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MAGNETIC FIELD Magnetic Force ● For centuries, humans observed “strange” force – ● Force couldn't be gravity or electric – ● Between iron and special stones called “lodestones” Not enough mass or electric charge to explain force Iron and lodestones must have something – – Other than mass and charge Humans called these “North” and “South” (like + and –) Magnetic Poles ● Like poles repel; opposite poles attract S N S N S N N S Repulsive force ● Attractive force Curious fact: “North” and “South” always come in pairs (unlike + and –) Magnetic Field ● Magnetic Force → Action-at-a-distance – Just like gravity and the electric force ● Solution: The “magnetic field” ● Magnetic poles affect the space around them – – If another pole comes into this space, it feels a force This still doesn't explain the origin of the magnetic field... Magnetic Materials ● Experiments → possible to “magnetize” iron – – – ● Most other metals don't show this behavior – ● By touching an unmagnetized piece of iron to a lodestone The iron holds its magnetism permanently Iron can be “de-magnetized” by heating it Iron must be magnetically “special” For centuries, iron was used to make: – – Compasses Bar magnets Magnets and Electric Current ● Experiments on magnets – – Found that electric current can deflect a compass needle Must be a link between electricity and magnetism! creates Electric Magnetic Field Current exerts force ● exerts force creates Electric Current Currents create magnetic fields of many sizes – – – Atoms (due to current produced by orbiting electron) Medium size (e.g. coil of wire → electromagnet) Huge (Sun is made of rotating plasma → magnetic field) Atoms and Magnetism ● Individual atoms act like tiny bar magnets – ● Usually pointing in random directions When many atomic magnets “line up”: – – – An object is “magnetized” Requires special conditions Only iron and a few other materials are easily magnetized Earth's Magnetic Field ● Earth itself acts like a huge “bar magnet” – ● Lodestones – – – ● Humans have observed the magnetic field for centuries Naturally-formed magnetic rocks Made as iron-rich lava cooled Earth magnetized the iron inside the rocks Origin of magnetic field not fully understood – – Can NOT be due to Earth's iron core (too HOT!) Must be due to electric currents in molten core Cosmic Radiation ● Dangerous high-energy particles hit Earth – – From the Sun and from deep space Charged particles are deflected by the magnetic field The deflected charged particles produce the aurora borealis Uncharged particles must be stopped by the atmosphere Electromagnetic Induction ● Magnetic fields resist changes to their state – ● Similar to a large mass resisting changes in velocity Result: New method for creating electric current – – – Use a changing magnetic field This is called electromagnetic induction And it creates electric current without a battery! This is called “alternating current” (AC) because the current switches direction... Batteries produce direct current (DC) Generators and Motors ● Generator – ● Motor – ● Heat energy or KE → electrical energy Electrical energy → KE Both devices are made up of: – – – Magnet (to produce a magnetic field) Rotating coil of wire (to carry electric current) Both use electromagnetic induction