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PHY132_L14

# PHY132_L14 - Classical Physics II Classical Physics II...

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Classical Physics II PHY132 Lecture 14 M ti II M ti t Magnetism II: Magnetic torque Lecture 14 1 03/03/2010

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B Force on a Moving Charge Force on a Moving Charge W ’ th t th ti fi ld B We’ve seen that the magnetic field – is generated by currents (i.e. vectors), e.g. wire: and likewise interacts with currents and i h (i t ) 0 2 r I r B moving charges (i.e. vectors). Thus we expect the magnetic FORCE vector F B exerted on a moving charge to depend on TWO vectors: B and q v VECTOR PRODUCT: F B = q v × B (Lorentz Force) – Force F is perpendicular to both B and v ! B In a wire carrying a current I = dq/dt , charge dq in the wire travels a short distance d l in time dt : dq v dq = dq d l /dt = I d l Th th t ib ti t th t t l f f h t i d l f Thus, the contribution to the total force from a short piece of a current-carrying wire is: d F B = dq v dq × B = Id l × B (Lorentz Force) Lecture 14 2 for a straight wire of length l in a uniform B field: F B = I l × B 03/03/2010