Hanley the Science Guy! ch?v=ak8Bh9Zka50

Magnets  Iron objects are strongly attracted to the ends (poles) of a magnet.  North points north  Technological applications   Motors, meters, loudspeakers, maglev trains  Like poles repel  Magnetic poles cannot be isolated – poles always occur in pairs  Magnetically soft – easily magnetized, also loses its magnetism easily – iron  Magnetically hard – difficult to magnetize, retain their magnetism – cobalt and nickel

BASIC LAW OF MAGNETISM Like poles repel; unlike poles attract. Although it is possible to isolate positive and negative charges, it is impossible to isolate NORTH and SOUTH magnetic poles. Magnetic monopoles do not exist.

Even though magnetic properties are similar to electric properties, it is erroneous to directly relate positive and negative electric charges to north and south magnetic poles. The north magnetic pole is not caused by protons, nor is the south magnetic pole caused by electrons!!!!

The force between magnetic poles follows a law similar to the Law of Universal Gravitation and Coulomb’s Law for Electrostatics. Universal Gravitation F = G (m 1 m 2 )/d 2 Electrostatics F = k (q 1 q 2 )/d 2 Magnetism F = k (M 1 M 2 )/d 2

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic Fields  Greatest closest to poles  North pole of compass is north-seeking  Magnetic field lines show magnitude and direction of the magnetic field.  SI Unit = Tesla

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic fields surround magnets.  Field lines point from North to South outside the magnet. NS

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic Domains  A microscopic magnetic region composed of a group of atoms whose magnetic fields are aligned in a common direction  When a substance is unmagnetized, its domains are randomly oriented.  Soft vs. hard magnets  You can strengthen a solenoid by inserting an iron rod into the coils center. The magnetic field produced by the solenoid will cause alignment of the domains in the iron.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 The Earth  The north pole of a compass points towards the magnetic south pole which is the geographic north pole.  Scientists are still unsure of the source of the Earth’s magnetic field.  Magnetite achieved magnetism because it was subjected to Earth’s magnetic field over long periods of time.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic Field of Current Carrying Wire  A wire that carries a strong, steady current will produce a magnetic field around the wire.  The magnetic field forms concentric circles around the wire.  The direction of the magnetic field can be found using the right hand rule.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic Field of a Current Loop  A solenoid (form of an electromagnet) is a long helically wound coil of insulated wire.  It’s magnetic field lines are similar to that of a bar magnet because it acts like a magnet when it is carrying current.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Magnetic Force  A charge moving through a magnetic field will experience a magnetic force.  SI Unit = Newtons  When a charge moves parallel to a magnetic field, there is no magnetic force on the charge.  When a charge moves perpendicular to a magnetic field, the magnetic force has a maximum value.  F magnetic = qvB  B=Magnetic field strength in Teslas  The Earth’s magnetic field near the surface is about 50  T. Conventional laboratory magnets are about 1.5T.  To find the direction of the force, use another right hand rule.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Moving charge  A charge moving through a magnetic field will follow a circular path if the velocity is perpendicular to the field.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Wires…  A current carrying conductor in a magnetic field experiences a force that is perpendicular to the direction of the current.  F=IℓB  Right hand rule

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Two parallel wires  If the current is in the same direction, the wires are attracted to each other.  If the current is in opposite directions, the wires repulse each other.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Cathode Ray Tube  Is a vacuum tube in which electric fields are used to form a beam of electrons  Phosphor on the TV screen glows when it is struck by the electrons in the beam.  Without magnetism, only the center of the screen would be illuminated by the beam.  The direction of the beam is changed by two electromagnets, one deflecting the beam horizontally and one deflecting the beam vertically.  This happens because of the changing current direction in each electromagnet.  A color television has three different phosphors (red, green, blue) to make up the screen. Three electron beams scan over the screen to produce the color picture.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Loudspeakers  When the direction and magnitude of the current in the coil of wire change, the paper cone attached to the coil moves, producing sound waves.

Serway, Raymond and Faughn, Jerry. Physics. Holt, Rinehart and Winston, 2002 Galvanometers  Are used in the construction of ammeters and voltmeters.  A torque acts on the current loop in the presence of a magnetic field. The amount of deflection is proportional to the current in the coil.

MYTHBUSTERS – MAGNETS!  U7M U7M