Introduction To Electricity

INTRODUCTION TO ELECTRICITY After reading this section you will be able to do the following: • •

Define electricity and identify the origins of the term. Discuss how electricity can be observed in the world.

What is Electricity? Electricity is a naturally occurring force that exists all around us. Humans have been aware of this force for many centuries. Ancient man believed that electricity was some form of magic because they did not understand it. Greek philosophers noticed that when a piece of amber was rubbed with cloth, it would attract pieces of straw. They recorded the first references to electrical effects, such as static electricity and lightning, over 2,500 years ago.

It was not until 1600 that a man named Dr. William Gilbert coined the term “electrica,” a Latin word which describes the static charge that develops when certain materials are rubbed against amber. This is probably the source of the word “electricity." Electricity and magnetism are natural forces that are very closely related to one another. You will learn a little about magnetism in this section, but there is a whole section on magnetism if you want to learn more. In order to really understand electricity, we need to look closely at the very small components that compose all matter.

Review 1. Electricity occurs naturally and has been observed for thousands of years. 2. Electricity and magnetism are very closely related.

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

Explain the differences between electrons and protons. Predict what happens when protons and electrons interact with other protons or electrons.

Electrons Electrons are the smallest and lightest of the particles in an atom. Electrons are in constant motion as they circle around the nucleus of that atom. Electrons are said to have a negative charge, which means that they seem to be surrounded by a kind of invisible force field. This is called an electrostatic field.

Protons Protons are much larger and heavier than electrons. Protons have a positive electrical charge. This positively charged electrostatic field is exactly the same strength as the electrostatic field in an electron, but it is opposite in polarity. Notice the negative electron (pictured at the top left) and the positive proton (pictured at the right) have the same number of force field lines in each of the diagrams. In other words, the proton is exactly as positive as the electron is negative. Like charges repel, unlike charges attract Two electrons will tend to repel each other because both have a negative electrical charge. Two protons will also tend to repel each other because they both have a positive charge. On the other hand, electrons and protons will be attracted to each other because of their unlike charges. Since the electron is much smaller and lighter than a proton, when they are attracted to each other due to their unlike charges, the electron usually does most of the moving. This is because the protons have more mass and are harder to get moving. Although electrons are very small, their negative 2

electrical charges are still quite strong. Remember, the negative charge of an electron is the same as the positive electrical charge of the much larger in size proton. This way the atom stays electrically balanced. Another important fact about the electrical charges of protons and electrons is that the farther away they are from each other, the less force their electric fields have on each other. Similarly, the closer they are to each other, the more force they will experience from each other due to this invisible force field called an electric field.

Review 1. Electrons have a negative electrostatic charge and protons

have a positive electrostatic charge. 2. A good way to remember what charge protons have is to remember both proton and positive charge start with "P." 3. Like charges repel, unlike charges attract, just like with magnets.

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

Explain how an electrical current is produced.

Electricity is a term used to describe the energy produced (usually to perform work) when electrons are caused to directional (not randomly) flow from atom to atom. In fact, the day-to-day products that we all benefit from, rely on the movement of electrons. This movement of electrons between atoms is called electrical current. We will look at how electrical current is produced and measured in the following pages.

Review 1. Electricity is a word used to describe the directional flow of

electrons between atoms. 2. The directional movement of electrons between atoms is called electrical current.

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CONDUCTORS AND INSULATORS After reading this section you will be able to do the following: • • •

Contrast the characteristics of conductors and insulators. List examples of common conductors and insulators. Explain how insulators provide protection from electricity.

In the previous pages, we have talked a bit about “conductors” and “insulators”. We will discuss these two subjects a little more before moving on to discuss circuits. Conductors Do you remember the copper atom that we discussed? Do you remember how its valence shell had an electron that could easily be shared between other atoms? Copper is considered to be a conductor because it “conducts” the electron current or flow of electrons fairly easily. Most metals are considered to be good conductors of electrical current. Copper is just one of the more popular materials that is used for conductors.

Other materials that are sometimes used as conductors are silver, gold, and aluminum. Copper is still the most popular material used for wires because it is a very good conductor of electrical current and it is fairly inexpensive when compared to gold and silver. Aluminum and most other metals do not conduct electricity quite as good as copper. Insulators Insulators are materials that have just the opposite effect on the flow of electrons. They do not let electrons flow very easily from one atom to another. Insulators are materials whose atoms have tightly bound electrons. These electrons are not free to roam around and be shared by neighboring atoms. Some common insulator materials are glass, plastic, rubber, air, and wood.

