Electromagnet: Encyclopedia II - Electromagnet - Force on ferromagnetic materials

Electromagnet - Force on ferromagnetic materials

Computing the force on ferromagnetic materials is, in general, quite complex. This is due to fringing field lines and complex geometries. It can be simulated using finite element analysis. However, it is possible to estimate the maximum force under specific conditions. If the magnetic field is confined within a high permeability material, such as certain steel alloys, the maximum force is given by:

Where:

• F is the force in newtons
• B is the magnetic field in teslas
• A is the area of the pole faces in square meters
• μ is the permeability of free space

See energy in a magnetic field for more details on the derivation.

In the case of free space (air), , the force per unit area (pressure) is:

or @ B = 1 tesla

or @ B = 2 teslas

In a closed magnetic circuit:

Where:

• N is the number of turns of wire around the electromagnet
• I is the current in amperes
• L is the length of the magnetic circuit

Substituting above,

In order to build a strong electromagnet, a short magnetic circuit, with large area is preferred. Note that most ferromagnetic materials saturate around 1 to 2 teslas. This occurs at a field intensity of:

or

For this reason, there is no point in building an electromagnet with a higher field intensity. Industrial lifting electromagnets, like the one pictured above, are designed with both pole faces at one side (the bottom). This confines the field lines to maximize the magnetic field. It's like a cylinder within a cylinder. Many speaker magnets use a similar geometry, although the field lines are radial from the inner cylinder rather than perpendicular to the face. Notice the large surface area compared with the height. With pole faces of one square foot or more, thousands of pounds can be lifted with drive currents of just a few amperes or less.

Adapted from the Wikipedia article "Force on ferromagnetic materials", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki