Recall that a coil carrying a steady current I

1
Recall that a coil carrying a steady current I
generates a uniform magnetic field within its
enclosed area of
BmoI
B
I
This circular loop encloses a total FLUX of
? ? ? ? ? ? ? ? ? ? ? ? ?
?BA (moI)pr2
2
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)pr2.
• 2) ½?.
• 3) 2?.
• 4) 3?.
• 5) zero.

Here assume ?BA (moI)pr2
3
A
B
6 V
Compared to the voltage drop across coil A, the
voltage across coil B is 1) less than 6
V 2) 6 V 3) greater than 6 V
4
120 Watt bulb
120 V
120 V
240 V
What is the voltage across the light
bulb? 1) 60 V 2) 120 V 3) 240 V
What is the current through the light
bulb? 1) 1/2 Amp 2) 1 Amp 3) 2 Amps
What is the current through middle
circuit? 1) 1/2 Amp 2) 1 Amp 3) 2 Amps
5
Resonance occurs when XCXL (and they cancel!)
Z
R
f ?
fo
6
Resonance occurs when XCXL (and they cancel!)
Z
R
f ?
fo
IV/R
IV/Z
f ?
fo
7
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



8
Fully charged capacitor
I
I

Current starts to flow
I
Maximum positive current
I
Maximum current
I
9
Current begins to die down
I
Capacitor fully charged
I
Current flows (opposite direction!)
I
10
LC Circuits
Inductor
Capacitor
Capacitor Stores energy in E field Energy ½
CV2 Inductor Stores energy in B field Energy
½ LI2
Two ways to store energy ? Oscillations
1
1
Frequency of oscillation
2
p
LC
Adjusting the capacitance of an LC circuit is the
basic was a radio tuner works.
Question What are the two ways to store energy
in a pendulum? Answer Gravitational potential
and kinetic energy
11
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
12
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 adhered 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
13
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.
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3) 2?.
The too black loops reinforce one another,
combining to create a magnetic field B moI.
1) less than 6 V
The voltage across B is zero Only a changing
magnetic flux induces an EMF. Batteries provide
DC current.
2) 120 V
Twice as many loops voltage doubles. THEN half
as many loops, voltage halves
Supposed to be an easy one. P IV, so 120 watts
I (120 volts).
2) 1 Amp
Twice the voltage, means half the current.
1) 1/2 Amp
C ? 2C then f ? (1/?2) f ?2 1.41 141 MHz /
1.41 100 MHz
B) doubled