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Electricity and Magnetism
Electricity exists because most elementary particles of matter (such as electrons and protons) have a negative or positive charge. Like charges (two positives or two negatives) repel each other. Unlike charges attract. This electrical force is responsible for many natural phenomena. Technology has found many uses for it.
Electricity can be classified as static or current. In static electricity, the charges are “at rest.” They remain in certain positions on objects. Static electric charges can build up when two different materials rub together, as when you walk on a carpet. If you then touch metal, the built-up charges suddenly flow away. They are no longer static, and you feel a shock. Lightning is another example of the sudden discharge (flowing away) of static electricity.
A flow of electricity is a current. A material through which current flows easily is called a conductor. A material through which almost no electricity can flow is called an insulator. Some materials can behave either as conductors or insulators, depending on what other materials are mixed with them. These are called semiconductors. They can be used to control the flow of electricity. For example, semiconductors are used to make computer chips.
Excerpts from Chapter 8: Electricity to Electronics from the textbook Introduction to Technology 2010 Glecoe/McGraw-Hill
What is Electricity?
Can static electricity be dangerous?
Have you ever touched a doorknob, and received a shock? In this chapter, you will learn why these 'shocks' are not only annoying and why lightning, which is another form of static electricity, is extremely dangerous. You will also learn why some devices require very little electricity while others require a lot. Every electric circuit contains voltage, amperage, and resistance. They determine what electricity can do and how dangerous it can be.
The Atom
To understand what electricity is and how it functions, you need to understand more about atoms. Atoms are building blocks from which all things are made. They are made of several atomic particles:
- Protons
- Electrons
- Neutrons
Atoms contain an equal number of positively charged protons and negatively charged electrons. Most atoms also contain the atomic particles called neutrons, which have no charge at all.
Negatively charged electrons travel around the nucleus. The nucleus contains positively charged protons and neutral neutrons.
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Electrons and Protons
An atom is in balance when it contains the same number of electrons and protons. Atoms obey the same rules of repulsion (pushing away) and attraction (moving together) as magnets. When an atom has too many or too few electrons, it pushes or pulls extra electrons off to its neighboring atoms. This flow of electrons from one atom to another is called electricity.
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Electrons in a circuit move from atom to atom. When an atom loses electrons, it becomes positively charged. When an atom gains electrons, it becomes negatively charged.
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Matter
Our world consists of energy and matter. Energy is the ability to do work. Matter is the “stuff” that things are made of.
An atom is the smallest unit of matter, so small that trillions of atoms fit on a pencil point. An atom has a positively charged core called the nucleus and one or more negatively charged electrons. These orbit the nucleus. Electrons respond to nearby atoms to form bonds. For example, atoms may donate electrons to neighboring atoms. The hardness of quartz comes from this kind of bond. Plastics share electrons in covalent bonds. Metals have their own form of electron sharing, known as the metallic bond. All these bonds involve the attraction of electrons to neighboring atoms. They are chemical bonds.
Types of Electricity
What are some different types of electricity?
Is the electricity that comes out of the wall outlets of your home the same as the electricity in batteries? Are static electricity and lightning different from the electricity that powers your home?
Static Electricity
Static electricity occurs when atoms have built up "extra" electrons. These extra electrons are ready to jump to new atoms. When super-charged atoms come close to an object that can conduct electricity, a spark occurs. When you receive a shock from ordinary static electricity, you are not hurt because the static charge lacks power. Static electricity can, however, destroy sensitive electronic circuits and ignite flammable liquids and gases.
Lightning is a very strong discharge of static electricity. If it hits a power line, the electricity surge can destroy electrical equipment in the area and cause blackouts.
Direct Current
DC, or direct current, is the flow of electrons in only one direction. At one time, all of the electricity used in the United States was direct current. Your local phone company still supplies DC current. Many people keep their original local phone service because the phones almost always work during a power failure.
Devices powered by batteries also use direct current. When plugged in. these devices convert wall outlet electricity to direct current. Batteries contain stored electricity. When batteries are in use, electrons flow out from the negative terminal of the battery, and then back through the battery's positive terminal. A circuit is the pathway electricity takes. If the circuit is broken, electricity stops flowing, and all of the systems stop working.
Alternating Current
The wires that come into your home can be traced all of the way back to a power generating station. The electrons that flow through these wires change direction 120 times per second [60 Hertz]. This is the reason that the electricity is called AC, or alternating current. Alternating current is easier to transmit and control than direct current.
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Each back and forth motion is called a cycle. The electricity that powers your home is measured at 60 cycles per second. The number of cycles per second is the frequency of the alternating current. In metric system, cycles are called hertz or Hz.
