Download Magnetism (High School)

Magnetism
Properties of Magnets
 All magnets have two areas of strongest force, called
poles.
 Each magnet has one north pole and one south pole.
 Like poles repel, and opposite poles attract.
 The magnetic region where you can “feel the force” is
called a magnetic field.
Magnetic Poles
If you break a bar magnet in
half, each half still behaves as
a complete magnet.
Break the pieces in half again,
and you have four complete
magnets.
Even when your piece is one
atom thick, there are two
poles. This suggests that
atoms themselves are
magnets.
Magnetic Materials
 What makes some things

magnetic, while other things
can’t be magnetized?
 Spinning electrons cause small
magnetic fields around each
atom.
 Magnetic materials have atoms
whose magnetic fields can be 
lined up in the same direction.
 Areas where atoms’ magnetic
fields line up are called
magnetic domains.
magnetic domain
Randomly arranged domains =
No magnet!
Magnetic domains lined up =
Magnet!
Magnetic Fields
Iron filings sprinkled on a sheet of paper over a bar magnet will
tend to trace out a pattern of lines that surround the magnet.
The space around a magnet, in which a magnetic force is
exerted, is filled with a magnetic field.
The shape of the field is revealed by magnetic field lines.
Magnetic Fields
Magnetic field lines spread out from one pole, curve around
the magnet, and return to the other pole.
Magnetic Fields
• The direction of the magnetic field
outside a magnet is from the north
to the south pole.
• Where the lines are closer together,
the field strength is greater.
• The magnetic field strength is
greater at the poles.
• If we place another magnet or a
small compass anywhere in the
field, its poles will tend to line up
with the magnetic field.
Magnetic Fields
Magnetic Fields
Electric Fields
Electric fields arise from voltage.
Are a vector.
Their strength is measured in Volts per
meter (V/m)
An electric field can be present even when a
device is switched off
Can do work.
Particles may change speed
Particles may change direction
Electric Fields
Field strength decreases with distance from
the source.
Most building materials shield electric fields
to some extent
Begin on positive charges and end of
negative charges.
Electric force acts along the direction of the
electric field.
Acts on a charged particle regardless of
whether the particle is moving.
Magnetic Fields
Magnetic fields arise from current flows.
Are a vector.
Their strength is measured in Tesla (T).
Magnetic fields exist as soon as a device is
switched on and current flows.
CANNOT do work
Particle speed is constant
Particle direction can change.
Magnetic Fields
Field strength decreases with distance from
the source.
Magnetic fields are not attenuated by most
materials.
Have no beginning and no end, form
continuous circles.
Magnetic force acts perpendicular to the
magnetic field
Acts on a charged particle ONLY when the
particle is in motion.
Electric Fields vs. Magnetic
Fields
Electric Fields vs. Magnetic
Fields
Electric Fields vs. Magnetic
Fields
Electric Fields vs. Magnetic
Fields
Electric Currents &
Magnetic Fields
• A moving charge produces a magnetic field.
• An electric current passing through a conductor
produces a magnetic field because it has many
charges in motion.
• The magnetic field surrounding a current-carrying
conductor can be shown by arranging magnetic
compasses around the wire.
• The compasses line up with the magnetic field
produced by the current, a pattern of concentric
circles about the wire.
• When the current reverses direction, the compasses
turn around, showing that the direction of the
magnetic field changes also.
Electric Currents &
Magnetic Fields
a. When there is no current in the wire, the compasses
align with Earth’s magnetic field.
Electric Currents &
Magnetic Fields
a. When there is no current in the wire, the compasses
align with Earth’s magnetic field.
b. When there is a current in the wire, the compasses
align with the stronger magnetic field near the wire.
Electric Currents &
Magnetic Fields
A currentcarrying coil of
wire is an
electromagnet.
The Earth is a magnet!
magnetic north pole
geographic north pole
magnetic south pole
geographic south pole
 Magnetic lines of force around
the earth are like the field
lines around a giant bar
magnet.
 The magnetic north pole and
the geographic north pole are
not located in the same place!
 The north pole of a compass
points to the earth’s magnetic
north pole.
Electricity to Magnetism
 In 1820, H.C. Oersted
discovered that an electric
current flowing through a wire
had a magnetic field around it.
 Electricity can cause
magnetism!
 Electromagnets are powerful
magnets that can be turned on
and off.
 You can make an
electromagnet stronger by (1)
putting more turns of wire in
the coil or (2) making a larger
soft iron core, or (3)
increasing the current through
the wire.
Uses for electromagnets
 A simple DC electric motor
contains a permanent magnet, an
electromagnet, and a commutator.
When current flows through the
electromagnet, it turns within the
magnetic field of the permanent
magnet, changing electricity to
mechanical energy.
 Current meters also use permanent
magnets and electromagnets.
When current flows through a wire,
it makes an electromagnet. The
force between the electromagnet
and the permanent magnet makes
a needle move on the meter.
Magnetism to Electricity
 Joseph Henry and Michael
Faraday discovered that
magnetism could also produce
electric current. This is called
 If a magnet is moved back and
forth through a coil of wire,
current can be made to flow
