Electromagnetism

ELECTROMAGNETISM

In 1820, a Danish scientist named Hans Oersted discovered that a magnetic compass could be deflected from its resting position if a wire carrying electric current were placed near the compass. This deflection of the compass only occurred when current was flowing in the wire. When current was stopped, the compass returned to its resting position.

Magnetic Field

This graphic seems to indicate that any wire in which an electric current is flowing is surrounded by an invisible force field called a magnetic field. For this reason, any time we deal with current flowing in a circuit, we must also consider the effects of this magnetic field. We have all probably had experiences with magnets at one time or another. Magnets attract certain types of material like iron but almost nothing else.

Electromagnetism

The term electromagnetism is defined as the production of a magnetic field by current flowing in a conductor. We will need to understand electromagnetism in greater detail to understand how it can be used to do work.

Coiling a current-carrying conductor around a core material that can be easily magnetized, such as iron, can form an electromagnet. The magnetic field will be concentrated in the core. This arrangement is called a solenoid.The more turns we wrap on this core, the stronger the electromagnet and the stronger the magnetic lines of force become.

Electromagnet

We have created an electromagnet, which behaves just like a regular permanent bar magnet when the current is flowing. Notice that all of the lines of force pass through the center of the core material, regardless of how they extend outside the coil of wire. The direction of magnetic polarity is determined by the direction of current flowing in the coil of wire. The direction that the wire is coiled around the core also determines the direction of magnetic polarity. This is important to know if we want to use the electromagnet to apply a force to another material.

In the next sub-unit you will learn how the electrostatic field and field intensity are related to electromagnetism.

Review

1. A magnetic field is generated anytime an electrical current flows through a conductor.
2. The magnetic field around the conductor flows in closed loops.
3. Wrapping the wire into a coil creates an electromagnet.
4. Wrapping the wire around a piece of iron creates a solenoid.

ELECTROSTATIC FIELD

Electrostatic Field

Remember that electrons have a negative electrostatic field surrounding them. When energy from a power source such as a battery is applied to a circuit, making the electrons flow through a conductor, a new type of field is developed around the wire. This is called an electromagnetic field. You can learn more about why this field develops in the materials about magnetism.

As we can see in the diagram below, the magnetic field that surrounds a current-carrying conductor is made up of concentric lines of force. The strength of these circular lines of force gets progressively smaller the further away from the conductor we get. Also, if a stronger current is made to flow through the conductor, the magnetic lines of force become stronger. As a matter of fact, we can say that the strength of the magnetic field is directly proportional to the current that flows through the conductor.

Field Intensity

The term field intensity is used to describe the strength of the magnetic field. From now on we will use this new term to describe this field that is developed around a conductor that is carrying electrical current.

We have determined that this magnetic force field is a result of current flowing in a conductor. We have also shown that the field is circular in shape. What we do not yet know is what direction the circular field is in.

Field Direction (The Right-hand Rule)

A number of different rules have been developed to help determine the direction of the magnetic field relative to the current.  “The right-hand rule” is the simplest to remember and can be used to determine the direction of the electromagnetic field around a current carrying conductor.  With this rule when the thumb of the right-hand is pointing in the direction of current flow, the fingers will be pointing in the direction of the magnetic field.

Review

1. Field intensity is a term used to describe the strength of the electromagnetic field.
2. Field intensity is determined by the amount of electrical current flowing in the wire.
3. The right-hand rule can be used to describe the direction of the electromagnetic field.

ELECTROMAGNETIC INDUCTION

We have now seen that if electrical current is flowing in a conductor, there is an associated magnetic field created around the wire. In a similar manner, if we move a wire inside a magnetic field there will be an electrical current that will be generated in the wire.

Induction

Current is produced in a conductor when it is moved through a magnetic field because the magnetic lines of force are applying a force on the free electrons in the conductor and causing them to move. This process of generating current in a conductor by placing the conductor in a changing magnetic field is called induction. This is called induction because there is no physical connection between the conductor and the magnet. The current is said to be induced in the conductor by the magnetic field.

One requirement for this electromagnetic induction to take place is that the conductor, which is often a piece of wire, must be perpendicular to the magnetic lines of force in order to produce the maximum force on the free electrons. The direction that the induced current flows is determined by the direction of the lines of force and by the direction the wire is moving in the field.  In the animation above the ammeter (the instrument used to measure current) indicates when there is current in the conductor.

If an AC current is fed through a piece of wire, the electromagnetic field that is produced is constantly growing and shrinking due to the constantly changing current in the wire. This growing and shrinking magnetic field can induce electrical current in another wire that is held close to the first wire. The current in the second wire will also be AC and in fact will look very similar to the current flowing in the first wire.

