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Attraction and Repulsion Magnetism and Its Uses Magnetism • Discovered over 2000 years ago in Magnesia, Turkey • Magnetic Force – You can feel the magnetic ability before magnets touch • Magnetic Field – Surrounds the magnet – North and South pole is strongest point • Round magnet has poles on the top and bottom – Field has direction north to south • Unlike poles attract – Produce new magnetic field • Like poles that are the same – Repel • Magnetic Field Direction – Goes from north to south Earth’s Magnetic Field South Seeking Pole North Geographic Pole North Seeking Pole South Geographic Pole Reason for earth’s magnetism is unknown Magnetic Materials • Iron, cobalt and nickel are naturally magnetic – Electron’s magnetic properties not cancelled in these metals – Each atom magnetic – Not always magnetic – Groups of atoms must be aligned • Called Magnetic Domain Magnetism 8.1 Lining Up Domains • Even though each domain behaves like a magnet, the poles of the domains are arranged randomly and point in different directions. • As a result the magnetic fields from all the domains cancel each other out. Magnetism 8.1 Lining Up Domains • If you place a magnet against the same nail, the atoms in the domains orient themselves in the direction of the nearby magnetic field. • The like poles of the domains point in the same direction and no longer cancel each other out. • Domain alignment can be temporary or permanent – Magnet held next to metal will align domain – Vibrations/Heat will knock domain out of alignment • Permanent magnets have strong magnetic domain not influenced by atomic motion Electricity and Magnetism Electricity and Magnetism • Electric currents creates a magnetic field • Magnetic field reverses when the current reverses • Electromagnet – – – – Temporary Solenoid-cylindrical wire coil Strength determined by coil Speakers and motors (Electric to Mechanical) Electricity and Magnetism 8.2 Moving Charges and Magnetic Fields • It is now known that moving charges, like those in an electric current, produce magnetic fields. • Around a current-carrying wire the magnetic field lines form circles. Increasing Coils Increasing Current Larger Magnetic Field Faraday’s Law • Flux in magnetic force=flux in electric voltage Electricity and Magnetism 8.2 Using Electromagnets to Make Sound • The electromagnet changes electrical energy to mechanical energy that vibrates the speaker cone to produce sound. Electricity and Magnetism 8.2 Using Galvanometers • In a galvanometer, the electromagnet is connected to a small spring. • Then the electromagnet rotates until the force exerted by the spring is balanced by the magnetic forces on the electromagnet. Electricity and Magnetism 8.2 A Simple Electric Motor • A simple electric motor also includes components called brushes and a commutator. • The brushes are conducting pads connected to the battery. • The brushes make contact with the commutator, which is a conducting metal ring that is split. • The brushes and the commutator form a closed electric circuit between the battery and the coil. Electricity and Magnetism 8.2 Making the Motor Spin • Step 1. When a current flows in the coil, the magnetic forces between the permanent magnet and the coil cause the coil to rotate. Electricity and Magnetism 8.2 Making the Motor Spin • Step 2. In this position, the brushes are not in contact with the commutator and no current flows in the coil. • The inertia of the coil keeps it rotating. Electricity and Magnetism 8.2 Making the Motor Spin • Step 3. The commutator reverses the direction of the current in the coil. • This flips the north and south poles of the magnetic field around the coil. Electricity and Magnetism 8.2 Making the Motor Spin • Step 4. The coil rotates until its poles are opposite the poles of the permanent magnet. • The commutator reverses the current, and the coil keeps rotating. • Superconductors-mercury, lead and tin • Conductors-copper and aluminum • Semiconductors-silicon • Credit Card Magnetic Strip Producing Electricity • From Mechanical to Electrical • Electromagnetic induction • Generator – Coil rotates between the poles of a permanent magnet – Rotations change the direction of the current – Rotate 60 times a second-How many times per minute? Types of Electrical Currents • Direct Current – Battery – Current in one direction • Alternating current – – – – Household current Current changes direction 2x per second Produce at 60 Hz per second How many times does direction change per minute? • Transformers – 150,000 volts generated by power plants – Too much for house – Voltage needs to be lowered