Transcript Loudspeaker and Microphone_ppt_RevW10

Physics 106 Lesson #22
Magnetism:
Speakers and Microphones
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review: The Magnetic Force on a Wire
•The force on a wire
carrying current is
perpendicular to both
the wire and the
magnetic field
•The magnitude of the
force is the product of
the current, the wire
length and the
component of the field
perpendicular to the wire
•The force on a wire
carrying current parallel
to the magnetic field is
therefore zero!
B
Fingers point in the
direction of the current
flowing in the wire
F
Right Hand
IL
F  ILB for B to the wire
Review: Motors Exploit the Magnetic
Force on Current-Carrying Wires
• A current is established in a loop by a
battery.
• When the current-carrying loop is placed in a
magnetic field the field exerts a torque on the
loop causing it to rotate.
A rotating switch
(commutator) switches
the current direction to
maintain the torque in
the same direction as
the loop spins.
Review: A Changing Magnetic
Flux Induces a Voltage
• Faraday demonstrated that a
voltage was induced if the
magnetic flux through a loop
changed with time
• Qualitatively, magnetic flux
is the number of field lines
passing through the loop
• Move a magnet near a loop
or a loop near a magnet and
you can generate electricity!
• Faraday Induction turned on
the lights for all mankind!
Demonstration
Review: Quantifying Induced
Voltage (EMF) With Magnetic Flux
• Define Magnetic Flux Φ as the product
of the area of the loop and the
component of the magnetic field
perpendicular to the loop.
N Loops
• Φ is quantitatively equivalent to the
number of field lines passing through
the loop.
B
  B A

EMF   N
t
Review: How to Generate EMF
To change the magnetic flux
with time you can:
N Loops
1) Change the magnetic field with time.
2) Change the loop area with time.
3) Change the angle between the loop
and the field with time (spin the loop).
4) Any combination of 1 – 3.
Review: Generating
Alternating (ac) Current
Rotating a coil of
wire in a magnetic
field at a constant
rate produces
alternating current
that varies with time
as a sine function
Loudspeakers: Another Type of Motor
Applying an alternating
voltage to the coil of a
loudspeaker causes an
alternating current to flow
in the coil (I = V/Rcoil) which
in turn causes an
alternating magnetic force
on the coil and speaker
cone. This alternating
force causes the cone to
vibrate. The vibrating
cone produces a sound
wave which is a copy of
the alternating input
voltage (the input signal).
Note that the simple speaker we
built today has the magnet
attached to the cone and the
coil stationary, but it functions
in exactly the same way.
Microphones: Another Type of Generator
A microphone generates an
alternating voltage across a coil of
wire by exactly the same process
of Faraday Induction that
generates electrical power for the
power grid. In this case the
vibrations from a passing sound
wave cause a magnet to vibrate
near a coil (or vise-versa) to
produce an alternating voltage
signal which is a copy of the
original sound. The signal is tiny
compared to the AC EMF from a
generator, but the physics is
identical—changing magnetic flux
induces a voltage across a coil.