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Physics 121: Electricity & Magnetism – Lecture 10 Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research Magnetic Field Due to a Current 0 ids r 4 r 3 Biot–Savart’s law dB 0 4 107 Tm/A B Permeability constant 0i 4 R Circular arc March 28, 2007 0i 2 R Long straight wire B Semi–infinite straight wire 0i B 4 R Center for Solar-Terrestrial Research Force Between two Parallel Currents To find the force on a current– carrying wire due to a second current–carrying wire, first find the field due to the second wire at the site of the first wire. Then find the force on the first wire due to that field. Parallel currents attract, and anti– parallel currents repel. March 28, 2007 Center for Solar-Terrestrial Research Ampere’s Law B ds i 0 enc Ampere’s law Curl your right hand around the Amperian loop, with the finger pointing in the direction of integration. A current through the loop in the general direction of your outstretched thumb is assigned a plus sign, and a current generally in the opposite direction is assigned a minus sign. March 28, 2007 Center for Solar-Terrestrial Research Solenoids and Toroids Ideal solenoid B 0in Toroid 0iN 1 B 2 r March 28, 2007 Center for Solar-Terrestrial Research Current–Carrying Coil as a Magnetic Dipole Current–carrying coil March 28, 2007 B z 0 2 z 3 Center for Solar-Terrestrial Research