MAGNETIC FORCES, FIELDS,
AND OERSTED’S PRINCIPLE
Lesson 10

A magnet is a material or object that produces a
magnetic field.
 The
distribution of a magnetic force in the region of a
magnet.

Magnets are labelled North and South and have
the same field laws as electric charges.
 Similar
attract.
magnetic poles repel and dissimilar poles

To map a magnetic field a test compass can be
used. Much like a test charge, a test compass will
point north in line with the south field.

Magnetic forces can act between some metals that
are not magnetic. These metals are called
Ferromagnetic metals and include metals such as
cobalt, iron, nickel, or mixtures of the three.

The atomic structure of these metals seems to make
them strongly magnetic. Think of magnetic materials,
being made up of a lot of smaller magnets.
Domain Theory of Magnets

All large magnets are made up of many smaller
and rotatable magnets, called dipoles, which can
interact with other dipoles close by. If dipoles line
up, then a small magnetic domain is produced.
Electromagnets


Force at a distance is the common element between
electrostatics and magnetism. Hans Oersted studied
this and came up with the following principle.
Charge moving through a conductor produces a
circular magnetic field around the conductor.


Mapping the magnetic field of a conductor enables
one to be able to predict the direction of the
electromagnetic force from the current.
There are several hand signs developed to predict
how magnetic forces will act.
Left-hand rule # 1 for Conductors
(LHR #1)

Grasp the conductor with your left hand such that
the thumb points in the direction of the electron (-)
current flow. The curved fingers point in the direction
of the circular magnetic field around the conductor.


Using this rule allows us to produce a weak magnet
that we can turn on and off.
To make the electromagnet stronger and straighten
out the field so that it is more like a bar magnet, the
wire conductor is made into a coil. The individual
field lines fall on top of each other strengthening
the entire field. Coiling the wires also straightens
out the field.
Left hand rule # 2 (LHR#2)

Grasp the coiled conductor with the left
hand such that the curled fingers point in the
direction of the electron (-) current flow
through the conductor. The thumb points in
the direction of the magnetic field within the
coil. Outside the coil, the thumb represents
the north (N) end of the electromagnet
produced by the coil.

This allows us to create a magnet that acts like a
bar magnet but that can be tuned off when it needs
to be. The other advantage is that strength of the
electromagnet can be controlled by the following
factors.
1. Current in the coil
 The
greater the current flow, the greater the field
strength. Strength varies directly as the current in
the coil.
2. Number of turns in the coil
 The
greater the number of coils, the greater the
field strength. Strength varies directly as the
number of turns in the coil of the current is constant.
3. Type of material in the coils centre
 The
more ferromagnetic the material within the coil,
the greater the magnets strength. Iron is one of the
better materials to use.
4. Size of the coil
 the
smaller the diameter of the coil, the stronger the
magnetic field.
Questions

Given the direction of current flow in the conductor seen below,
find the direction of the magnetic field.
Current


Given the direction of the magnetic field, find the direction of
the current in the conductor seen to the right.
What happens to the strength of the magnetic field around a
coil if the current through the conductor is increased from 1.0 A
to 2.5 A? (Hint: Look at the 4 factors that affect
electromagnetic strength.)

What would happen to the field strength if the number
of turns in the coil of the electromagnet were reduced
by half and the current remained the same? (Hint: Look
at the 4 factors that affect electromagnetic strength.)


What would happen to the strength of an
electromagnet if over time the conductor started to
corrode and increased the resistance of the conductor?


What would happen to the strength of the
electromagnet if the coiled conductor started to unwind
causing the diameter of the coil to increase?