21-8 Applications of Induction: Sound Systems, Computer Memory and the Seismograph Some microphones operate with induction…as the ‘plunger’ moves deeper from sound waves striking it, as the coil moves, voltage is produced. Recording and playback in magnetic tape uses induction. (analog) Digital induction involves 0 and 1 signals instead of a continuous variation. Movement of the earth can cause a suspended coil to move in a magnetic field to cause induction and conversion of that movement to electric current.

21-9 Inductance Mutual inductance…If 2 coils are placed close to one another, a changing current in one will induce a current in the other one. E =-M ( D I 1 / D t) Inductance has SI units of henrys 1H=Vs/A= W s (Joseph Henry) The value of M depends on whether iron is present or not, and on geometry. M=constant of proportionality

21-9 Inductance (cont’d) Self inductance can occur when a lone coil has a current that produces a change in magnetic flux, which causes induction on the coil. E =-L ( D I 1 / D t) … L=constant of proportionality or self-inductance or simply inductance. Any ac circuit contains some inductance…if there are many coils then there may be considerable inductance…often called an inductor or choke coil. See symbol on page 639. Winding backwards can cancel the effect.

21-9 Inductance (cont’d) An inductance acts something like a resistance to impede the flow of current…we call this impedance. Impedance depends on L and on f. See Example 21-11pg. 640

21-10 Energy Stored in a Magnetic Field We have seen that energy can be stored in a capacitor… U=1/2 CV 2 For inductors, energy can be stored… … U=1/2 L I 2 See formulas on page

21-11 LR Circuit Any inductor will have some resistance. An LR circuit has some properties similar to an RC circuit. See formula and diagrams on page 641. See Example p 641.