21.4

Magnetic Field Produced by a Solenoid

Student Notes

In this simulation, you can examine the magnetic field created by the current in a solenoid, which is a cylindrical coil of wire. Instead of using a spiral-shaped coil, the simulation approximates the coil with a stack of seven single loops. The plane of each loop is parallel to the x-z plane, with a radius of either 0.6 m or 0.25 m, and the displayed field is in the x-y plane.
Here are some things to consider. How does the magnetic field depend on the magnitude of the current? What happens to the magnetic field if the direction of the current is reversed? Is the direction of the magnetic field consistent with what you obtain using the right-hand rule? A solenoid is often used when a uniform magnetic field is required. How uniform is the field inside the solenoid? Does the magnitude of the field significantly different at the center of the loop from what it is near the edge of the loop? Compare the uniformity of the field in the solenoid to the field inside a single coil (see the previous simulation). As with a single current-carrying loop, doubling the current in a solenoid doubles the magnitude of the magnetic field. Does doubling the current affect the shape of the magnetic field in any way? What is the shape of the magnetic field determined by?

Self-test

The shape of the magnetic field from a solenoid is essentially the same as the magnetic field from a bar magnet that has the same dimensions as the solenoid. Even in a bar magnet, the magnetic field is produced by a current, which is a set of flowing charges. In a bar magnet, what are the charges that produce the magnetic field? You'll probably need to do some research on this question - how do the charges act collectively to generate the field? Do they actually flow around the bar magnet? The textbook states that the magnetic field inside an infinitely long solenoid is given by m on I, where n is the number of turns per unit length. In this simulation n is 1/the distance between the turns. For a range of solenoid geometries, compare the value of the magnetic field obtained using the equation to the magnetic field obtained by the simulation. The two values can be significantly different, depending on the geometry of the solenoid. This tells you something about the validity of the equation - when does the equation B = m on I apply?

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