Full Notes for Electricity

AMPERAGE.

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

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, orEMF. 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?

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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

RESISTANCE

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 restrict the flow of electricity and protect 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.

CONDUCTORS AND INSULATORS

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.

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 in a circuit can be calculated by dividing the voltage by the resistance. In other words, the current is directly proportional to the voltage and inversely proportional to the resistance. So, an increase in the voltage will increase the current as long as the resistance is held constant. Alternately, if the resistance in a circuit is increased and the voltage does not change, the current will decrease.

The second version of the formula tells us that the voltage can be calculated if the current and the resistance in a circuit are known. It can be seen from the equation that if either the current or the resistance is increased in the circuit (while the other is unchanged), the voltage will also have to increase.

The third version of the formula tells us that we can calculate the resistance in a circuit if the voltage and current are known. If the current is held constant, an increase in voltage will result in an increase in resistance. Alternately, an increase in current while holding the voltage constant will result in a decrease in resistance. It should be noted that Ohm's law holds true for semiconductors, but for a wide variety of materials (such as metals) the resistance is fixed and does not depend on the amount of current or the amount of voltage.

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.

SERIES AND PARALLEL CIRCUITS

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.

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 power-generating 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.

CIRCUIT DIAGRAMS

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.

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.

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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.

THE SERIES CIRCUIT

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 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 loadas 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 resistorsinstead 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.

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

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.

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

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.

THE 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

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

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.