14 04 2009

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 “electrical,” 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.

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 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.

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.

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.

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.

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.

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 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.
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.

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 = I x R
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. 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.

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 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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