Magnetic Forces

Definition of the Magnetic Field

With a brief history of electromagnetism, and a general understanding of what conditions give rise to a magnetic field, we may now precisely define the magnetic field.

Magnetic Field Acting on a Charge

When we defined the electric field, we first established the electric charge, and related the interaction of electric charges through Coulomb's Law. Unfortunately we cannot do the same for magnetic fields, because magnetic charges do not exist. Whereas electric fields originate from a single point charge, magnetic fields come from a wide variety of sources: currents in wires of varying shapes or forms, permanent magnets, etc. Instead of beginning with a description of the field created by each of these examples, we must define the magnetic field in terms of the force exerted by the field on a moving point charge.

Consider a point charge q moving with a velocity v that is perpendicular to the direction of the magnetic field, as shown below.

Figure %: A point charge moving perpendicular to a uniform magnetic field, B. The force on the charge is perpendicular to both the motion of the charge and the direction of the magnetic field.

In this very simple case, the force felt by the positive point charge has magnitude

 F =

where B is the magnitude of the magnetic field, and c is the speed of light. The force points in the positive z direction, as shown in the figure. Because we are now working in three dimensions, it is often difficult to determine the direction of this force. The easiest way to do this is to use your hands, as we will explain.

First Right Hand Rule

Take your right hand (it is important not to use the left one), and stick your thumb, your index finger and your middle finger in mutually perpendicular directions. Each one of these fingers represents a vector quantity: the thumb points in the direction of the velocity of the positively charged particle, the index finger points in the direction of the magnetic field, and the middle finger points in the direction of the force felt by the moving charge. Try it out on the above figure: point your thumb in the negative x direction and your index finger in the negative y direction. Hopefully you will find that your middle finger points in the positive z direction, which is exactly the direction of the force. This is known as the first right hand rule.

Magnetic Force when Moving Charges are not Perpendicular

We disucussed the special case in which the moving charge moves perpendicular to the magnetic field. This perfectly perpendicular situation is uncommon. In more normal circumstances the magnetic force is proportional to the component of the velocity that acts in the perpendicular direction. If a charge moves with a velocity at an angle θ to the magnetic field, the force on that particle is defined as:

 F =

If you are familiar with vector calculus, you will notice that this can be simplified in terms of cross products:

forceequation*

 =

This last equation is the most complete; the cross product of two vectors is always perpendicular to both vectors, providing the correct direction for the direction of our force.

Having established this equation, let us take a moment to analyze its implications. First, it is clear that a charge moving parallel to the magnetic field experiences no force, as the cross product is zero. Second, the magnitude of the force on the charge varies directly not only with the magnitude of the charge, but of the velocity as well. The faster a charged particle travels, the more force it will feel in the presence of a given magnetic field.

This equation forms a basis for our study of electromagnetism. From it we will be able to derive the fields created by various wires and magnets, and derive some properties of the magnetic field.

Relating Magnetic and Electric Forces

Using the definition of the magnetic field we have just developed, we become able to generate a complete expression for the force exerted on a charged particle, q , in the presence of both electric and magnetic fields. Recall that in the presence of an electric field alone the force felt by a point charge q is simply proportional to the field at that point, or F = qE . Thus, if this point charge is in the presence of both an electric field and a magnetic field, we can find the total force on the charge by simple vector addition:

 = q +

This equation only applies to vector quantities--usually the force due to the electric field and the magnetic field are not in the same direction, and cannot be added algebraically.

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