Chapter 22 Magnetism and Its Uses

Magnetism  Discovered over 2000 years ago by the Greeks. Named after Magnesia, Turkey.  Magnetic Force –You can feel the magnetic ability before magnets touch  Magnetic Field –Surrounds the magnet –Strongest at the North and South poles –Round magnets have poles on the top and bottom

Magnetic Fields  Unlike poles that are the opposite will attract –Produce new magnetic field  Like poles that are the same will repel  Magnetic Field Direction –Goes from the north to the south

The Earth is a Large Magnet Earth’s Magnetic Field South Seeking Pole  North Geographic Pole North Seeking Pole  South Geographic Pole

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 ( magnetic domains ) must be aligned

Even though each domain behaves like a magnet, the poles of the domains are arranged randomly and point in different directions. Lining Up Domains Magnetism As a result the magnetic fields from all the domains cancel each other out.

If you place a magnet against the same nail, the atoms in the domains orient themselves in the direction of the nearby magnetic field. Lining Up Domains Magnetism 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 –Intense vibrations/heat can knock the domains out of alignment  Permanent magnets have strong magnetic domain not influenced by atomic motion

Electricity and Magnetism  Electric currents creates a magnetic field  Magnetic field reverses when the current reverses  An electromagnet is a temporary magnet.

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. Electricity and Magnetism

Increasing Coils Increasing Current  Larger Magnetic Field

Faraday’s Law  Flux in magnetic force = flux in electric voltage

Using Galvanometers In a galvanometer, the electromagnet is connected to a small spring. Electricity and Magnetism Then the electromagnet rotates until the force exerted by the spring is balanced by the magnetic forces on the electromagnet.

A Simple Electric Motor A simple electric motor also includes components called brushes and a commutator. Electricity and Magnetism 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.

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

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. Electricity and Magnetism The inertia of the coil keeps it rotating.

Making the Motor Spin Step 3. The commutator reverses the direction of the current in the coil. Electricity and Magnetism This flips the north and south poles of the magnetic field around the coil.

Making the Motor Spin Step 4. The coil rotates until its poles are opposite the poles of the permanent magnet. Electricity and Magnetism The commutator reverses the current, and the coil keeps rotating.

Producing Electricity  A generator produces electricity from mechanical energy to electrical energy. –A coil rotates between the poles of a permanent magnet –The Rotations change the direction of the current

Types of Electrical Currents  Direct Current = D/C –Battery –Current in one direction  Alternating current = A/C –Household current –Current changes direction 2x per second –Produce at 60 Hz per second  Transformers ( Step-up / Step-down ) –150,000 to 200,000 volts is generated by power plants –This is too much voltage, so it must be lowered for use in our homes.

 Superconductors are materials that have no electrical resistance, such as mercury, lead and tin  Conductors, good examples are copper and aluminum  Semiconductors, such as silicon which is used in computers.  Credit Cards work by using a Magnetic Strip to store information.