through the wire. This is the
idea behind electric generators
and transformers.
Current moves left in wire.
Current moves right in wire.
electromagnetic induction.
Electromagnetic Induction
No battery or other
voltage source was
needed to produce a
current—only the
motion of a magnet
in a coil or wire loop.
Voltage was induced
by the relative motion
of a wire with respect
to a magnetic field.
Electromagnetic Induction
• The production of voltage depends only on the relative
motion of the conductor with respect to the magnetic field.
• Voltage is induced whether the magnetic field moves past
a conductor, or the conductor moves through a magnetic
field.
• The results are the same for the same relative motion.
Electromagnetic Induction
• The amount of voltage induced depends on how quickly
the magnetic field lines are traversed by the wire.
• Very slow motion produces hardly any voltage at all.
• Quick motion induces a greater voltage.
• Increasing the number of loops of wire that move in a
magnetic field increases the induced voltage and the
current in the wire.
• Pushing a magnet into twice as many loops will induce
twice as much voltage.
Electromagnetic Induction
Twice as many loops as another means twice as much
voltage is induced. For a coil with three times as many loops,
three times as much voltage is induced.
Electromagnetic Induction
• We don’t get something
(energy) for nothing by
simply increasing the number
of loops in a coil of wire.
• Work is done because the
induced current in the loop
creates a magnetic field that
repels the approaching
magnet.
• If you try to push a magnet
into a coil with more loops, it
requires even more work.
Electromagnetic Induction
Work must be done to move the magnet.
a. Current induced in the loop produces a magnetic field
(the imaginary yellow bar magnet), which repels the bar
magnet.
Electromagnetic Induction
Work must be done to move the magnet.
a. Current induced in the loop produces a magnetic field
(the imaginary yellow bar magnet), which repels the bar
magnet.
b. When the bar magnet is pulled away, the induced
current is in the opposite direction and a magnetic field
attracts the bar magnet.
Electromagnetic Induction
• The law of energy conservation applies here.
• The force that you exert on the magnet multiplied by the
distance that you move the magnet is your input work.
• This work is equal to the energy expended (or possibly
stored) in the circuit to which the coil is connected.
Electromagnetic Induction
• If the coil is connected to a resistor, more induced voltage
in the coil means more current through the resistor.
• That means more energy expenditure.
• Inducing voltage by changing the magnetic field around a
conductor is electromagnetic induction.
Uses for Electromagnetic
Induction
 Generators produce AC current
for home and industrial use.
Water, wind, or steam are
used to move large
electromagnets through the
coils of wire to produce
current.
 Transformers are used to step
up voltage of electricity that
must travel long distances
through wires. Other
transformers then step down
the voltage before it enters
our homes.
Faraday’s Law
• Faraday’s law describes the relationship between
induced voltage and rate of change of a magnetic field:
• The induced voltage in a coil is proportional to the
product of the number of loops, the cross-sectional area
of each loop, and the rate at which the magnetic field
changes within those loops.
Faraday’s Law
• The current produced by electromagnetic
induction depends upon
• the induced voltage,
• the resistance of the coil, and the circuit to
which it is connected.
• For example, you can plunge a magnet in and out
of a closed rubber loop and in and out of a closed
loop of copper.
• The voltage induced in each is the same but the
current is quite different—a lot in the copper but
almost none in the rubber.
Fleming’s Hand Rules
 If a current carrying conductor placed in a magnetic field,
it experiences a force due to the magnetic field. On the
other hand, if a conductor moved in a magnetic field, an
emf gets induced across the conductor (Faraday's law of
electromagnetic induction).
 John Ambros Fleming originated two rules to determine
the direction of motion (in electric motors) or the
direction of induced current (in electric generators). The
rules are called as, Fleming's left hand rule (for
motors) and Fleming's right hand rule (for
generators).
Left Hand Rule
 Fleming's left hand rule is applicable for
electric motors. Whenever a current
carrying conductor is placed in a
magnetic field, the conductor
experiences a force. According to
Fleming's left hand rule, if the
thumb, fore-finger and middle finger of
left hand are stretched perpendicular to
each other as shown the figure, and if
fore finger represents the direction of
magnetic field, the middle finger
represents the direction of current, then
the thumb represents the direction of
force.

Right Hand Rule
 Fleming's right hand rule is applicable for electrical
generators. As per Faraday's law of electromagnetic
induction, whenever a conductor is moved in an
electromagnetic field, and closed path is provided to the
conductor, current gets induced in it.
Right Hand Rule
 According to Fleming's right
hand rule, the thumb, fore
finger and middle finger of
right hand are stretched
perpendicular to each other as
shown in the figure at right,
and if thumb represents the
direction of the movement of
conductor, fore-finger
represents direction of the
magnetic field, then the middle
finger represents direction of
the induced current.
Right Hand Rule