L 28 Electricity and Magnetism [5] - Physics & Astronomy

L 28 Electricity and Magnetism [5] - Physics & Astronomy

29:006 EXAM 3 Wednesday April 11 Lectures 19 through 28 29:006 FINAL EXAM FRIDAY MAY 11 3:00 5:00 PM IN LR1 VAN 1 L 27 Electricity & Magnetism [5] Magnetism Magnets permanent magnets Electromagnets

The Earths magnetic field magnetic forces applications 2 Magnetism two sources of magnetism permanent magnets electromagnets the earths magnetic field how does a compass work the north pole is really a south pole! Van Allen radiation belts 3

Permanent magnetism certain minerals (magnetite, Fe3O4) are naturally magnetic a piece of loadstone will attract bits of iron a magnet produces a magnetic field in the space around it, just like the Sun has a gravitational field that holds the planets in their orbits the magnetic field can be visualized with iron filings 4

Earths magnetic and gravitational force fields Gravitational field Magnetic field 5 Permanent magnets Are made from alloys of some of the rare earth elements like neodymium and samarium and cobalt. Always have a north and a south pole

like poles repel and unlike poles attract if you break a magnet in half you get 2 magnets cannot have just a north or just a south pole S NS NN 6 ELECTROMAGNETISM I N

S Hans Christian Oersted in 1820 observed that current flowing in a wire near a compass caused the compass needle to move. Andr-Marie Ampre, also in 1820, discovered the law relating the magnetic field and current. 7 Bar Magnet Electromagnets Magnetic field lines Solenoid coil Oersted

Long straight wire 8 Homemade magnets COIL Iron nail Duracell + You can think of the nail as a collection of little magnets that are randomly aligned. The magnetic

field of the coil aligns these little magnets giving a larger field than that of the coil alone. We say that the nail becomes magnetized, but the effect is not permanent. 9 N S N S N S PARTIALLY MAGNETIZED N S

N S S N N S N S N S N S S N

S N S N S atomic magnets N S N S N S N S

SOUTH MAGNETIZED NORTH N S N S N S

S UNMAGNETIZED N N N S N S Inside a piece of iron Spinning

electrons 10 Magnetic materials some materials are naturally magnetic or can be magnetized and retain their magnetism ferromagnetic materials other materials (iron) can be magnetized temporarily by placing them near magnets some materials have essentially no magnetic properties copper, aluminum, plastics... heat can destroy magnetism (Curie effect) 11

The earth is a big magnet The earths north geographic pole is the south pole of a big magnet. A compass needle is attracted to the earths north geographic pole The earths magnetism is the magnetic north pole is due to currents flowing in inclined about 14 from the its molten core (not geographic north pole, or

entirely understood!) by about 600 miles. 12 CNN Report Dec. 12, 2003 Earth's magnetic field fading Slight chance of flipping magnetic poles The strength of the earths magnetic field decreased 10% over the last 150 years. at this rate, the field will disappear altogether in 1,500 to 2,000 years. N

S North time South S N 13 Sun Earth Connection: space weather earth solar eruption

SUN solar eruption Space weather can have a large effect on communications, and it can cause damage to orbiting satellites. Northern Lights 14 Solar eruptions 15

Charges stay on magnetic field lines electron that originated on the sun part of the solar wind. The earths magnetic field traps charged particles. During periods of intense solar activity, satellites can be at risk. 16

Van Allen Radiation Belts 2 regions where charged particles are trapped in the earths Magnetic field 17 Magnetic forces Magnetic fields exert sidewise forces on charges A charge is turned around by the magnetic force There is NO magnetic force if the charge is not moving +q 18 Application:

Magnetic deflection of electrons in a TV tube. 19 Magnetic forces on wires Magnetic fields exert forces on the electrons moving in a wire (current) N S Wire pushed OUT

N S Wire pulled IN 20 Forces on current carrying wires The current in one wire makes a magnetic field that exerts a magnetic force on the current in the other wire Opposites repel

Likes attract 21 Torque on a current loop in a magnetic field A loop of wire (coil) carrying current experiences a torque when placed in a magnetic field. The torque makes the loop rotate. 22

Application: The electric motor When a current is present in a coil, it experiences a torque and rotates. 23 Application: Magnetic force in a speaker The force between the permanent magnet and the voice coil moves the speaker cone 24 Application: MAGLEV Trains

Magnetic levitation can be used to keep the cars on the track, and to propel them without touching 25

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