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Insulators are used to protect us from the dangerous effects of electricity flowing through conductors. Sometimes the voltage in an electrical circuit can be quite high and dangerous. If the voltage is high enough, electric current can be made to flow through even materials that are generally not considered to be good conductors. Our bodies will conduct electricity and you may have experienced this when you received an electrical shock. Generally, electricity flowing through the body is not pleasant and can cause injuries. The function of our heart can be disrupted by a strong electrical shock and the current can cause burns. Therefore, we need to shield our bodies from the conductors that carry electricity. The rubbery coating on wires is an insulating material that shields us from the conductor inside. Look at any lamp cord and you will see the insulator. If you see the conductor, it is probably time to replace the cord. Recall our earlier discussion about resistance. Conductors have a very low resistance to electrical current while insulators have a very high resistance to electrical current. These two factors become very important when we start to deal with actual electrical circuits.

Review 1. Conductors conduct electrical current very easily because of

their free electrons. 2. Insulators oppose electrical current and make poor conductors. 3. Some common conductors are copper, aluminum, gold, and silver. 4. Some common insulators are glass, air, plastic, rubber, and wood.

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

Define amperes and name the instrument that is used to measures amperage. Construct an experiment to determine the amount of amps flowing in a circuit.

It is very important to have a way to measure and quantify the flow of electrical current. When current flow is controlled it can be used to do useful work. Electricity can be very dangerous and it is important to know something about it in order to work with it safely. The flow of electrons is measured in units called amperes. The term amps is often used for short. An amp is the amount of electrical current that exists when a number of electrons, having one coulomb (ku`-lum) of charge, move past a given point in one second. A coulomb is the charge carried by 6.25 x 10^18 electrons. 6.25 x 10^18 is scientific notation for 6,250,000,000,000,000,000. That is a lot of electrons moving past a given point in one second! Since we cannot count this fast and we cannot even see the electrons, we need an instrument to measure the flow of electrons. An ammeter is this instrument and it is used to indicate how many amps of current are flowing in an electrical circuit.

Review 1. Amperage is a term used

to describe the number of electrons moving past a fixed point in a conductor in one second. 2. Current is measured in units called amperes or amps.

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

Define EMF and explain how it is measured. Explain why EMF is important to the flow of electrical current. List several examples of sources of electromotive force.

We also need to know something about the force that causes the electrons to move in an electrical circuit. This force is called electromotive force, or EMF. Sometimes it is convenient to think of EMF as electrical pressure. In other words, it is the force that makes electrons move in a certain direction within a conductor. But how do we create this “electrical pressure” to generate electron flow? There are many sources of EMF. Some of the more common ones are: batteries, generators, and photovoltaic cells, just to name a few.

Batteries are constructed so there are too many electrons in one material and not enough in another material. The electrons want to balance the electrostatic charge by moving from the material with the excess electrons to the material with the shortage of electrons. However, they cannot because there is no conductive path for them to travel. However, if these two unbalanced materials within the battery are connected together with a conductor, electrical current will flow as the electron moves from the negatively charged area to the positively charged area. When you use a battery, you are allowing electrons to flow from one end of the battery through a conductor and something like a light bulb to the other end of the battery. The battery will work until there is a balance of electrons at both ends of the battery. Caution: you should never connect a conductor to the

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two ends of a battery without making the electrons pass through something like a light bulb which slows the flow of currents. If the electrons are allowed to flow too fast the conductor will become very hot, and it and the battery may be damaged. We will discuss how electrical generators use magnetism to create EMF in a coming section. Photovoltaic cells turn light energy from sources like the sun into energy. To understand the photovoltaic process you need to know about semiconductors so we will not cover them in this material. Take this link to learn more about the volt: What is a volt? How does the amp and the volt work together in electricity? To understand how voltage and amperage are related, it is sometimes useful to make an analogy with water. Look at the picture here of water flowing in a garden hose. Think of electricity flowing in a wire in the same way as the water flowing in the hose. The voltage causing the electrical current to flow in the wire can be considered the water pressure at the faucet, which causes the water to flow. If we were to increase the pressure at the hydrant, more water would flow in the hose. Similarly, if we increase electrical pressure or voltage, more electrons would flow in the wire. Does it also make sense that if we were to remove the pressure from the hydrant by turning it off, the water would stop flowing? The same is true with an electrical circuit. If we remove the voltage source, or EMF, no current will flow in the wires. Another way of saying this is: without EMF, there will be no current. Also, we could say that the free electrons of the atoms move in random directions unless they are pushed or pulled in one direction by an outside force, which we call electromotive force, or EMF.

