Download induction

The black post has a coil
inside which produces a
constantly-changing magnetic
field around it.
This constantly changing
magnetic field induces a
current to form in the circular
aluminum rings which
surround it.
This current forms its own
magnetic field – which
opposes (pushes against!) to
one formed by the black post.
Why doesn’t the bottom ring
“work”?
FIGURE 25-1. When a wire is moved in a magnetic field,
an electric current flows in the wire, but only while the wire
is moving. The direction of the current flow depends on the
direction the wire is moving through the field. The arrows
indicate the direction of conventional current flow.
FIGURE 25-2. The right-hand rule can be used to
find the direction of the forces on the charges in a
conductor that is moving in a magnetic field.
FIGURE 25-3. Schematic of a
moving coil microphone. The
aluminum diaphragm is
connected to a coil in a
magnetic field. When sound
waves vibrate the diaphragm,
the coil moves in the magnetic
field, generating a current
proportional to the sound wave.
FIGURE 25-4. An electric current is generated in a wire loop as
the loop rotates (a). The cross-sectional view (b) shows the
position of the loop when maximum current is generated. The
numbered positions correspond to the numbered points on the
graph in Figure 25-5.
FIGURE 25-6. Alternating current generators transmit current
to an external circuit by way of a brushslip-ring arrangement
(a). The alternating current varies with time (b).
ELECTRICIANS Immediate openings!
Resid'l journeyman/ foreman, rough &
finish. Top wages & bnfts – start at
$45,000 for Journeyman. Fax resume
916 294-0168 or apply in person
11855 White Rock Road Rancho C,
916-294-0140
Published to the Web on Sun, 04/17/05
Real ad from Sacamento, CA!
FIGURE 25-7. The magnet approaching the coil
causes an induced current to flow. Lenz's law predicts
the direction of flow shown.
FIGURE 25-9. As the current in the coil increases
from left to right, the EMF generated by the current
also increases.
Lenz’s Law
Back-EMF can be explained another way. As Faraday showed, EMF is
induced whenever a wire cuts lines of magnetic flux. Consider the coil of
wire shown in Figure 25-9. The current through the wire in-creases as we
move from left to right. Current generates a magnetic field, shown by
magnetic field lines. As the current and magnetic field increase, new
lines are created. As the lines expand, they cut through the coil wires,
generating an EMF to oppose the current changes. This induction of
EMF in a wire carrying changing current is called self-inductance. The
size of the EMF is proportional to the rate at which flux lines cut through
the wires. The faster you try to change the current, the larger the
opposing EMF, and the slower the current change. If the current reaches
a steady value, the magnetic flux is constant, and the EMF is zero. When
the current is decreased, an EMF is generated that tends to prevent the
reduction in magnetic flux and current.
FIGURE 25-10. For a transformer, the ratio of input
voltage to output voltage depends upon the ratio of the
number of turns on the primary to the number of turns on
the secondary.
How DOES a transformer work ???
FIGURE 25-11. If the
input voltage is connected
to the coils on the left,
with the larger number of
turns, the transformer
functions as a step-down
transformer. If the input
voltage is connected at the
right, it is a step-up
transformer.
FIGURE 25-12. Transformers
are used to reduce voltages to
consumer levels at the points of
use.