Download Magnetism, Electromagnetism, & Electromagnetic Induction

Magnetism,
Electromagnetism, &
Electromagnetic Induction
Magnetic Fields
• The source of all magnetism is moving
electric charges.
• Iron is the element with the most magnetic
properties due to its net electron spin of 4.
• Magnetic field lines are vectors with a
direction from North to South.
• Magnetic field lines must not cross each
other.
• Magnetic fields are measured in Teslas and
is represented with the symbol B.
Earth’s Magnetic Field
The angle between the magnetic and
geographic poles is called the magnetic
declination.
Compasses
• Compass needles are magnetized and line up
along magnetic field lines.
• The North magnetic pole of a compass points
to the geographic north.
• Since opposites attract, the magnetic pole in
the Northern Hemisphere is actually a South
magnetic pole.
• The North pole of a compass points in the
direction of the field lines.
Magnetic Field around a
straight current-carrying wire
• A current moving through a wire
creates a magnetic field around
that wire.
• The magnetic field forms
concentric circles around the wire.
• Right hand rule (see picture):
thumb – points in direction of the
current in the wire
fingers curl – in direction of the
magnetic field
Electromagnets
• Electromagnets are
temporary magnets
formed by wrapping
wire around an iron
core.
• The iron becomes
magnetized when the
current is flowing due
to the magnetic field
being concentrated
inside the coil of wire.
Right-hand Rule:
Fingers curl – in direction
of the current (+ to -)
Thumb – points in
direction of the North pole
N
S
Force of a magnetic field on a
charged particle
• A charged particle moving
through a magnetic field will
experience a force that will
cause it to move in a circular
path.
• The force is  to both the
velocity and the magnetic
field direction.
F  qvB sin
F = force(N), q = charge(C), v = velocity(m/s),
B = mag. field strength(T), =angle between v & B
Right-hand Rule (F=qvB)
• Flat fingers – point in direction of the magnetic
field (B)
• Thumb – points in the direction the charged
particle is moving (v)
• Coming out of palm – direction of the force on the
charged particle (F)
• Note – This rule is for a positively charged
particle. For a negatively charged particle, force is
negative and so the direction of the force is in the
opposite direction of the right-hand rule.
Magnitude of Force (F=qvB)
• If Ө is zero, F = 0. So there is no force on the
charged particle if the particle moves parallel to
the magnetic field.
• If Ө is 90°, F = maximum. So there is a maximum
force on the charged particle if the charged particle
moves perpendicular to the magnetic field.
• If Ө is between 0 and 90°, the force will between
0 and a maximum. For the right-hand rules we will
assume 90°.
Force of a magnetic field on a
current-carrying wire
• A conductor with
a current flowing
through it in a
magnetic field
will experience a
force.
F  BIl sin
F = force(N), I = current(A), l = length of wire(m),
B = mag. field strength(T), =angle between l & B
Right-hand Rule (F  BIl )
• Flat fingers – point in the direction of the
magnetic field (B)
• Thumb – points in the direction the current
or moving charges (I)
• Out of palm – direction of the force on the
wire (F)
• Remember: magnetic field is from N to S,
current is from + to -
Magnitude of Force
( F  BIl )
• If Ө is zero, F = 0. So there is no force on the wire if
the wire moves parallel to the magnetic field.
• If Ө is 90°, F = maximum. So there is a maximum
force on the wire if the wire moves perpendicular to
the magnetic field.
• If Ө is between 0 and 90°, the force will between 0
and a maximum. For the right-hand rules we will
assume 90°.
Force between 2 currentcarrying wires
• When 2 wires carry current near
each other there will be an
interaction (force) between the
magnetic fields produced by
each individual wire.
• Current going the same way –
the wires are attracted.
• Current going the opposite waythe wires are repelled.
Induced EMF (Voltage)
• A conductor in a
changing magnetic
field will have an
EMF (voltage)
induced .
• Either the conductor
can be moving across
field lines or the
magnetic field can EMF = electromotive force or
itself be changing. voltage(V), B = magnetic field strength
EMF
EMF
EMF  Blv
(T), v = velocity of wire (m/s)
Induced Current
• When a EMF (voltage, or potential
difference) is present in a closed loop of
conducting material current will flow.
EMF
V
I

R
R
I=current(A), EMF = V = Voltage(V), R = resistance
Lenz Law
The voltage (and thus current) induced
when a wire or conductor is moved
through a magnetic field is induced in
such a way that the magnetic field
created by the induced current opposes
the original magnetic field that induced
the voltage or current.
Motors vs. Generators
• Motors
– Electric current is
changed to motion.
– A coil of wire with
a current through it
will be forced to
turn in a magnetic
field.
• Generators
– Motion is changed
to electric current.
– Turning a coil in a
magnetic field will
induce an EMF
(voltage), thus
current is
produced.
• As the loop of wire is turned
in the magnetic field, one side
is moving up while the other
is moving down, therefore a
current is induced in opposite
directions in the different
sections of the loop.
• As the loop continues to turn,
the sections of wire change
places and so the current
switches direction.
• This causes the current to
change constantly as shown
in the graph.
AC Generator
AC/DC
• Alternating Current (AC) • Direct Current (DC)
– current that switches
– current that flows in
direction of flow on
only one direction
regular time intervals
through a circuit
– 60 Hz in US
– supplied by batteries or
electrochemical cells
– created by EMF
induced in a coil of wire
– created by a chemical
turning in a magnetic
reaction that produces
field
a potential difference
(voltage) between the
two electrodes
(terminals)
Effective vs. Maximum with AC
Current
• DC values are comparable to Effective AC values.
• AC circuits do not get the effect of the maximum
current and voltage produced
• The power equivalent of AC to DC voltage is half.
Peff 
1
PAC 
1
2
Pmax
2 PDC
I eff  .707  I max
Veff  .707  Vmax
Transformers
• An alternating current flows through the
primary coil creating an alternating magnetic
field.
• This changing magnetic field induces an EMF
(Voltage) in the secondary coil and thus
current flows.
• In an ideal transformer, Power in = Power out
To solve Transformer Problems
The ratio of
voltages on the
two coils is equal
to the ratio of the
number of turns
in the coils.
Vs  Ns
Vp Np
Pin  Pout
VpIp  VsIs