Electrical Flow
What are voltage, amperage, and resistance?
You cannot see electricity flowing through a circuit. However, its movement through a circuit is similar to that of water through a pipe. You need water pressure to push water through a pipe, and you need electrical pressure to push electricity through a wire.
The pressure and strength of the electrical current as well as the opposition to the current are known as:
- Voltage
- Amperage
- Resistance
Voltage
Voltage is the pressure that pushes electricity through an electric circuit. Voltage pressure is measured in units called volts. A transformer is a device that can change alternating current from one voltage to another. Some equipment and appliances require less voltage than others require. High voltages can be sent over distances inexpensively.
Amperage
Amperage is the strength of the electrical current. It is measured in units called amperes. Amperage is measured in electrons per second. In cases of electric shock, it is the amount of current (amperage) that makes electricity dangerous enough to kill.
Resistance
Resistance is anything that opposes or slows the flow of electricity. It is measured in units called ohms. Resistance in a water pipe is determined by the diameter, length, bends, and kinks in the pipe. The resistance in an electric circuit is determined by the electric wire's diameter, length, and temperature. For example, the resistance of a wire decreases as the wire gets fatter. The resistance increases as the wire gets longer. If you add a component to a circuit, like an electrical appliance, resistance increases.
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Ohm's Law
What can you determine by using Ohm's Law?
Building an electric device often means joining many different electrical systems together. These separate systems are all needed to fulfill the requirements of the device. The components must get the proper amount of electricity or the device will burn out, not function properly, or refuse to work. When you design an electric circuit, you must know the power needs of the components.
Ohm's law is a mathematical formula that is used to determine the voltage (E), amperage (I), or resistance (R) of an electric circuit. Ohm's law states:
Voltage = Amperage x Resistance or E = I x R
The law gives the electrical designer a way of determining exactly what the circuit will need to make it work efficiently.
Ohm’s Law
Current is the amount of charge that flows past a point in a circuit during a given time. Current is measured in amperes (A). For a current to exist in a conductor, there must be an electromotive force (emf), or potential difference, between the two ends of a conductor. This electromotive force is measured in volts (V).
Even conductors resist the flow of current somewhat. The greater the resistance, the less current that flows. Resistance is measured in ohms (?). The voltage, amperage, or resistance of an electrical circuit can be calculated by using Ohm’s law (named for German physicist George S. Ohm). This law states that electric current equals the ratio of voltage and resistance.
I = E/R
In this formula, I stands for the intensity of the current, measured in amperes; E stands for electromotive force, measured in volts; and R stands for resistance, measured in ohms.
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The Electric Meter
What does an electric meter do?
Your local electric company keeps track of the electricity that your family uses. When you turn on an electric appliance in your home, the wheels of your electric meter turn. The moving wheels are measuring the electricity that is being consumed. The more appliances that you run, the faster the wheels turn, and the bigger your electric bill becomes.
Measuring Electricity
The electricity that your appliances use is measured in watts. Wattage (W) is a measurement of the electrical power an appliance will need to run. It is the voltage of the electric circuit multiplied by the amount of amperage needed to run the appliance (W = E x I).
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Your electric meter measures kilowatt hours. Your lights and other appliances consume the equivalent of one kilowatt hour of electricity when they consume 1,000 watts of electricity.
Energy Efficiency
You can compare wattage efficiency of an appliance to gasoline efficiency in a car. The more efficient the car or electrical appliance, the less you will pay to keep it running.
Most electrical appliances, including light bulbs, indicate how much electricity (wattage) they will consume. You want to purchase the most energy efficient appliances possible. If you buy less efficient appliances, you will spend more on electricity over time. Look for Energy Star ratings on packages or labels and purchase the appliance or light bulb that is the most energy efficient.
Electric utility companies encourage people to use less electricity during high-demand periods. When the utility company cannot keep up with the demand, an electrical blackout is likely to occur.
See Smart Grid
Conductors and Insulators
How is the flow of electricity controlled?
Conductors are materials that contain atoms that have a very weak hold on their electrons. Copper, aluminum, gold, and silver are excellent conductors of electricity. Think of electrical conductors as highways on which electricity travels.
Insulators are materials that contain atoms that have a very tight grip on their electrons. Rubber, plastic, and ceramics make good insulators. Insulation prevents the electrons from leaving their intended path.
Semiconductors and Superconductors
What are semiconductors and superconductors?
Semiconductors are materials that can act as either conductors or insulators. Most semiconductors are made using the silicon found in ordinary sand. Manufacturers add different ingredients to the silicon to control their electrical conducting properties.
