Download Lecture38_Radio

The moment this switch is closed
energizing this
electromagnet
-
+
we expect the current jumps from zero to
I = V/R
time switch
is closed
time
However…
-
as each coil of this
electromagnet
starts to pull
current
+
its contribution to the magnetic field
shoves flux down the center
of all the neighboring coils.
each resists with an induced voltage
to slow this change down...
a small voltage countering the battery.
Which means the current ramps up!
-
+
We reach the full current V/R only after
a few moments:
I = V/R
time switch
is closed
time
A light bulb is run by a battery
in series with an inductor.
-
+
Current winds through the inductor
as shown, creating an electromagnet
with its North Pole on its
1) right end.
2) left end.
3) top surface.
4) bottom surface.
A light bulb is run by a battery
in series with an inductor.
-
+
When the switch is opened, current
1) stops abruptly.
2) gradually dies out.
3) continues flowing until the
bulb cools.
4) reverses direction, returning
charge to the battery.
When plugged into an active household
outlet, which of the light bulbs above
will be brighter?
1) A
2) B
3) both the same
Self-Induction in Daily Life
When you turn off your toaster by unplugging it
 current changes rapidly to zero
 rapidly changing B field produced
 very large voltage induced in toaster’s wires
 spark
voltage difference large enough for
electrons to flow through air
If you unplug your
computer while it is
still turned on,
large voltages
may be generated,
possibly destroying
the CPU chip.
+
-
e
++
++
-- -
- - + +
- - + +
-
The electric field in
this region points:
+
A.
B.
C.
D.
up.
down.
into the screen.
out of the screen.
Equilibrium position …antenna current zero..
E field points down
E field momentarily zero
E field points up
E field points down
strong
weak
E field zero
zero
E-field
pointing
down
E field points up
Also, this OSCILLATION generates a B-Field!
slowing to zero speed
maximum speed
As this positive
moves up, the
in this region p
INTO the scree
Also, this OSCILLATION generates a B-Field!
slowing to zero speed
maximum
speed
As this positive
charge moves up,
the B-field in this
region points
INTO the screen
B field is oscillating, continuously flipping the magnetic field
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
        
    
    

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
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in & out & in & out & in & out & · · ·
and at the same time…
E field is oscillating, continuously flipping the electric field
up & down & up & down & up & down & · · ·
c is used
to stand for the
“speed of light”
Before the days of cable, television
sets often had two antennae on
them, one straight, and one circular.
One antenna picked up electric field
oscillations, and the other picked up
magnetic field oscillations.
Which antenna picked up the
magnetic oscillations?
1) the circular antenna
2) the straight antenna
Carrier Frequency
Amplitude Modulation
Frequency Modulation
QUESTION 1
1) right end.
QUESTION 2
2) gradually dies out.
The loss of flux induces a current in the coil to partially replace the sudden
loss. Though it soon fades, this current continues to flow in the same direction
as the current from the battery had.
One more example of induction easing sudden changes.
The inductor slows down sudden changes
…even the fluctuations of AC current!
This means with the inductor in place, the AC current never makes it
to quite as high as maximum values (before reversing direction) when
with the inductor.  the rms average current is also lower then, so
I2R, the power consumed in the light-bulb is much less!
QUESTION 3
QUESTION 4
QUESTION 5
2) B
A. up.
Electric field lines point away from the positive
charge, into the negative. At the moment pictured,
the field right of the antenna points UP (see next slide).
1) the circular antenna
Set up to monitor the local magnetic fields…whenever the
magnetic field passing through this circular loop CHANGES, a
current is induced in it.
This is the type of antenna (see slide 23) your radio probably uses to pick up
FM stations.