2 minute read
Magnetic Field and Currents
by AudioLearn
Figure 132.
MAGNETIC FIELD AND CURRENTS
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What you have learned so far is that currents and magnetism are related. Currents themselves can create a significant magnetic field as can be demonstrated by trying to use a compass near overhead electric power lines. Current loops create magnetic fields, as has been demonstrated in the way motors work. What about a straight wire and the current applied to it? Will this cause a significant electric field. Look back at figure 127. It shows the direction of the magnetic field as it applies to a wire and a current. This is referred to as the right-hand rule two. The current will wrap around counterclockwise if the current is going upward as is identified by the curvature of the fingers and the direction of the thumb.
For a long, straight wire with a current applied, the magnitude of the magnetic field B will be proportional to the current inversely proportional to r, which is the shortest distance to the wire. The actual equation is shown in figure 133. It involves a constant called the permeability of free space, which is related to the speed of light. The assumption is that the wire is very long with the constant being four pi x 10-7 Teslameters per ampere.
Figure 133.
So, what about a current-carrying circular loop? There will be a field produced in the center of the loop that has a certain strength. At the center the magnetic field will be the permeability of free space multiplied by the current divided by two times the radius of the circle enclosed by the loop. It is only valid at the center of the loop but it is similar to the findings seen in a straight wire. The larger the loop, the smaller will the field be at the center of the loop.
A solenoid is a long coil of wire that can create a strong, uniform magnetic field with very little field created outside of the coils. A battery is applied to the solenoid, creating a field inside the tube of coils. Only at the ends of the tube will the field strength be diminished. This will be a uniform field as is seen in figure 134:
