SUMMARY: Oersted showed that magnetic effects
could be produced by moving electrical charges;
Faraday and Henry showed that electric currents
could be produced by moving magnets
from forces between
electric charges in
What We Will Learn About Magnetism
1. There are North Poles and South Poles.
2. Like poles repel, unlike poles attract.
3. Magnetic forces attract only magnetic materials.
4. Magnetic forces act at a distance.
5. While magnetized, temporary magnets act like permanent
6. A coil of wire with an electric current flowing through it becomes
7. Putting iron inside a current-carrying coil increases the strength
of the electromagnet.
8. A changing magnetic field induces an electric current in a
9. A charged particle experiences no magnetic force when
moving parallel to a magnetic field, but when it is moving
perpendicular to the field it experiences a force perpendicular
to both the field and the direction of motion.
10. A current-carrying wire in a perpendicular magnetic field
experiences a force in a direction perpendicular to both the
wire and the field.
Every magnet has at least one north pole and one south pole. By
convention, we say that the magnetic field lines leave the North end
of a magnet and enter the South end of a magnet.
If we take a bar magnet and break it into two pieces, each piece will
again have a North pole and a South pole. If we take one of those
pieces and break it into two, each of the smaller pieces will have a
North pole and a South pole. No matter how small the pieces of the
magnet become, each piece will have a North pole and a South pole.
It has not been shown to be possible to end up with a single
North pole or a single South pole, which is a monopole ("mono"
means one or single, thus one pole).
Note: Some theorists believe that magnetic monopoles may
have been made in the early Universe. So far, none have been
We will say that a moving charge sets up in the space
around it a magnetic field,
it is the magnetic field which exerts a force on any other
charge moving through it.
Magnetic fields are vector
quantities….that is, they have a
magnitude and a direction!
Magnetic Field vectors as written as B
Direction of magnetic field at any point is defined
as the direction of motion of a charged particle on
which the magnetic field would not exert a force.
Magnitude of the B-vector is proportional to the
force acting on the moving charge, magnitude of the
moving charge, the magnitude of its velocity, and the
angle between v and the B-field. Unit is the Tesla or
the Gauss (1 T = 10,000 G).
The Concept of “Fields”
realized that ...
A magnet has a
the surrounding space
Magnetic field lines describe the structure of magnetic fields
in three dimensions.They are defined as follows. If at any
point on such a line we place an ideal compass needle, free to
turn in any direction (unlike the usual compass needle, which
stays horizontal) then the needle will always point along the
Field lines converge where the magnetic force is strong, and
spread out where it is weak. For instance, in a compact bar
magnet or "dipole," field lines spread out from one pole and
converge towards the other, and of course, the magnetic
force is strongest near the poles where they come together.
Action at a Distance Explained
Although two magnets
may not be touching,
they still interact
This explains the
‘action at a distance’,
say of a compass.
Right Hand Rule!
Put your fingers in the direction of motion of
the charge, curl them in the direction of the
magnetic field. Your thumb now points in the
direction of the magnetic force acting on the
charge. This force will bend the path of the
moving charge appropriately.
Since moving charges experience a force in a magnetic field, a currentcarrying wire will experience such a force, since a current consists of
moving charges. This property is at the heart of a number of devices.
An electric motor, is a
machine which converts
electrical energy into
mechanical (rotational or
A current is passed
through a loop which is
immersed in a magnetic
field. A force exists on
the top leg of the loop
which pulls the loop out
of the paper, while a
force on the bottom leg
of the loop pushes the
loop into the paper.
The net effect of these forces is
to rotate the loop.
An electromagnet is simply a coil of wires which, when a
current is passed through, generate a magnetic field, as
In other words….materials which produce
magnetic fields with no apparent circulation of
All substances - solid, gas, and liquid - react to
the presence of a magnetic field on some level.
How much they react causes them to be put into
several material “types”.
When a ferromagnetic material is
placed near a magnet, it will be attracted toward the region of
greater magnetic field. This is what we are most familiar with when
our magnet picks up a bunch of paperclips. Iron, cobalt, nickel,
gadolinium, dysprosium and alloys containing these elements exhibit
ferromagnetism because of the way the electron spins within one
atom interact with those of nearby atoms. They will align
themselves, creating magnetic domains forming a permanent magnet.
If a piece of iron is placed within a strong magnetic field, the
domains in line with the field will grow in size as the domains
perpendicular to the field will shrink in size.
• domains in which the magnetic
fields of individual atoms align
• orientation of the magnetic
fields of the domains is random
• no net magnetic field.
• when an external magnetic
field is applied, the magnetic
fields of the individual domains
line up in the direction of the
• this causes the external
magnetic field to be enhanced
If we look at a solenoid, but rather than
air, wrap it around a nice iron core. What
happens to the change in flux for a given
Can you see why ferromagnetic materials
are often put in the middle of currentcarrying coils?
When a diamagnetic material is placed
near a magnet, it will be repelled from the region of greater
magnetic field, just opposite to a ferromagnetic material. It is
exhibited by all common materials, but is very weak. People and
frogs are diamagnetic. Metals such as bismuth, copper, gold,
silver and lead, as well as many nonmetals such as water and most
organic compounds are diamagnetic.
When a paramagnetic material is
placed near a magnet, it will be attracted to the region of greater
magnetic field, like a ferromagnetic material. The difference is
that the attraction is weak. It is exhibited by materials
containing transition elements, rare earth elements and actinide
elements. Liquid oxygen and aluminum are examples of