Download EM-3 Powerpoint (Scannell)

 Forcing
a wire through a magnetic field
produces (generates) a current in the wire


Example: Generator
Converts mechanical energy into electrical
energy
 Generators
spin a bundle of wire attached to
a motor through magnets causing current or
electricity to be produced
 Electromagnetic
induction- a current is
produced in a wire by passing a magnet near
it

The magnetic field lines (flux) crossing the wire
cause the electrons in the wire to move
(electricity)
 Force
is greatest when the charge moves
perpendicular to the magnetic field
 At other angles the force is less
 The force is 0 when the charge moves
parallel to the magnetic field
 Magnetic
Flux ( Φ ) - The # of magnetic field
lines passing through a given area
 Magnetic
Flux( Φ ) =BA
 Where
is the greatest concentration of
magnetic flux lines?

Near the poles (ends)
A
current carrying wire placed in a magnetic
field experiences a force
 Example: Motors

Converts electrical energy into mechanical
energy
 1.
Keep hand flat
 2. Thumb in the direction of the current
 3. Fingers in the direction of the magnetic
field
 4. Palm in direction of the force

All are perpendicular to each other
3
factors affect the magnitude of the force
on a current-carrying wire placed in a
magnetic field
 1.
Strength of the magnetic field
 2. Amount of current (velocity)
 3. Length of the wire
 F=B
Il
 F=Force
(N)
 B=magnetic field strength (N/am)
 I=Current in wire (a)
 l=Length of the wire perpendicular to the
field (m)
A
segment of wire .040 m long is
perpendicular to the magnetic field inside a
solenoid. When a current of 3.0 amps flows
through the wire, it takes a force of 0.020
Newtons to balance the wire. What is the
magnetic field inside the solenoid.
F= B I L
.020N = B (3.0 amps) (.040m)
B = .17 N / amp m
 The
formula can also be written for any
single charge moving through a magnetic
field
 F=Bqv
 F=Force
(N)
 B=Magnetic field strength (N/am)
 q=charge (C)
 v=velocity of charge (m/s)
 When
not just any charge, but an electron
 F=Bev
 e=charge
for an electron (1.6 X 10^-19 C)
 An
electron moves through a magnetic field
of .20 N/Am at a speed of 300,000 m/s.
What is the force on the charge due to this
field?
F=Bev
F = .20 N/Am (1.6 x 10-19 C)(300,000 m/s)
F = 9.6 x 10-15 N
 Voltage-Energy
needed to move a charge
 Helps
explain how motors work
 Motors convert electrical energy into
mechanical energy
 Faraday’s
Law- Voltage (current) induced in
a wire is proportional to the rate of magnetic
flux cutting across the wire
 What happens when you increase voltage?

Increase magnetic flux (the # of magnetic field
lines/area)
 Relative
Motion-The movement of one
object with respect to another object
 Lenz’s
Law-The current induced in a wire is
in such a direction that its magnetic field
opposes the changing field that induced it
 Emf
= -B l v
 Emf=electromotive
force (volts)
 B=Magnetic Field Strength
 l=length of wire
 v=velocity
A
wire of length 50 cm is moving at a speed
of 2.0 m/s perpendicular to a magnetic field
of 0.75 N/A m. What emf is induced in the
wire?
Emf = -B l v
Emf = - (.75 N/A m) (.50 m) (2.0 m/s)
Emf = 0.75 Volts
 Transformers-
A device to increase or
decrease voltage
 http://phet.colorado.edu/en/simulation/far
aday
 N1/N2
= V1/V2
 N=number
of turns of wire
 V=voltage (volts)
 If
the primary coil having 5 turns of wire
contains 20 volts, what is the voltage in the
secondary coil having 10 turns?
N1/N2 = V1/V2
5 / 10 = 20 / V2
V2 = 40 Volts
 1.
A segment of wire 25 cm long is in a
magnetic field of .75 N/A m. The force on
the wire is 0.30 N. What is the current
flowing through the wire?
 2.
A charge (2.3 X 10^-15 C) moves through a
magnetic field at 225,000 m/s. What is the
magnetic field strength if the force is 4X10^4 N?