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Magnetism Notes I. Magnetism – refers to the properties and interactions of magnets A. Magnetic force is the interaction between two magnets. • A magnet is surrounded by a magnetic field that exerts the magnetic force. • The magnetic field is strongest closer to the magnet. B. Magnetic Field of a Bar Magnet • Un-magnetized iron brought near a magnet becomes magnetized. • Iron filings line up around the bar magnet. This shows the shape of the magnetic field. C. Magnetic Poles – regions where magnetic force is the strongest • All magnets have a north and south pole. • Magnetic field lines always connect the north pole and the south pole of a magnet. D. Earth’s Magnetic Poles • Earth is like a bar magnet, with its south magnetic pole near its geographic south pole. • Scientists are NOT sure what produces Earth’s magnetic field. E. Compass Needles • A compass contains a small bar magnet for a needle. • If you place a bar magnet near a compass, north and south poles will attract. The earth's magnetic field is just like the field of any magnet - only LARGER and STRONGER. A compass is simply another magnet. And the principles of attraction and repulsion govern the earth magnet and the compass magnet. The earth magnet is considered stationary. Therefore, the compass magnet's north pole is attracted to the earth's south pole and the compass' south is attracted to the earth's north. Which means that the compass' magnet, which is free to turn, always points north. The confusing part of this is that the NORTH POLE of the compass points to the NORTH POLE of the earth. This apparently says "North attracts North." Of course, this is NOT true. The magnetic pole near the north geographic pole is ACTUALLY A SOUTH MAGNETIC POLE. Common usage has named this "the North Pole" - just remember that MAGNETICALLY it's a SOUTH pole. F. Magnetic Materials • Only a few metals such as iron, cobalt, and nickel are attracted to magnets or can be made into a permanent magnet. • These particular metals are magnetic because the magnetic properties of the electrons don’t cancel out. II. Magnets A. Making a Metal a Magnet • In order for a metal to become a magnet, the atoms in the metal must align their magnetic domains in the same direction. • If you place a permanent magnet near a nail, the magnet aligns the magnetic domains and the nail becomes a temporary magnet. • A permanent magnet can be made by placing the metal in a strong magnetic field. • Permanent magnets can lose their magnetic behavior if heated or dropped. B. What happens when a magnet is broken? • In un-magnetized iron, the domains are randomly oriented. • In slightly magnetized iron, there is incomplete alignment of domains. • In strongly magnetized iron, virtually all of the domains are aligned. • Even if the magnet were divided into bits as small as a single domain, it would still have north and south poles. C. North Poles Cannot Be Separated from South • Each piece of broken magnet still has a north and south pole. • No matter how many times a bar magnet is cut in half, there is always a north and south pole, even in the smallest piece. III. Electromagnets A. Moving charges and Magnetic Fields • When electric current flows through a wire, a magnetic field forms around the wire. • The direction of the magnetic field depends on the direction of the current in the wire. B. Electromagnet – temporary magnet made by placing a piece of iron inside a current • The more coils there are, the more current the electromagnet has. • The gauges in a car use galvanometers, which are devices that use an electromagnet to measure electric current. C. Electric Motors – a device that changes electrical energy into mechanical energy. • A battery causes an electric current to flow through the coil of the electromagnet. • Unlike poles of the two magnets attract to each other and the like poles repel. This causes the coil to rotate until the opposite poles are next to each other. • If the current in the coil is switched, the direction of the coils magnetic field also switches. The north and south poles of the magnet trade places. • The coil is repelled by and attracted once again to the poles of the permanent magnet. D. Generators • A generator produces electric current by rotating a coil of wire in a magnetic field. • Mechanical energy is turned in to electrical energy E. Current Direction • A direct current (DC) flows through only one direction through a wire • An alternating current (AC) reverses the flow of the current flow F. Transformers • A transformer is a device that increases or decreases voltage of an alternating current. • Made of two coils of wire called primary and secondary coils. • If the secondary coil has more wires, then the transformer, increases, or steps up the voltage. • If the primary coil has more wires, then the transformer steps down, or reduces voltage. *** This is how electricity gets to houses in power lines. G. Power Transmission and Transformers • In the U.S., power lines can carry up to 750,000 volts. • This amount of power is dangerous and cannot be used with household appliances. • Step down transformers convert voltage to 120 V—a usable amount of power for household appliances.