MAGNETIC FORCES, FIELDS, AND OERSTED’S PRINCIPLELesson 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 magnetic poles repel and

dissimilar poles attract.

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.  

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.)

Current

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?