Phy 203: General Physics III Ch 21: Magnetic Fields & Magnetic Forces Lecture Notes

Magnetic Fields The movement of electric charge produces a magnetic field (a B field) Magnetic field lines point from North (N) to South (S) The units of magnetic field are called Tesla (T) 1 Tesla (T) = 1 N. s/C. m S N B

Magnets & magnetic materials Magnetism always exists as a dipole never as a point “charge” A single magnetic point charge is called a magnetic monopole (& has never been discovered in nature) Magnetic materials have both north and south poles

The Earth as a Magnet The Earth has a magnetic field and acts like a 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 (B earth ) is B earth ~ 60 x T

Magnetic Fields (moving charges) Moving charges produce magnetic fields The magnitude of the produced magnetic field depends: –Magnitude of charge (q) –Speed of the charge (v) –Distance from charge (r) Direction of magnetic field is determined by the “right hand rule” –Point thumb in direction of v (or –v for negative charge) –Curl fingers around the thumb –The direction of the fingers is the direction of magnetic field Examples: What is the direction of the B field? + v - v

Magnetic Force Magnetic fields exert force on moving charges (the magnetic force) The direction of the magnetic force is always –Perpendicular to the direction of movement –Perpendicular to the direction of magnetic field Magnetic force exerted on a charge depends on: –The magnitude of the moving charge (q) –The speed of the moving charge (v) –The magnitude of the magnetic field (B) –The angle (  ) between v and B To calculate magnetic force on a moving charge: F magnetic = q. v. B. sin 

Magnetic Force on a Current- Carrying Wire Current carrying wires have moving charge When placed in a magnetic field, the field can exert a force on these moving charges The magnetic force exerted on a wire of length, L, is: F magnetic = I. L. Bsin  Example:

Ampere’s Law The magnetic analogy to Gauss’ Law –Ampere’s Law is a tool to calculate the magnetic field For a closed loop of length, L: B //. L closed loop =  o I enclosed To find B: B // = (  o I enclosed )/ L closed loop Example: a current carrying wire

Andre Marie Ampere ( ) 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