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Phy 103: Fundamentals of Physics. Chapter 24: Magnetism Lecture Notes. Magnetic Poles. N S. Since magnetism is created by moving charge, there is no single source of magnetism (such as a single electron) All magnets have both a north pole a south pole Magnetic poles are never
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Phy 103: Fundamentals of Physics Chapter 24: Magnetism Lecture Notes
Magnetic Poles N S • Since magnetism is created by moving charge, there is no single source of magnetism (such as a single electron) • All magnets have both • a north pole • a south pole • Magnetic poles are never occur themselves • Aside from the obvious geographic reference, the south and north poles are magnet equivalent to positive and negative charge
Magnetism & Magnetic Force N N S S S N N S • The magnetic poles interact similarly to electric charge • “like” poles repel • “unlike” poles attract
Magnetic Fields S B 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 • The field lines originate at the North pole • The field lines terminate at the South pole • The units of magnetic field are called Tesla (T) 1 Tesla (T) = 1 N.s/C.m
Electric Currents & Magnetic Fields • Wires carrying electric current produce magnetic fields • The magnitude of the magnetic field is related to: • Current in wire (I) • Distance from wire (r) B ~ I/r • To calculate the B field: • The magnetic field “curls” around the wire!!
Andre Marie Ampere (1775-1836) • First person to describe current as the flow of electricity along a wire • Published his derivation of the electrodynamic (electric) force law in 1826 • Considered to be the “Newton of electricity” • Not formally educated (well did not attend school) • Reputed to have mastered all known mathematics by the age of 12 (probably untrue) • Worked on an electrical & magnetic wave theory of light
Electromagnets • 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) • The number of coils (N) • The length of the coil (L) Bcoil ~ I.N/L
Magnetic Force (on charged particles) • 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: • 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: • The direction of the field • The direction of motion (v)
Magnetic Force(on current carrying wires) • 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: I1 I1 F F Repulsive: Attractive: F F I2 I2
Earth’s Magnetic Field • 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 • 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
