Chapter 21 Magnetism Magnets and Magnetic Fields Greeks discovered magnetism almost 3000 years ago – Magnesia Lodestone – iron containing magnetic rock – used for ancient travel Magnetite – mineral (Fe3O4)

All magnets have 2 poles - north (N) and south (S). Magnetism strongest at the poles of a magnet. Like poles repel – unlike (opposites) attract If a magnet is cut in half, ….2 smaller magnets are created with a new N and S pole.

Magnets produce a magnetic field Magnetic Field Lines are used to model the magnetic field around a magnet. The field lines form closed loops Strongest at the poles Lines close together – stronger; farther apart – weaker Like electric fields and gravitational fields – magnetic field lines weaken as distance increases. Draw Picture

Types of magnets: Temporary magnets and Permanent magnets Rare-earth elements - samarium and neodymium – very strong magnetic properties.

In Non-magnetic materials – domains not aligned Domains are regions within a magnetic material where the atoms have aligned. atoms with unpaired electrons can align so their e-spins are in the same direction. (unfilled D orbitals) In Non-magnetic materials – domains not aligned Any action that causes the electrons to align can create magnetism: Hammering Rubbing against another magnet Placing in an intense magnetic field http://micro.magnet.fsu.edu/electromag/java/compass/index.html

Magnetic Field Lines

21-2 Electromagnetism and Magnetic Domains Electromagnetism – relationship between magnetism and electricity Oersted – (1820) Danish physicist who first discovered that an electric current produces a magnetic field. http://www.magnet.fsu.edu/education/tutorials/java/magwire/index.html

21-3 Magnetic Force A stationary charged particle is not affected by a magnetic field – the charge must be moving – ‘current carrying wire’. A charge moving through a magnetic field experiences a force. The direction and strength of the magnetic field is strongest when perpendicular to the wire. A moving charge is defected when it crosses magnetic field lines but not when it travels parallel to the field lines.

Right Hand Rule – used to find the direction of the magnetic field around a current carrying wire. When your thumb is pointing up towards the direction of the conventional current (+ to -) Then your Finger Tips point in the direction of the magnetic field. The Magnetic Field around a current carrying wire – forms concentric circles. neg - Pos +

The direction of conventional current: from the positive terminal to the negative terminal (opposite the direction of electron flow). Symbol for Magnetic Field = B Symbol for Current = I

Solenoid – coil of wire with an electric current passing through it. (Wrapping the wire in a coil increases the magnetic field) North and South Pole as in a magnet. Strongest at the ends North Pole South Pole

3 Ways to Increase the strength of the magnetic field: increasing the amount of coils and/or increasing the amount of current. OR by adding magnetic material between the wires. An electro-magnet – solenoid with magnetic material (iron nail) inside the coils. Greatly increases the strength. Used in Motors and generators. Transformers, Relays, Telephones Electric bells and buzzers. Loudspeakers and earphones.

Galvanometers – instruments used to detect small currents (coil of wire connected to an electric circuit and a needle) – Measures both strength and direction of a current Examples: ammeters and voltmeters

Earth has a magnetic field surrounding the planet Most likely caused by liquid outer core – iron (Fe) and nickel (Ni) Magnetic South Pole located NE Canada – 1500 km from Geographic North Pole

Earth’s Magnetosphere (magnetic sphere that surrounds earth) protects us from the sun’s harmful rays Ionized particles from the sun get trapped in the magnetosphere and cause Aurora Borealis - Northern / Southern Lights

Aurora Borealis – Northern Lights

Convection currents within the mantle bring hot magma to surface. Ocean Floor provides evidence of movement of Earth’s Lithospheric Plates and that Earth’s Magnetic Field has reversed many times