A wire coiled into two loops carries a current, I. It faces a

1
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
• ?(moI)2pr.
• 2) 2?.
• 3) zero.

2
Resonance occurs when XCXL (and they cancel!)
Z
R
f ?
fo
IV/R
I V/Z
f ?
fo
3
LC Oscillations
Current starts to flow Energy some E, some B
Fully charged capacitor Energy all E
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
Capacitor charged Energy all E
I
I



4
Fully charged capacitor
I
I

Current starts to flow
I
Maximum positive current
I
Maximum current
I
5
Current begins to die down
I
Capacitor fully charged
I
Current flows (opposite direction!)
I
6
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
C ? 2C then f ? (1/?2) f ?2 1.41 141 MHz /
1.41 100 MHz
7
changing current produces changing B field
Recording on Tape
AC current matching frequency of voice
coil Changing current in the coil creates a
changing magnetic field in the iron
recording head (iron ring) Guides the magnetic
field created by the current in the coil around
to to the recording tape.
direction of tape movement
N
S
magnetic tape Thin, plastic tape with millions of
tiny permanent magnets glued to it
magnetized areas of tape
Gap Gap in recording head lets the magnetic
field out, so it can align the magnetic dipoles
on the tape
8
changing B field induces changing current
Playback
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)
direction of tape movement
S
N
Of course, there are amplifiers and other stereo
components which, for clarity, are not shown.