Download An example of electromagnetic induction: Chapter

Chapter 25 Electromagnetic Induction
and Electromagnetic Waves
Wednesday, March 24, 2010
3:16 PM
An example of electromagnetic induction:
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Faraday's further investigations of electromagnetic induction:
Here's another example of EM induction:
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And here's a final example of EM induction:
Faraday concluded from his investigations that a changing
magnetic field induces an electromotive force (i.e., emf,
which is another way of saying a potential difference) in a
nearby electric circuit, which ultimately causes electric
current to flow in the circuit.
To make this relationship quantitative, we need the
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To make this relationship quantitative, we need the
concept of magnetic flux, which we'll discuss next.
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There are three ways the magnetic flux through a coil of wire
can change: The strength of the magnetic field can change,
the area of the coil can change, or the relative orientation of
the coil and the magnetic field (i.e., the angle theta) can
change.
The following two diagrams illustrate Lenz's law:
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There is also an alternative version of the right-hand
rule that is convenient when the vectors are not
perpendicular. For example, to determine the direction
of the force on a positively charged particle when the
velocity of the particle and the magnetic field are not
perpendicular, it's simpler to curl your fingers from the
velocity vector to the magnetic field vector through the
acute angle; then the thumb points in the direction of
the force.
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CP 3 A 10-cm-long wire is pulled along a U-shaped
conducting rail in a perpendicular magnetic field. The total
resistance of the wire and rail is 0.20 Ω. Pulling the wire with
a force of 1.0 N causes 4.0 W of power to be dissipated in the
circuit. (a) Determine the speed of the wire. (b) Determine
the strength of the magnetic field.
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CP 14 Patients undergoing an MRI scan occasionally report
seeing flashes of light. Some practitioners assume that this
results from electrical stimulation of the eyes by the emf
induced by the rapidly changing fields of an MRI solenoid.
We can do a quick calculation to see if this is a reasonable
assumption. The human eyeball has a diameter of about 25
mm. Rapid changes in current in an MRI solenoid can
produce rapid changes in the magnetic field, with B/ t as
large as 50 T/s. How much emf would this induce in a loop
circling the eyeball? How does this compare with the 15 mV
necessary to trigger an action potential?
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necessary to trigger an action potential?
CP 15 A 1000-turn coil of wire 2.0 cm in diameter is in a
magnetic field that drops from 0.10 T to 0 T in 10 ms. The
axis of the coil is parallel to the field. Determine the emf in
the coil.
CP 18 A 5.0-cm-diameter loop of wire has resistance 1.2 Ω.
A nearby solenoid generates a uniform magnetic field
perpendicular to the loop that varies with time as shown
in the figure. Graph the magnitude of the current in the
loop over the same time interval.
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CP 21 A microwave oven operates at 2.4 GHz with an
intensity inside the oven of 2500 W/m 2. Determine the
amplitudes of the oscillating electric and magnetic fields.
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CP 29 At what distance from a 10 W point source of
electromagnetic waves is the electric field amplitude
(a) 100 V/m, and (b) 0.010 V/m.
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CP 31 Only 25% of the intensity of a polarized light wave
passes through a polarizing filter. What is the angle between
the electric field and the axis of the filter?
CP 43 The spectrum of a glowing filament has its peak at a
wavelength of 1200 nm. Determine the temperature of the
filament in degrees Celsius.
CP 57 A 100-turn, 8.0-cm-diameter coil is made of 0.50-mm
diameter copper wire. A magnetic field is perpendicular to the
coil. At what rate must B increase to induce a 2.0 A current in
the coil?
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CP 58 The loop in the figure is being pushed into the 0.20 T
magnetic field at a speed of 50 m/s. The resistance of the loop
is 0.10 Ω. Determine the direction and magnitude of the
current in the loop.
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