PHY132_L17 - Classical Physics II Classical Physics II...

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lassical Physics II Classical Physics II PHY132 Lecture 17 Induction Lecture 17 1 03/10/2010
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F B between Two Parallel Wires Current-carrying wires exert a forces on each other: Consider the simple case of two parallel long straight wires, carrying currents I and I : 1 2 – the first wire’s current I 1 generates a magnetic field B 1 everywhere, in particular along the location of the second wire… – the second wire’s current I 2 interacts (via the Lorentz force) with the local magnetic field and experiences a force F 2 –Ca lcu la t ing : I 1 (at wire 2) B 01 () I  22 1 I  Fl B I 2 d B 1 F 2 F 1 21 IlB  I 1 2 d – Note the reaction force F 1 = – F 2 ! 2 l F I B 12 0 2 II d Lecture 17 2 – The unit of current A (mpere) is defined in terms of SI units N and m this way, and therefore μ 0 4 π ×10 7 N/A 2 exactly! 03/10/2010
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Magnetic Materials i k d i lt i m ti l s i n i f i l d s m l s f t n h Like dielectric materials in electric fields, materials often have magnetic properties as well, characterized by the relative magnetic permeability K m multiplying μ 0 : hus: the formulae! – Thus: μ 0 μ K m μ 0 in the formulae! We distinguish three categories: Paramagnetic materials: trinsic tomic magnetic moments lectron orbital and electron intrinsic intrinsic atomic magnetic moments (electron orbital and electron intrinsic spin) align with the external magnetic field; alignment is only approximate as thermal “jitter” disturbs alignment: K m 1.0001 – 1.001 (small) Diamagnetic materials: some materials without intrinsic magnetic moment can acquire an INDUCED magnetic moment (rotating charge distributions that are pulled apart) which anti-aligns with the external magnetic field: K m 0.998 – 0.999 (small) Ferromagnetic materials: in materials like Fe, local “domains” of mutually aligned intrinsic spins arise Lecture 17 4 spontaneously. Even a moderate external field will align these domains, which gives an enormous boost to the magnetic field: K m 1000 – 10 5 ! (very large) 03/10/2010
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Amp Ampère’s re’s Law Law Consider again Ampere’s law: – For a circle around a wire carrying a current I : –symmetry tells us that B
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PHY132_L17 - Classical Physics II Classical Physics II...

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