You can use your right hand as follows to determine the direction of the magnetic field relative to the current. Imagine grasping the wire with your right hand, with your thumb pointing in the direction of the current. Your remaining fingers will circle the wire giving you the direction of the magnetic field. This is known as the second right-hand rule. Once you have found the direction of the magnetic field relative to the current, you can use the right hand rule to determine the direction of the magnetic force acting on the wire. Which way is the current in the wire traveling if the magnetic field is directed out of the screen We can see that the magnetic field that is created when current flows through a long straight wire are represented by circles around the wire. How does the magnetic field strength vary with the strength of the current I, in the wire and the distance r, from the wire? The concentric rings showing the magnetic field get weaker with increasing distance from the wire. With double the distance from the wire, the magnetic field strength is ½ as strong. Therefore, we can deduce that the magnetic field strength is inversely proportional to the distance from the wire. But near the ends of a long wire, the magnetic force is no longer inversely... ...proportional to the distance from the wire because of distortions to the symmetry of the field. The field strength is inversely proportional to the distance as long as the distance from the wire is less than the distance to an end of the wire. Another generalization that can be made is that, when the current flowing in the wire increases, the magnetic field strength also increases. In fact, the magnetic field is directly proportional to the strength of the current in the wire. From these generalizations, we can conclude that the magnetic field B is directly... ...proportional to the current I and inversely proportional to the distance r from the wire, which can be written B is proportional to I divided by r. But remember, this relation holds only when the distance from the wire is less than the distance to the ends of the straight section of wire. To make this relation an equation, a proportionality constant k, must be added. The proportionality constant k equals mu sub zero divided by 2 times pi. The mu sub zero is called the Permeability of Free Space and is equal to 4 pi times ten to the negative seventh Tesla-meters per Amperes. The formula becomes mu sub zero divided by two pi; times the quantity current divided by the distance from the wire. This is equal to two times ten to the negative seventh Tesla meters divided by Amperes times the quantity I divided by r. The accuracy of a magnetic compass in an airplane is affected by magnetic fields that are generated nearby. What is the strength and direction of the magnetic field created as indicated by a... compass by a wire that is 10.0 centimeters behind the compass and carrying 1.5 Amperes of current?

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