ENGLISH ELECTRONIC LEARNING

WEB SPACE FOR ENGLISH STUDENTS

electricity

Basic Electrical Concepts

 

Charge

 Electricity is a rather multi-faceted subject and a good understanding of the basics is important. We begin with a “SCI200” review of charge. Charge is a basic property possessed by electrons and protons. Charge comes in two types, negative (the type on an electron) and positive (the type on a proton.) Opposite charges attract and like charges repel one another. Atoms are formed by light, negatively-charged electrons orbiting around positively-charged protons in the much heavier nucleus. In general, matter has equal numbers of protons and electrons and is neutral overall. An object becomes charged only when electrons are added to or stripped away from the object.

Current

Generally, a current is any movement or flow of charge. In household applications, it is specifically the movement of electrons through wires and electrical devices. There are several factors that determine the flow of current. First, we classify all substances into two broad categories, according to how well current can flow through them.

Insulators are substances in which all the electrons in the atoms of the substance are tightly bound. The electrons do not easily move from one atom to the next. It is very difficult to get current to flow through insulators. Examples of insulators include: ceramics, rubber, plastic, and surprisingly, pure water.

Conductors are those substances in which the outer electrons (typically only one or two in each atom) can move freely from one atom to the next. Current flows easily through conductors. Examples of conductors include: metals such as copper, iron, aluminum, and water‹that contains a lot of dissolved minerals.

Although most substances are easily classified as one or the other, there is no absolute dividing line between insulators and conductors. There are substances which lie somewhere in between. Semi-conductors, important for electronics, are an important example. The glowing filaments in light bulbs and toasters are often classified as conductors, but they are actually not very good conductors. As we will see below, they shouldn’t be. The electrical wiring in the walls of your home and the power cords for electrical devices, such as lamps and refrigerators, are much better conductors.

Consider the diagram of a conducting wire, in which a few of the outer electrons are shown. A current (denoted I) exists in the wire as the electrons move along the wire. (Note that the wire stays neutral as current flows. As one electron jumps to a neigboring atom, another moves in to take its place.) The more charge that passes per unit time, the greater the current. The idea is simple. Current is a measure of the number of electrons that flow past any point along the wire during any time interval, divided by that time interval.

I    =    charge / time

The units of current are amperes or just amps (denoted A). One amp represents a flow of about 6 x 1018 electrons per second! Is one amp a lot of current? Despite the incredible number of electrons per second, one ampere is roughly the amount of current that flows in the common household 100 Watt incandescent light bulb. It is also roughly the amount of current that flows in small flashlight. Are you surprised? There’s more to learn.

Electric Potential

Electric potential is what drives current. You may know electric potential by another term that we will use … voltage. This name comes from the unit of potential, which is the volt (denoted V). When you buy an AA battery, you are buying a device that provides a potential of 1.5 V between its positive and negative terminals. Your car battery maintains about 12 V between its terminals. And the potential between the two slots in a household electrical outlet is about 120 V. (Although not important here, the different nature of the potential of the outlet will be considered in the AC section.) You are probably already familiar with a basic truth about electric potential. All other things being equal, a greater potential will create a greater current. But what is electric potential?

 Water can provide a good analogy (although far from perfect!) for both current and potential. Consider a pipe that comes out of the bottom of a large tank of water, such as shown above. You open the spigot and water flows. The flow rate of the water is analogous to current. Common sense tells you that the higher the water level in the tank, the higher the flow rate in the pipe. (We will investigate this further in the Plumbing module!) The height of the water level is analagous to electric potential. A greater potential will cause a greater current.

Where this analogy fails is with the battery. The tank stores water and as the height slowly decreases, so does the water flow. A battery does not store charge! It is always electrically neutral and for whatever amount of charge leaves one terminal, an equal amount must come into the other. (As we will see in the next section, a complete circuit is required for this to happen.) A battery is more analogous to the water pump shown in Figure 1-3. A battery, therefore, is an electron pump! It has the ability to push electrons directly proportional to its voltage rating. And, it does this through a chemical reaction. The battery becomes “discharged,” (an unfortunately misleading term), when the chemicals in the battery are used up. Most batteries maintain a fixed potential until near the end of their life. The 120 V potential of a household outlet is produced in a very different way. There will be more on this topic in an upcoming section.

Resistance

How much current flows when a given potential is present? That depends upon the resistance to the flow of charge. For a given potential, low resistance results in a higher current and high resistance results in a lower current. The resistance of an object depends upon both the material used and it’s shape. A good conducting material has lower resistance while an insulating material has higher resistance. A long wire has more resistance than a short one. A thick wire (having a large cross section) has less resistance than a skinny one.  Resistance (R) is actually defined by the ratio of potential (V) to current (I).

R    =    V / I

The unit for resistance is the volt/amp, called an ohm, and is denoted by the greek symbol omega (W). Associated with this definition is Ohm’s Law, which is represented by the same equation, but usually written as V = IR. We will use Ohm’s Law in the next section on circuits.

Resistance and Heat Energy

 Resistance in a material arises from the collision of electrons with the atoms and with each other as they move. The collisions produce heat, increasing the temperature of the material. Consider the ordinary toaster shown* in Figure1-4. Current flows through the wires of the power cord and through the toaster’s filament (the glowing wire you see inside). The same current must flow in the power cord as flows through the filament. The cord has very little resistance, while the filament has considerably more. Since the filament has a much higher resistance than the cord, it produces much more heat. That’s as it should be. You want the heat for your toast, but you do not want the power cord getting hot! The standard incandescent light bulb is another example. The filament in the light bulb glows white hot (hence, the word “incandescent”) to produce light and a lot of heat as well. But, the low-resistance power cord stays cool.

Toasters and light bulbs are called resistive devices. They convert electrical energy into heat and light energy. Electrical devices with motors, such as refrigerators or blenders, are more complicated than simple resistive devices. They are designed to convert elecrical energy into mechanical energy. (We will study this in more detail later.) Nevertheless, they have an effective resistance. In general, the power cords and electrical wiring in your home should have much less resistance than the devices to which they supply current. Power cords and electrical wiring are rated by the maximum current they can carry without significant heating. That brings us to another important concept.

Power

How much energy does your toaster use? That depends upon how many pieces of bread you toast. Devices are not rated by the energy they consume, but by the rate at which they consume energy, the power.

Power is energy per time. The standard unit used in electricity is the Watt (W) = 1 Joule / second. (Need a review of energy?) A 100 W light bulb will consume 100 joules of energy every second that it is in operation. Batteries (and WAPA) supply power, while electrical devices such as light bulbs and refrigerators consume power. There is a simple relationship between power and all the other quantities we have discussed so far. For all electrical devices, the power that they supply or consume is the product of the potential across the device and the current that flows through the device.

P = I V   all devicesIf a device has a well-defined resistance such as a light bulb or resistor (a device purposely designed to limit current), we can also use our expression V = I R to get

P = I2 R   resistorsIn this form, we can see more clearly the importance of the current in power consumption. If we were to double the potential across a resistor, the current would also double (V = I R). But the power consumption of the resistor would increase by a factor of 4! We’ll see the importance of this later when we consider energy losses in high voltage powerlines

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March 18, 2008 - Posted by | Uncategorized

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