PHY132_L13 - Classical Physics II Classical Physics II...

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lassical Physics II Classical Physics II PHY132 Lecture 13 Magnetism II: Magnetic torque Lecture 13 1 02/24/2010
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Magnetic Force MAGNETISM is yet another force that has been known since a very long time. – Its name stems from the mineral magnetite, which is naturally magnetic. We now know that magnetism is the action-at-distance orce felt between urrents ie moving charges force felt between currents , i.e. moving charges – magnetic materials contain currents, i.e. rapidly spinning and orbiting electrons, which “align” to give a macroscopic MAGNETIC FIELD B – because the magnetic forces are between (moving) charges, there must be a deep relationship with electricity… – Because motion is dependent on the reference frame of the observer, we expect connections with relativity as well… Again, we use the FIELD concept: – Currents (sources of B ) generate a Magnetic field B everywhere in Lecture 13 2 (f )g g f y space, –and B interacts with the “test”-currents… 02/24/2010
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Magnetic Force and Sources Note, that currents are by nature not “point-like”, but we will start with looking at the force between two “ ery short” pieces of “wire” carrying currents nd very short pieces of wire carrying currents I 1 and I 2 – This is necessarily an idealization! A current can only flow in a LOOP! – By integration over the loops, we can calculate the total force etween the currents and compare with experiment between the currents… and compare with experiment Currents in a wire have both a MAGNITUDE and a DIRECTION (// wire), i.e. currents are VECTORS by nature; unlike charges! Thus, the Magnetic Field B ( r ) generated by a current I must depend on TWO vectors: – position vector r and the current vector I VECTOR PRODUCT Lecture 13 3 – Note that experiment will ultimately decide if we are on the right track! 02/24/2010
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Magnetic Field from a Wire Current Before we skip to the EFFECTS of a magnetic field B ( r ) , we shortly describe the GENERATION of B by a current; e’ll turn to this in great detail later we ll return to this in great detail later. .. – the simplest (?) case is the generation of a B field by a infinitely long straight wire carrying a onstant urrent d l constant current I Biot-Savart Law : 0 2 4 I r d d l B r I d B d B right-hand rule!
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PHY132_L13 - Classical Physics II Classical Physics II...

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