How Do You Use Magnetic Fields in Your Everyday Life Physics 2102 Aurora Borealis Jonathan Dowling Lecture 18 FRI 26 FEB Magnetic fields Ch 28 1 5 I ll be back Second Exam Review 6 7PM WED 04 MAR Nicholson 130 Second Exam Chapters 24 28 6 7PM THU 05 MAR Lockett 6 Electric vs Magnetic Fields Electric fields are created microscopically by electric charges fields of elementary particles electrons protons macroscopically by adding the field of many elementary charges of the same sign Magnetic fields are created microscopically by magnetic moments of elementary particles electrons protons neutrons macroscopically by adding many microscopic magnetic moments magnetic materials or by electric charges that move electric currents Magnetic Field Direction B E FROM North Poles TO South Poles Compare to Electric Field Directions Ritchie s Rule for Magnets Opposite Poles Attract Like Poles Repel Magnetic vs Electric Forces We know that an electric fields exists because it accelerates electric charges with a force independent of the velocity of the charge proportional to the electric charge FE qE We know that a magnetic field exists because it accelerates electric charges in a direction perpendicular to the velocity of the charge with a magnitude proportional to the velocity of the charge and to the magnitude of the charge FB q v x B Magnetic forces are perpendicular to both the velocity of charges and to the magnetic field electric forces are parallel to the field Since magnetic forces are perpendicular to the velocity they do no work W F r Speed of particles moving in a magnetic field remains constant in magnitude ONLY the direction changes Kinetic energy is constant no work Magnetic vs Electric Forces Electric Force on Charge Parallel to E FE qE Magnetic Force on Charge Perpendicular to B and v FB qv B FE q F B q E B v Definition of Magnetic Field Definition of Electric Field Definition of Magnetic Field FE E q FB B qv N Newton Newton Newton Units B Coulomb meter sec Coulomb sec meter Ampere meter A m Thompson Experiment Forces Balance v E B L III vB E I E 0 y B 0 II E 0 B 0 m L2 B 2 q 2yE E 0 B 0 qE FE ma a FE m qE m 1 2 2 L vt y at qEL2 Solve y 2mv 2 qEL2 v E B y 2mv 2 qEL2 B 2 qL2 B 2 y 2 2mE 2mE
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