Review 1. EMF is electromotive force. EMF causes the electrons to move

in a particular direction. 2. EMF is measured in units called volts.

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THE VOLT What is a volt? Technically (very technically), one volt is defined as the electrostatic difference between two points when one joule of energy is used to move one coulomb of charge from one point to the other. We already know that a coulomb is a lot of electrons flowing past any point in one second, but what is a joule? A joule is a measurement of energy. It is the amount of energy that is being consumed when one watt of power works for one second. This is also known as a wattsecond. For our purposes, just accept the fact that one joule of energy is a very, very small amount of energy. For example, a typical 60-watt light bulb that is used in a desk or floor lamp is consuming about 60 joules of energy each second it is on. In not quite such technical terms, a volt is the difference in the electrostatic charge that exists between two points. It is this imbalance in the electrostatic charge that causes electrons to flow from one point to the next.

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OHM'S LAW After reading this section you will be able to do the following: • •

Identify Ohm's law and discuss why it is important. Calculate the amount of electric current in a circuit using Ohm's law.

Probably the most important mathematical relationship between voltage, current and resistance in electricity is something called “Ohm’s Law”. A man named George Ohm published this formula in 1827 based on his experiments with electricity. This formula is used to calculate electrical values so that we can design circuits and use electricity in a useful manner. Ohm's Law is shown below. OHM'S LAW I = V/R, I = current, V = voltage, and R = resistance *Depending on what you are trying to solve we can rearrange it two other ways. V=IxR R = V/I *All of these variations of Ohm’s Law are mathematically equal to one another. Let’s look at what Ohm’s Law tells us. In the first version of the formula, I = V/R, Ohm's Law tells us that the electrical current flowing in a circuit is directly proportional to the voltage and inversely proportional to the resistance. In other words, an increase in the voltage will tend to increase the current while an increase in resistance will tend to decrease the current. The second version of the formula tells us that if either the current or the resistance is increased in the circuit, the voltage will also have to increase.

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The third version of the formula tells us that an increase in voltage will result in an increase in resistance but that an increase in current will result in a decrease in resistance. As you can see, voltage, current, and resistance are mathematically, as well as, physically related to each other. We cannot deal with electricity without all three of these properties being considered. (The symbol for an Ohm looks like a horseshoe and is pictured after the "100" in the diagram above.)

Review 1.

Ohm's Law is used to

describe the mathematical relationship between voltage, current, and resistance.

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

Define resistance and how we measure it. Discuss the similarities between resistance in a wire and the resistance in a water hose.

There is another important property that can be measured in electrical systems. This is resistance, which is measured in units called ohms. Resistance is a term that describes the forces that oppose the flow of electron current in a conductor. All materials naturally contain some resistance to the flow of electron current. We have not found a way to make conductors that do not have some resistance. If we use our water analogy to help picture resistance, think of a hose that is partially plugged with sand. The sand will slow the flow of water in the hose. We can say that the plugged hose has more resistance to water flow than does an unplugged hose. If we want to get more water out of the hose, we would need to turn up the water pressure at the hydrant. The same is true with electricity. Materials with low resistance let electricity flow easily. Materials with higher resistance require more voltage (EMF) to make the electricity flow. The scientific definition of one ohm is the amount of electrical resistance that exists in an electrical circuit when one amp of current is flowing with one volt being applied to the circuit. Is resistance good or bad? Resistance can be both good and bad. If we are trying to transmit electricity from one place to another through a conductor, resistance is undesirable in the conductor. Resistance causes some of the electrical energy to turn into heat so some electrical energy is lost along the way. However, it is resistance that allows us to use electricity for heat and light. The heat that is generated from electric heaters or the light that we get from light bulbs is due to resistance. In a light bulb, the electricity flowing through the filament, or the tiny wires inside the bulb, cause them to glow

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white hot. If all the oxygen were not removed from inside the bulb, the wires would burn up. An important point to mention here is that the resistance is higher in smaller wires. Therefore, if the voltage or EMF is high, too much current will follow through small wires and make them hot. In some cases hot enough to cause a fire or even explode. Therefore, it is sometimes useful to add components called resistors into an electrical circuit to slow the flow of electricity and protect of the components in the circuit. Resistance is also good because it gives us a way to shield ourselves from the harmful energy of electricity. We will talk more about this on the next page.

Review 1. Resistance is the opposition to electrical current. 2. Resistance is measured in units called ohms.

