Phy 103 Fundamentals of Physics Chapter 24 Magnetism

Phy 103: Fundamentals of Physics Chapter 24: Magnetism Lecture Notes

Magnetic Poles n n Since magnetism is created by moving charge, there is no single source of magnetism (such as a single electron) All magnets have both n n a north pole a south pole n Magnetic poles are never occur themselves n Aside from the obvious geographic reference, the south and north poles are magnet equivalent to positive and negative charge N S

Magnetism & Magnetic Force n n The magnetic poles interact similarly to electric charge N N “like” poles repel S n S “unlike” poles attract S N N S

Magnetic Fields n n The movement of electric charge produces a magnetic field (a B field) Magnets have magnetic fields associated with them Magnetic fields are essentially the magnetic influence a magnet has on the space around it Magnetic field lines always point from North to South S n n n The field lines originate at the North pole The field lines terminate at the South pole B The units of magnetic field are called Tesla (T) 1 Tesla (T) = 1 N. s/C. m N

Electric Currents & Magnetic Fields n n Wires carrying electric current produce magnetic fields The magnitude of the magnetic field is related to: Current in wire (I) n Distance from wire (r) n B ~ I/r n To calculate the B field: n The magnetic field “curls” around the wire!!

Andre Marie Ampere (1775 -1836) n n Considered to be the “Newton of electricity” Not formally educated (well did not attend school) n n Reputed to have mastered all known mathematics by the age of 12 (probably untrue) Worked on an electrical & magnetic wave theory of light First person to describe current as the flow of electricity along a wire Published his derivation of the electrodynamic (electric) force law in 1826

Electromagnets n n n A coil of wire with current running through is an electromagnet The magnetic field inside the coil is constant and points long the length of the coil The strength of the electromagnet depends on: The current (I) n The number of coils (N) n The length of the coil (L) n Bcoil ~ I. N/L

Electromagnets (cont. )

Magnetic Force (on charged particles) n n When an electric charge (such as an electron) moves through an magnetic field, it experiences a magnetic force The magnitude of the magnetic force depends on: n n The charge (q) The velocity of the charge (v) The magnetic field (B) Fmagnetic = q. v. B The direction of the magnetic force is perpendicular to both: n n The direction of the field The direction of motion (v)

Magnetic Force (on current carrying wires) n n n When two current carrying wires are aligned parallel to each other they exert a magnetic force on each other Each wire is in the magnetic field of the other The direction of the force depends on the relative direction of the electric currents in te wires: I 1 Repulsive: F F Attractive: F I 2

Earth’s Magnetic Field n n The Earth has a magnetic field and acts like a giant big magnet We define the magnetic “north” direction as the direction the North end of a compass points n n n The geographic “North Pole” is really the South pole of the magnetic field The geographic “South Pole” is really the North pole of the magnetic field Although its value varies depending on location, the magnitude of the Earth’s magnetic field (Bearth) is Bearth ~ 60 x 10 -6 T

North is South & South is North…