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PSCI II – Chapter 8 – Magnetism and Its Uses (pages 224 – 255) Section 1 – Magnetism I. Magnets º The Greeks discovered a mineral that was a natural magnet. º They found it in a region called Magnesia so they called it magnetic. º Magnetism – refers to the properties and interactions of magnets. Magnetic Force º Have you ever played with magnets? º What happened? º Magnets exert a force on each other and other objects. º The magnetic force is one between magnets that you can feel before the magnets touch. º Its strength increases as the magnets get closer and gets weaker as they get farther apart. Magnetic Field º A magnet is surrounded by a magnetic field which exerts a magnetic force. º This field can be represented by lines of force. Magnetic Poles º Defn. – where the magnetic force exerted by the magnet is the strongest. º All have a north and a south pole. º Magnetic field lines always connect the north and south poles. PSCI II – Chapter 8 – Page 1 of 5 How Magnets Interact º Magnets can either attract or repel. º If you bring two north or south poles together you will feel them repel. º If you bring a north and a south together you will feel them attract. º When either of these happens a new magnetic field is formed. A Compass Needle º A magnet that is free to rotate when placed in a magnetic field. º When you place a compass in a magnetic field the north pole of the needle will align with the south pole of the magnet. º Picture on page 228. Earth’s Magnetic Field º With out landmarks to guide you how would you know which way to go? º The Earth acts like a big magnet so we could use a compass. Earth’s Magnetic Poles º Because we think that the Earth has an iron and nickel core and it’s rotating we have magnetic fields. º READ THIS SECTION II. Magnetic Materials º Only a few metals are magnetic: iron, nickel, and cobalt. º These are attracted to magnets or can be made into permanent magnets. º Remember electrons have magnetic properties and in most elements they cancel out. º In the three listed above they don’t cancel and each atom behaves like a mini magnet. º You can have temporary magnets also. PSCI II – Chapter 8 – Page 2 of 5 Magnetic Domains – A Model For Magnetism º Because each magnetic atom exerts a force on a nearby atom they align magnetically. º Magnetic Domains – group of atoms having aligned magnetic poles. º Each domain has billions of atoms yet we can’t see them. º Because individual domains are aligned the overall domain acts like a magnet. Lining Up Domains º If the domains are not aligned then the object is not magnetic because they’ll all cancel out. º If you place a magnet near the objects domain they will line up and form a temporary magnet. º Eventually they’ll go back and the object will no longer be magnetized. º Objects with iron are the most common type of magnet. Permanent Magnets º These can be made by placing a piece of magnetic material in a strong magnetic field. º This causes the material to become magnetized to create a magnet. º The strength of the magnetic field inside the material prevents the domains from changing. º Permanent magnets can lose their magnetism if heated or dropped enough. Can a Pole Be Isolated? º What happens if a magnet is broken in half? º The poles in the atoms are aligned so now you have two magnets with north and south poles. Section 2 – Magnetism I. Current and Magnetism º 1820 Hans Christian Oersted found electricity and magnetism to be related. º While teaching he found that a current affected a compass. Moving Charges and magnetic Fields º Moving charges create a magnetic field. º Forms a circular pattern (Figure 9 page 233) º Direction of the magnetic field depends on the direction of the current (same with the strength). PSCI II – Chapter 8 – Page 3 of 5 II. Electromagnets º Defn – temporary magnet made by placing a piece of iron inside a current carrying coil of wire. º Figure 10. º Inside a coil the magnetic field is strong so the iron is a strong magnet. Properties of Electromagnets º Temporary magnets. º Strength increases by adding more turns (loops) or by increasing the current. º Behaves like any other magnet. º Useful because properties can be controlled by changing the current. º Converts electrical energy to mechanical energy. Music to Your Ears – Stereo Speakers º Read this section. Galvanometers º Defn – devices that use an electromagnet to measure electric current. º Read this section. III. Electric Motors º Defn – device that changes electrical energy into mechanical energy. º Read this section. º Figure 15 A-D shows how these work. Section 3 – Producing Electric Current I. From Mechanical to Electrical Energy. º 1831 Michael Faraday and Joseph Henry found that moving a loop of wire through a magnetic field produces an electric current. º Mechanical energy producing electrical energy. º Electromagnetic induction – producing an electric current by moving a loop of wire through a magnetic field or moving a magnet through a loop of wire. Generators º Defn – produces electric current by rotating a coil of wire in a magnetic field. º Rotated by mechanical energy. Using Electric Generators º Used in cars (alternators) for lights and things. PSCI II – Chapter 8 – Page 4 of 5 Generating Electricity for Your Home º Our power comes from the power plant. º Turbine – a large wheel that rotates when pushed by water, wind or steam. II. Direct and Alternating Current º Is electricity from a battery the same as from a generator? º Direct current (DC) – flows in one direction through a wire (batteries). º Alternating current (AC) – reverses direction of the current flow in a regular way (generators). III. Transformers º Defn – device that increases or decreases the voltage of an alternating current. Stepping Up or Stepping Down. º Figure 22 A and B page 245. º Read the rest of the chapter. PSCI II – Chapter 8 – Page 5 of 5