PHY132_L20 - Classical Physics II Classical Physics II...

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lassical Physics II Classical Physics II PHY132 Lecture 20 Self Inductance Lecture 20 1 03/17/2010
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Summary: Mutual Inductance Relates a changing current in one coil, loop, or wire, to the induced emf in another coil, loop or wire… •U n i t H ; 1 Henry = 1 V/(A/s) = 1 ·s • Depends on geometrical quantities only (typically): coil dimensions, number of windings, relative distance and orientation… • Mutual Inductance is the principle behind TRANSFORMERS • The magnetic field, and thus the mutual inductance can be increased tremendously, ×10 5 or so, with the use of ferromagnetic cores (for high frequency: ferrites) • Mutual inductance is a nuisance in many electronic circuits, and careful circuit design is necessary to minimize it (e.g. distancing coils and mounting them mutually perpendicular…) • Mutual inductance is indeed mutual, i.e.: M 12 = M 21 ! – this can be easily shown for the examples above … Lecture 20 2 03/17/2010
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Self Inductance Whereas Mutual Inductance describes the effect of a changingcurrent in one coil on another coil, there is also an induced EMF in the first coil ITSELF! – That is (or should be) completely expected: • a coil carrying a changing current , itself experiences the changing flux as ell and thus should itself experience self- duced MF well, and thus should itself experience a self induced EMF… –the self-induced EMFimplies the existence of a voltage difference Δ V between the terminals of the coil … hat pposes the original current hange • that opposes the original current change Taking the definition of mutual inductance, we can easily define the SELF-INDUCTANCE L : here the elf- duced EMF ue to the changing 1 1c o i l c o i l di VV dt   E L (Note the "symbolic" sign!) Lecture 20 4 where E 1 is the self induced EMF due to the changing current i 1 03/17/2010
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PHY132_L20 - Classical Physics II Classical Physics II...

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