It is common to wrap the wire into a coil to concentrate the strength of the magnetic field at the ends of the coil. Wrapping the coil around an iron bar will further concentrate the magnetic field in the iron bar. The magnetic field will be strongest inside the bar and at its ends (poles).

Take this link if you want to learn how a transformer is created: Creating a Transformer

Review

1. If we move a conductor in a magnetic field, a current is induced in the conductor (wire).
2. An AC current in a coil of wire can induce an AC current in another nearby coil of wire.

EDDY CURRENTS

In the discussion on the previous page you learned about electromagnetic induction. You learned that anytime a conductor was placed in a changing magnetic field that electrical current was generated in the conductor. We talked about the conductor being a piece of wire that is often wrapped into a coil, but the conductor does not need to be in the shape of a coil and does not even need to be wire. It could be a piece of flat steel, aluminum plate, or any other conductive object. The only requirement is that the object must be able to conduct electrical current.

When current is induced in a conductor such as the square piece of metal shown above, the induced current often flows in small circles that are strongest at the surface and penetrate a short distance into the material. These current flow patterns are thought to resemble eddies in a stream, which are the tornado looking swirls of the water that we sometimes see. Because of this presumed resemblance, the electrical currents were namededdy currents.

Uses of eddy currents

Just like in our transformer experiment, these induced eddy currents generate their own magnetic field. After all, this is an actual electrical current and any current flowing in a conductor produces a magnetic field, right? The detection and measurements of the strength of the magnetic fields produced by the eddy currents makes it possible for us to learn things about conductive materials without even contacting them. For example, the electrical conductivity of a material can be determined by the strength of the eddy currents that form. Also since cracks and other breaks in the surface of a material will prevent eddy currents from forming in that region of the surface, eddy currents can be used to detect cracks in materials. This is referred to as eddy current testing in the field of nondestructive testing (NDT). NDT technicians and engineers use eddy current testing to find cracks and other flaws in part of airplanes and other systems where bad things can happen if the part breaks. On the next page you will learn more about eddy current testing and be able to try an inspection yourself.

Review

1. Any electrically conductive object will conduct an induced current if it is placed in a changing magnetic field.
2. Eddy currents are circular induced currents.
3. Eddy currents generate their own magnetic fields.

NDT AND EDDY CURRENT TESTING

Nondestructive testing (NDT) means exactly what the words say. NDT literally means testing materials without destroying them. In other words, we can look for defects in a variety of metallic materials using eddy currents and never harm the material that we are testing. This is important because if we destroy the material we are testing, it does not do much good to test it in the first place. NDT is very important because often the defects that we are looking for are not visible because paint or some other coating may cover them. There might also be defects that are so small they cannot be seen with our eyes or any other visual method of inspection. Therefore, inspection methods such as eddy current inspection have been developed to detect the defects.

Try an Eddy Current Experiment

In the experiment below you will use eddy current testing to detect cracks in a block of metal. You will notice that you are using a coil of wire wrapped around a piece of iron to generate the magnetic field that caused the eddy currents to form in the metal. In the field of NDT the coil is called the inspection probe. The magnetic field that is generated by the eddy currents can be detected using this same probe. We can monitor the magnetic field being produced by these eddy currents with an instrument called an eddyscope. If there is a change in the magnetic field from the eddy currents, we can tell that we have found some sort of defect in the material that we are testing. When the instrument sees a change in the magnetic field generated by the eddy currents, it displays a change in the signal on the screen.

As long as the material being tested is very uniform in every way, the eddy currents will be uniform and consistent. If there is some defect in the material such as a crack, the eddy currents will be disturbed from their normal circular shapes. NDT technicians use many different types of eddy current testing equipment. Some are simple coils that are held on a piece of metal. Others use special probes, like the one shown above, that are pushed inside of tube such as those in heat exchanger units.

The technicians on the right are performing an eddy current inspection on the tube of a heat exchanger. Heat exchangers are used in places like nuclear power plants. Radioactive water from the reactor is circulated through the tubes and cooling water that will be returned to a river or lake is circulated on the outside of the tubes. It is very important that the radioactive water and the cooling water do not mix. Therefore, technicians perform eddy current inspections on the tubes to find and defects that may be present before they become leaks in the tubing.

Review

1. NDT stands for "Nondestructive Testing."
2. NDT methods are used to test materials and parts without harming them.
3. Eddy current testing is just one of the methods used by technicians to find defects before they cause problems.