Semiconductors are found in most electrical and electronic devices. Without them, your TV, computer, MP3 player, and digital camera would not exist.
Superconductors have no measureable resistance to electricity. They make it possible to generate more productive and efficient electrical power. Superconductor technology is built into new electrical generators and transformers. They also play a critical role in magnetic resonance imaging [MRI] machines, new super-efficient industrial motors, and magnetic levitation trains.
Electrical Circuits
What makes up an electrical circuit?
An electrical circuit begins at the power source and ends back at that same power source. The circuit must have at least one device that consumes electricity, such as a buzzer or a light bulb.
An electric switch is used to open and close the circuit to turn things on or off. There are two types of electrical circuits:
- Series circuit
- Parallel Circuit
Series and Parallel Circuits
The path electricity takes as it flows is called a circuit. In a series circuit, individual components are connected end to end to form a single path for current flow. Series circuits have two major disadvantages. First, when connected in series, each circuit has to have its own switch and protective device. Second, if one component is open, the entire circuit is disabled.
Series Circuits
In a series circuit, the electricity takes one path through multiple electrical devices. A major disadvantage of series wiring is that if one device burns out, all of the devices in the circuit stop working. Devices cannot operate independently.
When batteries are connected in a series, each one increases the voltage [pressure] in the circuit. When you place three 1.5-volt batteries together in a series, the circuit is powered by 4.5-volts. Do you understand why?
Light bulbs and other electrical devices can also be wired together in a series. In this type of circuit, the electricity must pass through each device on its way to the next. Again, if any item in the circuit breaks down, power is lost to the entire circuit. Most of the circuits in your home are NOT series circuits.
Parallel Circuits
In a parallel circuit, electricity flows along separate paths to each individual energy-using device [load] in the circuit. If one device burns out, the electricity does not stop flowing to the other devices in the circuit.
In a parallel circuit, two or more loads are connected in separate branches. In most cases the parallel circuits are connected in series with a common switch and protective device [fuse]. Equal voltage is applied to each branch of a parallel circuit. Current flow divides as it reaches the parallel path. The amount of current flowing through each branch depends on the resistance in that path only.
Most of the circuits in your home are parallel circuits. All the electric outlets in your home are connected to a fuse box or circuit-breaker panel. These safety devices shut off the electricity in case of a power overload. A power overload can superheat electrical wires and start a fire.
Electronic Devices
What is an electronic device?
The words electric and electronic are often used interchangeably. To keep things simple, electric devices generate and utilize electricity as a source of power. The electricity lights electric light bulbs and rings door bells. An electronic device can change one form of energy, such as sound or light, into an electrical signal to be used in the device or transmitted.
Electronic devices allow for information to be processed and transmitted. This processing takes place in self-contained electronic games and in complex robotic systems. Your telephone, TV, radio, and computer are all electronic devices.
Signal Transmission
How can electronic signals be sent?
Electronic signals can be sent along a wire, through the air on electromagnetic waves, or as pulses of laser light through fiber optic cables.
Wire Transmission
Wire transmission includes all communication that takes place over wires. Telephones, cable TV, computer-to-printers, and even computer-to-monitors all use wire transmission.
To transmit a signal along a wire, the signal must be converted into a series of electric pulses. Alexander Graham Bell's 1876 telephone changed the sound of the speaker's voice into a varying electric current. Wires carried these electric pulses to the receiving phone. The electric pulses were then converted back into varying vibrations that replicated the sound of the sender's speech.
Today's land-line telephones have been modified and improved many times, but they still use the transmission principle developed by Bell more than 100 years ago. Your call is transmitted through a cable. Your phone signal reaches a computer exchange network, where it is directed to the proper receiver.
Atmospheric Transmission
The magnetic field created when electricity flows through a wire can also carry a message through the air. This very weak magnetic field is converted into electromagnetic waves by passing through a strong magnetic field. The electromagnetic wave that is produced carries the original signal off into the atmosphere.
The device used to send the electromagnetic wave that carries the signal into the atmosphere is called a transmitter. The transmitters in cell phones and personal remote controls are very small, so their signals must be picked up reasonably close to the location where they were sent.
Fiber Optic Transmission
Fiber optic cables are made from very pure glass. The term optic is used to describe something related to light. Each glass strand in a cable can be thinner than half the thickness of a human hair.
The glass strands have an outer glass coating with different reflective characteristics. This causes the rays of light traveling at the core to stay inside in spite of twists and turns. These cables can carry a signal for very long distances. If necessary, the sender can easily boost the power so messages will not be lost.
The signals that are sent along fiber optic cables are powered by lasers. Lasers are very powerful, very narrow beams of highly focused light. All of these light rays have exactly the same wavelength. The beam that is made by a laser can be visible light or invisible infrared light.