3. Resistance is sometimes desirable and sometimes undesirable.

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SERIES AND PARALLEL CIRCUITS After reading this section you will be able to do the following: • • • •

Explain how a circuit is formed. List examples of sources that the voltage for any electrical circuit can come from. Compare the differences between kinetic and potential energy. Discuss what would happen if a resistor was not included in a circuit.

When we connect various components together with wires, we create an electric circuit. The electrons must have a voltage source to create their movement and, of course, they need a path in which to travel. This path must be complete from the EMF source, through the other components and then back to the EMF source. The voltage for any electric circuit can come from many different sources. Some common examples are: batteries, power plants, fuel cells.

Power Plant

Flash Light Battery

Car Battery

Fuel Cell

When we plug an appliance into a wall outlet, voltage and current are available to us. That voltage is actually created in a power plant somewhere else and then delivered to your house by the power wires that are on poles or buried underground.

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As a matter of fact, since no current can flow unless there is a voltage source, we also refer to these sources as current sources. In other words, without the voltage source, there will be no current flowing. This makes it a current source instead of a voltage source. Batteries create voltage through a chemical process. Power plants generate electricity from numerous mechanical methods. Some burn coal or gas to create steam while others use water flowing through a dam on a lake. There are also nuclear-powered generating power plants. All of these powergenerating systems turn large turbines that turn the shaft on a generator. All of these sources of electricity convert something called potential energy to kinetic energy. The potential energy is stored in the fuel, whether it is coal, gas, uranium, water in a dam, etc. When we utilize these fuels to generate electricity, they become kinetic energy. We might say that potential energy is waiting to be used while kinetic energy is being used. In addition to the voltage source, we need to have wires and other components to build an electric circuit. Remember that copper wires are conductors since they can easily conduct the flow of electrons. We may also use resistors or other forms of loads to form a complete circuit. If we did not include resistors in our circuit, there may be too much current flowing to and from our voltage source and we could damage the voltage source.

Review 1. Wires and various components connected together form a circuit. 2. Power plants and fuel cells are some examples of sources that the voltage for any electrical circuit can come from.

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

Explain what circuit diagrams are used for. Identify what the symbols in the circuit diagrams stand for.

Circuit diagrams are a pictorial way of showing circuits. Electricians and engineers draw circuit diagrams to help them design the actual circuits. Here is an example circuit diagram.

The important thing to note on this diagram is what everything stands for. You see that there are straight lines that connect each of the symbols together. Those lines represent a wire. This is the Ammeter symbol. This is the Voltmeter symbol. This is the resistor symbol. This is the switch symbol. This is the battery symbol.

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The important thing to remember about this symbol is that the long bar on top represents the positive terminal on a battery while the short bar on the bottom represents the negative terminal. Below is the actual circuit made from the circuit diagram above. Pay close attention to see how similar the diagram and the real circuit looks.

--------In the next sub-unit you will be creating your own circuits from a circuit diagram as you learn about what series and parallel circuits are. However, before you do, there are two more symbols you will need to learn. This is the capacitor symbol. A capacitor is used to store electrical charge. An example would be a timer. We will not use this symbol but note that this symbol is very common in circuit diagrams.

This is the symbol for the light bulb.

Review 1. Circuit diagrams are used to show how all the components

connect together to make a circuit.

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THE SERIES CIRCUIT After reading this section you will be able to do the following: • • •

Define a series circuit, and list the components needed to make it. Construct a simple and complex series circuit. Define what a load is.

Try building this simple series circuit In the interactive box (applet) below, you will need to place the correct circuit components (i.e. battery, light bulb, etc.) on the correct diagram symbol by dragging them with your mouse.

Congratulations! You have just built an electric circuit. Notice that when you close the switch to complete the electrical circuit, the electrons start moving and the ammeter indicates that there is current flowing in this circuit. Also notice that the light bulb begins to glow. This happens because

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the electrons moving through the tiny wires in the bulb (or filament) make them become so hot that they glow. If there is any air inside the light bulb, the filament wires will burn up. What you have just created is something called a series circuit. This is called a series circuit because there is only one path for the electrons to take between any two points in this circuit. In other words, the components, which are the battery, the switch, the ammeter, and light, are all in “series” with each other. Load defined The light bulb is considered a load in this circuit. You might think of a load as anything that is using the energy that is being delivered by the electric current in a circuit. It could be anything from a light bulb to a computer to a washing machine and so on. Try building a series circuit with resistors Let’s build another series circuit, but this time we will use some resistors instead of a light bulb. Resistors are components that are used to control that amount of current flowing in a circuit. The light bulb in the first circuit was actually acting like a resistor because it only allowed a certain amount of current to flow through it. If there are no resistors or components that act like resistors to slow the flow of electrical current, too much current may flow through the circuit and damage its components or wires. Too much current flowing through a component results in the generation of heat that can melt the conductive path through which the electrons are flowing. This in known as a short circuit and is the reason fuses or circuit breakers are often included in a circuit.

