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EMF Induced in a Moving Conductor
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This figure shows another way the magnetic flux can change.
It can change if a conducting loop is
moved in a static magnetic field.
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The induced current in the figure is in a direction
that tends to slow the moving bar. That is,
It takes an external force to keep it moving.
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The induced emf has magnitude
This equation holds only if B, l, & v are mutually perpendicular.
If they are not, then it is true for their perpendicular components.
Example
Does a moving plane develop a large emf?
An airplane travels at speed
v = 1000 km/h in a region where the
Earth’s magnetic field is about
B = 5  10-5 T & is nearly vertical.
Calculate the potential difference induced
between the wing tips that are l = 70 m apart.
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Example
Electromagnetic blood-flow measurement
The rate of blood flow in our body’s vessels can be measured
using the apparatus shown, since blood contains charged ions.
Suppose that the blood vessel is 2.0 mm in diameter, the
magnetic field is 0.080 T, & the measured emf is 0.10 mV.
Calculate the flow velocity v of the blood.
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Example: Force on a rod.
To make the rod move to the right at speed v, you need to
apply an external force on the rod to the right.
Calculate
(a) The magnitude of the required force.
(b) The external power needed to move the rod.
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Electric Generators
A generator is the opposite of a motor. It transforms
mechanical energy into electrical energy. The figure
shows an ac generator:
The axle is rotated by an
external force such as
falling water or steam.
The brushes are in constant
electrical contact with the
slip rings.
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If the loop is rotating with constant angular
velocity ω, the induced emf is sinusoidal:
For a coil of N loops,
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Example: AC generator.
The armature of a 60-Hz ac generator rotates in a 0.15 T
magnetic field. The area of the coil is 2.0  10-2 m2, Calculate
the number of loops needed for the peak output to be E = 170 V.
A dc generator is similar to an ac generator, except
that it has a split-ring commutator instead of slip rings.
AC
DC
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Automobiles now use alternators rather than
dc generators, to reduce wear.
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