By transmitting signal beams that are on different wavelengths, engineers can send many messages along the same fiber optic strand at the same time. When they reach their destination, the different wavelengths are separated.
Analog and Digital Signals
Which signal is faster... analog or digital?
Electric analog signals change continuously, and they are used in all kinds of systems. Alexander Graham Bell learned how to turn electricity into analog signals. The current in his telephone circuit varied with the intensity of a person's speech. If your watch has hands that rotate smoothly, it is an analog watch. The second hand on an analog watch never stops moving.
A digital signal is analog information that has been converted into digital information. The changing analog information is sampled frequently and turned into distinct, separate values. Your digital watch represents the seconds as exact numbers. It starts and stops continuously. The time is never 'between' one second and the next.
Because digital signals can be compressed to take up less space in storage, sent faster, and stored longer than analog signals, they are preferred for electronics. For example, when you speak into a phone, your voice, which is analog information, is changed into a digital signal. However, this does not mean that digital signals are more accurate than analog signals.
Size and Speed
Will electronic devices continue to shrink in size?
Each new improvement in electronics is built on past knowledge. In the 1940's, vacuum tubes became the first electric 'switch' in electronic equipment. They resembled light bulbs, used a lot of electricity, and gave off a lot of heat.
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1950's
Starting in the 1950's, vacuum tubes were replaced by the transistor, a small, cool running device that could also act as an electric switch. The transistor was tiny compared to a vacuum tube. However, thousands of hand-soldered connections were needed to join transistors and other components into completed circuits.
1960's
The next size reduction took place in the 1960's. The first integrated circuit contained one transistor and two other electric components on a single chip of silicon. Very tiny passageways in the silicon connected the tiny components together.
1970's to the Present
In the 1970's, technologists were placing thousands of integrated circuit components on a single chip. Today these tiny microchips contain millions of components.
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Microchips may be incredibly small, but they can process huge amounts of information
The following information is from the text on Electricity from the book Questions and Answers about Science 2008 Arcturas Publishing Limited
Electricty is a secondary source of energy. We have to generate electricity from primary sources of energy like moving water, nuclear power, coal or natural gas. It can be converted into other forms of energy like light or heat. Electricity is used for lighting and heating or cooling of our homes. It runs machines to wash clothes and dishes and to cook. It brings us information through computers and television.
Alexander Volta was an eighteenth-century scientist who developed the voltaic pile. This was later developed into a battery. The measure of strength of current [voltage] is named after him
Q Who discovered electricty?
A Ancient Greeks knew that electricity could be produced by rubbing two pieces of felt together. BUT the first use of the word electricity was by Sir Thomas Browne in his 1646 book Pseudodoxia Epidemica (Vulgar Errors). In 1752, Bemjamin Franklin proved that lightning was created by electric charges. He tied an iron key to a kite string during a storm and showed that the lightning hit the key. For this reason, Franklin is said to have discovered electricity. Today, we know that lightning is the most commonly seen form of natural electricity. It is caused by clouds carrying a negative charge that bump into positevly charged objects on the Earth.
The lightning that transfers electrons from negatively charged clouds to positively charged substances on the surface of the Earth is the biggest source of natural electricity we know of. Lightning can be very dangerous to someone caught outdoors. Anyone caught outdoors during a thunderstorm should keep as low as possible.
Q What is electricity?
A Everything is made of atoms. At the center of the atom is the nucleus made of protons and neutrons. An atom also has tiny electrons which spin around the nucleus. Electrons have a negative electrical charge, and protons have a positive charge. The electrons don't stay in one place. They move around to different atoms, so some atoms have more protons, some have more electrons. An atom with more protons is positively charged; one with more electrons is negatively charged. When the lectrons pass from one atom to the next, it creates an electric current.
Q Can electricity make my hair stand on end?
A Static electricity is created when you rub against a charged surface. The extra electrons move from your body or the other way around, and a tiny spark of electricity is created. Static makes dry hair stand on end after you run a plastic comb through it.
Q How is electricity measured?
A Voltage is the measure of strength of an electric current. The unit for measuring voltage is the volt. A voltmeter tells us how many electrons are sent from one end of the circuit and how many are received at the other end. The distance that electricity travels affects its quality, especially if it is prevented from flowing freely because of resistance. Resistance is a material's opposition to the flow of electric current passing through it. Resistance is measured in Ohms. Scientists are always looking for materials like copper that are good conductors of electricity and have a low resistance. Silver is the best conductor, but it cannot be used in wires in our homes because it is too costly. Most metals are good conductors.
