(EAM) Electromagnets Lesson

Electromagnets

What do your doorbell, speakers at a rock concert, and heavy machinery at a wrecking yard have in common? They are all devices that use electricity to create a magnetic field, called electromagnets.  

Electromagnets find use in many practical applications. Electromagnets are used to lift large masses of magnetic materials such as scrap iron, rolls of steel, and auto parts. The overhead portion of this machine (painted yellow) is a lifting electromagnet. It is lowered to the deck where steel pipe is stored and it picks up a length of pipe and moves it to another machine where it is set upright and lowered into an oil well drill hole. A recent use for industrial electromagnets is to create magnetic levitation systems for bullet trains.

Let's look at how an electromagnet works.  First, it is important to know that a long coil of wire consisting of many loops of wire and making a complete circuit is called a solenoid. The magnetic field within a solenoid can be quite large since it is the sum of the fields due to the current in each individual loop.

If a piece of iron is placed inside the coil of wire, the magnetic field is greatly increased because the domains of the iron are aligned by the magnetic field of the current. The resulting magnetic field is hundreds of times stronger than the field from the current alone. This arrangement is called an electromagnet. The picture below shows an electromagnet with an iron bar inside a coil.

Two major advantages of electromagnets are that they are extremely strong magnetic fields and that the magnetic field can be turned on and off. When the current flows through the coil, it is a powerful magnet, but when the current is turned off, the magnetic field essentially disappears. 

Electric Motors

An electric motor is a device that uses an electromagnet to change electrical energy to kinetic energy. You can see a simple diagram of an electric motor below. The motor contains an electromagnet that is connected to a shaft. When current flows through the motor, the electromagnet rotates, causing the shaft to rotate as well. The rotating shaft moves other parts of the device. For example, in an electric fan, the rotating shaft turns the blades of the fan.

Why does the motor's electromagnet rotate?

• The electromagnet is located between the north and south poles of two permanent magnets. When current flows through the electromagnet, it becomes magnetized, and its poles are repelled by the like poles of the permanent magnets. This causes the electromagnet to rotate toward the unlike poles of the permanent magnets.
• A device called a commutator then changes the direction of the current so the poles of the electromagnet are reversed. The reversed poles are again repelled by the poles of the permanent magnets, which have not reversed. This causes the electromagnet to continue to rotate.
• These events keep repeating, so the electromagnet rotates continuously.

Electric Generators

These large machines are electric generators. This particular row of generators is installed in a hydroelectric power station. The insides of these generators are coils of wire spinning in a magnetic field. The relative motion between the wire and the magnetic field is what generates an electric current. In all generators, some mechanical energy is used to spin the coil of wire in the generator. In the case of hydroelectric power, the coil of wire is spun by water falling from a higher position to a lower position. Windmills and steam turbines are used in other types of power generators to spin the coil.

Electromagnetic induction is creating an electric current from a changing magnetic field. When a wire moves through a magnetic field, the electromagnetic effect takes place, creating an electric current through the wire. Electric generators are an application of electromagnetic induction.

Electric generators convert mechanical energy to electric energy. The generator consists of some number of wire loops wrapped around an iron core and placed in a strong magnetic field. The loops of wire and the iron core are called the armature. The armature is mounted so that it can rotate freely inside the magnetic field. Mechanical energy is used to spin the armature in the field so that the wire loops cut across the field and produce an electric current.

 

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