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Congratulations! You have just build a more complex series circuit. We cannot see any work being done since there is no light bulb, but there is current actually flowing inside. We know the current is flowing because the ammeter is indicating this. It is important to know that we may not be able to tell whether current is flowing through a circuit without test equipment, such as our ammeter connected to the circuit. Electricity can be very dangerous and experiments like these should never be conducted without adult supervision. Never work with electricity unless you are trained to know how to work with it safely.

Review 1. When all the components are in line with each other and the

wires, a series circuit is formed. 2. A load is any device in a circuit that is using the energy that the electron current is delivering to it

THE PARALLEL CIRCUIT After reading this section you will be able to do the following: • • •

Define a parallel circuit and explain how it compares to a series circuit. Construct a parallel circuit. Explain what a voltmeter does and how it is different from an ammeter.

Like the series circuit, parallel circuits also contain a voltage (current) source as well as wires and other components. The main difference between a series circuit and a parallel circuit is in the way the components are connected. In a parallel circuit the electricity has several paths that it can travel.

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Try building this simple parallel circuit

Congratulations! You have just built a parallel circuit. Notice that when you closed the switch, the electrons flowed through both loads at the same time. In our series circuit, all the electrons flowed through all the components in order. With the parallel circuit, some electrons go through one load and some go through the other load, all at the same time. At point A, the total current splits up and takes different paths before the circuit joins back together again at point B. A parallel circuit exists whenever two or more components are connected between the same two points. Those two points in this circuit are points A and B. Both resistors connect to both points A and B. Each parallel path is called a branch of the parallel circuit. Try building this parallel circuit, now including a voltmeter

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This parallel circuit contains 3 branches (two resistors and a voltmeter), which means the electron current goes through all three branches at the same time. We put a voltmeter on this second circuit to show an important fact. In the last 4 circuits you made, you included an ammeter into them. Ammeters must always be placed in series in a circuit, otherwise they will not work. The voltmeter we added in the last circuit has a different requirement in order to work. Voltmeters must be placed in parallel with the circuit in order to work. This is because voltage meters measure the difference in electromotive force (EMF) from one area to another. They are used to measure the difference in EMF on one side of a component compared to the other side of the component. In our homes, most circuits contain 120 volts of EMF.

Review 1. When some of the components are connected parallel with

each other, they form a parallel circuit. 2. A voltmeter must be wired in parallel in a circuit in order to measure the difference in EMF from one point to another.

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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 24

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.

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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

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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 27

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.

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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.

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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.

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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. 30

4. Wrapping the wire around a piece of iron creates a solenoid.

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

Compare the definitions of a magnetic field (from the previous page) and an electrostatic field. Describe what field intensity is and how it is determined. Explain the "right-hand rule."

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.

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

Explain how current can be induced in a conductor without making contact.

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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 our ammeter (the instrument used to measure current) only indicates when there is current in the conductor but it does not indicate current direction like most other ammeters.

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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.

CREATING A TRANSFORMER What is a transformer composed of?

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As you just read in the last page, 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). If we were to take this concept one step further and wind the second wire into a coil on the same iron bar as the first coil, we would create the strongest magnetic coupling of the two wires. When an AC current is flowing in one of the coils, a similar current is induced into the second coil. Even though no direct electrical connection exists between the two coils, we can induce electrical current in this manner. We often use this arrangement of coils to take the electrical current flowing in the first coil and change it in someway that is more useful for doing work. This is what is called a transformer.

As we can see in the above experiment, transformers have at least two windings or coils. One is called the primary, the other the secondary. The primary coil is where AC current is fed in. The secondary coil is where the current is induced to perform some sort of transfer of energy. In this case the current is used to light a light bulb. There are many types of transformers in existence. This is a very simple example. The iron bar core helps to transfer more of the magnetic energy from the primary coil to the secondary coil. How does mutual induction work?

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The secondary coil is also generating a magnetic field that is growing and shrinking just like the field in the primary coil. This coupling of magnetic energy between these two coils is called mutual induction. Mutual induction describes the fact that these two coils share the magnetic lines of force that are being generated by both coils. In other words, both coils are being affected by each other’s induced magnetic fields. The results of this mutual coupling can be quite complex. We will only deal with this concept as it applies to nondestructive testing principles.

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 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 36

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 named eddy 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.

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