electricity 2nd

THE SERIES/PARALLEL CIRCUIT

After reading this section you will be able to do the following:

• Explain what a series/parallel circuit is and what components are needed to complete it.
• Construct a series/parallel circuit.

When we have a circuit in which some of the components are in series and others are in parallel, we have a series/parallel circuit.

Try building a series/parallel circuit

Notice in this series/parallel circuit that the resistors R1, the switch, the battery, and the ammeter are in series with each other while resistors R2 and R3 are in parallel with each other. We might also say that the R2/R3 combination is in series with the rest of the components in this circuit. This is a very common circuit configuration. Many circuits have various combinations of series and parallel components.

If we apply Ohm’s law to any of these series or parallel circuits, we can calculate the current flowing at any point in the circuits.

Review

1. Some circuits contain series and parallel components. These are called series/parallel circuits.

DIRECT CURRENT

After reading this section you will be able to do the following:

• Explain what DC stands for and what it means.
• Define what a good source of DC would be.

Now that we have a fairly good understanding of basic electricity terms and concepts, let's take a closer look at some more details of the electrical current itself.

The battery we have been using for a current/voltage source generates direct current, which simply means the current flows in only one direction.

As long as electrons are flowing through the atoms of the circuit, work is being done. We can see that work is being done in this circuit because it lights the light bulb. The actual amount of electrons that are flowing is determined by the type and size of the battery as well as by the size and type of the light bulb. We could reverse the polarity of the battery by switching the contacts (wires), and the current would flow in the opposite direction and the bulb would still light.

Either way the battery is connected to the circuit, current can only flow in one direction. Direct current (DC) can also be generated by means other than batteries. Solar cells, fuel cells, and even some types of generators can provide DC current.

Review

1. DC, or direct current means the electrical current is flowing in only one direction in a circuit.
2. Batteries are a good source of direct current (DC).

ALTERNATING CURRENT

After reading this section you will be able to do the following:

• Define what AC stands for and what it means.
• Explain how AC is created and delivered to different places.
• Discuss the differences between AC and DC.

AC is short for alternating current. This means that the direction of current flowing in a circuit is constantly being reversed back and forth. This is done with any type of AC current/voltage source.

The electrical current in your house is alternating current. This comes from power plants that are operated by the electric company. Those big wires you see stretching across the countryside are carrying AC current from the power plants to the loads, which are in our homes and businesses. The direction of current is switching back and forth 60 times each second.

This is a series circuit using an AC source of electricity. Notice that the light bulb still lights but the electron current is constantly reversing directions. The change in direction of the current flow happens so fast that the light bulb does not have a chance to stop glowing. The light bulb does not care if it is using DC or AC current. The circuit is delivering energy to the light bulb from the source, which, in this case, is a power plant.

Review

1. AC, or alternating current means the electrical current is alternating directions in a repetitive pattern.
2. AC is created by generators in power plants, and other sources. This AC current is delivered to our homes and businesses by the power lines we see everywhere.
3. The frequency of repetition of this current is 60 Hertz. This means the direction of the current changes sixty times every second.

ELECTROMAGNETISM

After reading this section you will be able to do the following:

• Describe how a magnetic field is created.
• Explain how the electromagnet and the solenoid work together.

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.

ELECTROMAGNETIC INDUCTION

After reading this section you will be able to do the following:

• Explain how current can be induced in a conductor without making contact.
• Describe the process of 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.

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EDDY CURRENTS

After reading this section you will be able to do the following:

• Explain what an Eddy Current is.
• Discuss the one requirement necessary for a current to be induced into an object.

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

After reading this section you will be able to do the following:

• Explain how eddy currents are used in NDT.
• Explain why NDT is so important to our society.

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.

Electricity Unit Summary

So, what have we learned in this unit?

Photo Credit: Locke Karriker   Click for more info on photo.

We have learned that all matter is composed of atoms and that atoms are composed of protons, neutrons, and electrons. Protons have a positive charge, neutrons have a neutral charge, and electrons have a negative charge. Some materials, like copper, have electrons that are loosely held by their atoms and these "free" electrons easily move from atom to atom. Normally, these free electrons are moving from atom to atom in random directions so there is no net movement of electrons on one direction. However, we can force the electrons to move in on direction using a magnetic field, chemical reaction or other means. Electricity is the directional flow of electrons.

The directional flow of electrons (electrical current) is very useful because it can be used to do work. When electrons are moving, they produce a magnetic field and this magnetic field can be used to cause the shaft of a motor to spin and do work. Electrical current also generates heat that can be used to warm a home or produce light. The electrons moving through the filament of an incandescent light bulb cause the filament to heat up and glow. Only the fact that there is no oxygen inside the light bulb keeps the filament from burning up.

Electricity can also be used to make measurements and inspect conductive materials. When electricity is flowing in a coil, it produces a magnetic field in and around the coil. This magnetic field of the coil can then be used to generate electrical current in another conductor. These electrical currents are called eddy currents because they travel in circles like eddy currents in a stream. The strength of the eddy currents can be measured and this provides useful information about the material.

In this unit, we also learned about the various measures that are used to quantify electricity. Amperage is a measure of the electrical current strength and is the number of electrons moving past a fixed point in one second. Voltage is a measure of how many electrons are present in one area compared to another. When there is an imbalance of charge, the electrons want to move to the area of lower concentration until equilibrium is reached. The imbalance of charge is called electromotive force and it is measured using the volt as the unit.

You should also be aware that electricity can be very dangerous.  Electricity can cause burns and interfere with the normal electrical signals of the body and cause the heart to beat irregularly or even stop beating.  So, use electricity wisely and with caution.

Electricity is just one of the forms of energy that are useful to society.  Return the main menu and learn about one of the other energy forms discussed on this website.