. • Magnets range in strength from cheap refrigerator magnets to ones designed to pick up cars. Magnetic Fields • Region of space that surrounds a magnet and exerts a force on other magnets and objects made up of magnetic materials • Stronger field=stronger force • MRI is about 200x stronger than fridge magnet Magnetic Field Lines • The closer the lines, the stronger the force of the magnet. • Shows that the magnetic field is related to the magnetic force. • Lines point from the N to the S pole • If they were strong enough, all lines would reach the south pole • Earth has a magnetic field • The geographical North pole is actually the magnetic South pole • The magnetic poles reverse every few 1000 years Magnetic Poles • Magnetic fields are strongest near the ends of a magnet • These are called the magnetic poles • North and South • Like repel; opposites attract Compasses • The needle on a compass acts as a little bar magnet • The north pole of the magnet points “North” • Actually, the north pole is attracted to the Earth’s magnetic South pole • Remember, this is the geological North Pole Source of Earth’s Magnetic Field • Not certain • Probably has something to do with Earth’s core, which contains iron and nickel • The core is surrounded by liquid, and the interaction potentially creates a magnetic field Magnetic Materials • Electrons have magnetic properties • In most elements, the electron’s magnetic properties cancel one another out. • In magnetic materials, they do not Magnetic Domains • Groups of atoms with aligned magnetic poles • In non-magnetized materials, the domains point in random directions. • This allows the domains to cancel out • When the object becomes magnetized, the domains line up and point in the same direction • This causes a magnetic force • These domains act as tiny magnets themselves • Breaking a magnet into tiny pieces will result in pieces that each have a north and south pole. Permanent magnets • If you have a non-magnetic object and bring it near a magnet, it causes the domains to line up by attracting the opposite pole. This causes the magnet to be attracted to the object. • This is reversible because the field is not that strong. • Naturally, over time, the random movement of atoms will cause the domains to revert to random alignments and cancel one another out • Magnets can be made by stroking a magnet to a magnetic material Electric Current and Magnetism • Hans Christian Oersted • 1820 Danish physics teacher • Did a demonstration of current, and happened to have a compass nearby • Noticed that the current affected the compass needle • Determined that current through a wire creates a magnetic field • The magnetic fields are circular around the wire • The strength of the magnetic field increases as the current increases Electromagnetism • The interactions between electric charges and magnets • Electromagnetic force=attractive or repulsive force between electric charges and magnets Electromagnets • A temporary magnet created when there is charge in a wire coil • A single wire wrapped into a cylindrical wire coil is called a solenoid • Leads to many interacting magnetic fields, and strengthens the total • Electromagnets can me made very simply • Wrapping wire around a metal object, and connecting to a battery • Makes the metal object become temporarily magnetic • We’ll make some of these in lab • Electromagnets act like any other magnet when there is a current through it • There will be a north and south pole • There will be a magnetic force on objects • They are useful because the magnetic field can be controlled • • Adding more current or more turns increases the strength You can also cut the power, and it stops being magnetic Using electromagnets • Electromagnetic energy can be transferred into different forms • Speakers: • Electromagnetic energy is changed to mechanical energy that vibrates the speakers • Devices send current to the electromagnet in the speaker • The electromagnet becomes charged, and so has a magnetic field • This field interacts with a permanent magnet, and causes the electromagnet to move back and forth, causing the speaker cone to vibrate and emit sound waves • https://www.youtube.com/watch?v=AP2Nu4MZJRs Galvanometers • Devices that uses an electromagnet to measure electric current • Used in vehicles to determine amount of gas remaining Electric Motors • A device that changes electrical energy into mechanical energy • Used in many everyday devices • Simple electric motors • Magnetic repulsion and attraction between electromagnet and permanent magnet causes electromagnet to spin • https://www.youtube.com/watch?v=ziWUmIUcR2k Electromagnetic Induction • Generation of an electric current by changing magnetic field • Generators • Use electromagnetic induction to transform mechanical energy into electrical energy • Faraday Flashlight • https://www.youtube.com/watch?v=gfJG4M4wi1o Types of Current • Alternating Current (AC) • Directions reverse • Also causes magnetic field to reverse • Direct Current (DC) • Direction does not reverse • Magnetic field is always in one directions Transformers • • • • Device that increases or decreases the voltage of AC Made up of a primary coil and a secondary coil wrapped around the same iron core If the transformer increases voltage, it is called a step-up transformer If the transformer decreases voltage, it is called a step-down transformer Calculation output voltage • The ratio of the output voltage to the input voltage is equal to the ratio of the number of turns in the secondary and primary coils 𝑉𝑜𝑢𝑡 •𝑉 𝑖𝑛 = 𝑁2 𝑁1 • Where N2 is the number of turns in the secondary coil and N1 is the number of turns in the primary coil. • To solve for output voltage, • 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 𝑁2 𝑁1 Example Problem • A transformer has 150 turns in its primary coil and 50 turns in its secondary coil. If the input voltage is 12 V, what is the output voltage? • 4V Guided Practice • The input voltage into a device is 12 V. If there are 2x as many turns in the secondary coil as there are in the first, what will be the output voltage? • 24 V You Try it • A device converts 12 V to 48 V. Its secondary coil has 100 turns. How many turns does its primary coil have? • 25