MAG - Electromagnets and Their Applications

Electromagnets and Their Applications

Solenoids and Electromagnetics

As we ve seen, if you take a wire and make a loop, the magnetic field is directed through the center of the loop in a direction determined by the current flow. The strength of the magnetic field through the loop is a function of the strength of the current and the permeability of free space. If we continue to loop our wire, each loop adds to the string of the magnetic field. A coil of wire created in this manner is known as a solenoid. The strength of the magnetic field through a solenoid is given by:

N is the number of turns or loops in the solenoid and l is the length of the solenoid. As you can see from this equation you can strengthen the magnetic field inside the solenoid by increasing the current or the number of turns of wire. The direction of the magnetic field through the solenoid can be determined using our modified version of right hand rule #1 for loops of current. Simply coil your fingers in the direction of the current loops and your extended thumb points in the direction of the magnetic field. Switching the direction of current flips the magnetic field direction.

The strength of the magnetic field created in a solenoid can be increased dramatically by adding a ferromagnetic core. When the solenoid surrounds a metal like iron, the solenoid produces a magnetic field inside the ferromagnetic core. The total magnetic field strength is now due to both the solenoid and magnetic core. This field can be hundreds to thousands of times stronger than the field of the solenoid alone. A ferromagnetic core solenoid is known as an electromagnet. You can create simple electromagnets by winding wire around a nail and connecting the wire to a battery. Electromagnets have many everyday uses such as doorbells, speakers, and electric motors. Check out the Electromagnets resource link in the side bar for descriptions of how electromagnets are used in several common devices.

Galvanometers

A galvanometer is a device that can be used to measure current, voltage, and resistance in a circuit. The device works by placing a rotating coil of wire in between the poles of two permanent magnets. The coil is attached to a spring in order to hold it in position. When a current is run through the coil, the coil's magnetic field attempts to align with the external field provided by the permanent magnets. The spring works against this torque. A needle attached to the coil measures the position where the torque from the spring reaches equilibrium with the torque from the permanent magnets on the coil. The stronger the current in the coil, the greater the deflection of the needle.

Electric Motors

We've seen how a current carrying loop placed in an external magnetic field will rotate itself so that its magnetic field aligns with the external magnetic field. Once this alignment is complete, the loop will hold its position. If, however, you could change the direction of the electric current through the loop once it became aligned, the loop would spin 180 degrees as its magnetic field will have been reversed. Continuously changing the direction of the coils current and, therefore, its magnetic field would cause the loop to spin continuously. This is the principle behind the electric motor. An electric motor is a device used to convert electric energy to mechanical energy. Watch this video to get a clearer understanding of how an electric motor works.

As you can see in the video a device, known as a commutator, allows for the current in the loop to be reversed every time the loop spins 180 degrees. The electric motor is one of the two most prevalent applications of the relationship between electricity and magnetism. The second device, the electric generator, will be discussed in the next module.

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