A wire coiled into two loops carries a current, I. It faces a single independent loop enclosing the same cross-sectional area. The flux through the secondary (red) loop must be 1) =( o I)2 r. 2) 2 . 3) zero.
f fofo R Z “ Resonance ” occurs when X C =X L (and they cancel!) I =V/Z f fofo I=V/R
LC Oscillations Fully charged capacitor Energy: all E + + – – Capacitor charged Energy: all E Current starts to flow Energy: some E, some B Maximum current Energy: all B Current begins to die down induced C charges in opposite direction Energy: some E, some B Current flows ( opposite direction !) Energy: some E, some B + + – – + – II II + – + –
Fully charged capacitor Current starts to flow + + – – + – I I I I I Maximum current Maximum positive current
Capacitor fully charged Current flows ( opposite direction !) + + – – I + – I + – I Current begins to die down I I
A radio is tuned to 141 MHz. If the tuner is to be set for 100 MHz, the capacitance of the LC circuit in the radio must be A) quadrupled B) doubled C) not changed D) halved E) quartered 1 LC 2π 1 f = C 2C then f (1/ 2) f 2 = MHz / 1.41 = 100 MHz
N S NSSN magnetic tape Thin, plastic tape with millions of tiny permanent magnets glued to it AC current matching frequency of voice recording head (iron ring) Guides the magnetic field created by the current in the coil around to to the recording tape. coil Changing current in the coil creates a changing magnetic field in the iron Gap Gap in recording head lets the magnetic field out, so it can align the magnetic dipoles on the tape direction of tape movement magnetized areas of tape changing current produces changing B field
NSSN S N direction of tape movement Changing magnetic field inside the coil induces a current in the coil. changing current Tape induces a magnetic field in the recording head. (Just like holding a magnet near a nail makes it temporarily magnetic) Of course, there are amplifiers and other stereo components which, for clarity, are not shown. changing B field induces changing current