Practical Physics is a free textbook on basic laboratory physics. See the editorial for more information....  # Magnetic Field and Magnetic Force

Definition of magnetic field. - A portion of space throughout which magnetic effects are exerted by any distribution of magnetism is called the magnetic field due to that distribution.

Let us consider the magnetic field due to a given distribution of magnetism. At each point of the field a pole of strength m is acted on by a definite force. The Resultant Magnetic Force at each point of the field is the force which is exerted at that point on a positive pole of unit strength placed there.

This is also called the Strength of the Magnetic Field at the point in question.

If H be the strength of the field, or the resultant magnetic force at any point, the force actually exerted at that point on a pole of strength m is m H.

The magnetic force at each point of the field will be definite in direction as well as in magnitude.

Definition of line of magnetic force. - If at any point of the field a straight line be drawn in the direction of the magnetic force at that point, that straight line will be a tangent to the Line of Magnetic Force which passes through the point. A Line of Magnetic Force is a line drawn in such a manner that the tangent to it at each point of its length is in the direction of the resultant magnetic force at that point.

A north magnetic pole placed at any point of a line of force would be urged by the magnetic force in the direction of the line of force.

As we shall see shortly, a small magnet, free to turn about its centre of gravity, will place itself so that its axis is in the direction of a line of force.

A surface which at each point is at right angles to the line of force passing through that point is called a level surface or surface of equilibrium, for since the lines of force are normal to the surface, a north magnetic pole placed anywhere on the surface will be urged by the magnetic forces perpendicularly to the surface, either inwards or outwards, and might therefore be regarded as kept in equilibrium by the magnetic forces and the pressure of the surface. Moreover, if the pole be made to move in any way over the surface, since at each point of its path the direction of its displacement is at right angles to the direction of the resultant force, no work is done during the motion.

Last Update: 2011-03-27