Electricity
What is Electricity?
Electricity is the movement of electrons. It flows in a current like a river. It is measured in units called watts, which is power (energy per unit of time). The electrons flow through a circuit or path that creates power. The amount of electrical potential energy is measured in volts, which determines the amount of power that can be generated. The current is measured in amperes and the resistance is measured in ohms. The resistance can vary depending on the diameter or gauge of the wire and the length of the wire.
History of Electricity
Ancient civilizations were aware of electric shocks from fish. Later the ancient Greeks, Romans, and Arabs also mentioned knowledge of electric shocks. Early Mediterranean civilizations also described effects of static electricity and magnetism long before Benjamin Franklin would fly his kite. There were many important discoveries in the 18th century by Alessandro Volta and Michael Faraday to name a few scientists. The work of the 18th century and early 19th century really laid the groundwork for the discoveries of the 19th century. The work of Thomas Edison, Alexander Graham Bell, and Nikolai Tesla led to electricity becoming a necessary tool for life in the modern world. The discoveries that came out of the 19th century: the telegraph, the filament light bulb, and the distribution stations for electricity transformed the world and led to the Second Industrial Revolution.
As a result of Edison’s light bulb and Tesla’s AC power stations, businesses and homes could be lit up when it was dark outside. Once electricity could be delivered to factories, the opportunity for mass production began to take shape. The ability to distribute electricity led to many more inventions and modern conveniences. Today, technological advances such as the Internet, cell phones, and electronic devices continue to make our lives more convenient.
Sources of Energy
In 2005, the world used a vast amount, 16.3 TW of energy.12 Over 80% of world energy is fueled by carbon-based fuels, or combustibles.13 Electricity is generated from burning fossil fuels, running nuclear, hydroelectric, and geothermal power plants, and using solar and wind energy. 40% of the world’s electricity comes from coal.14 An important issue is that fossil fuels are readily available and are inexpensive. However, burning fossil fuels gives off carbon dioxide, a greenhouse gas that increases the average global temperature. “Most carbon dioxide emissions come from fossil-fuel burning.”15
Electricity is generated by two different methods: 1) harnessing energy by boiling water to create steam processed through a turbine; 2) Using the kinetic energy to spin a turbine. Fossil fuels, biomass, nuclear, and geothermal all generate electricity by heating water, whereas wind, tidal, and hydroelectric use the kinetic force for generation. The exact way they are all generated is slightly different, but this process is essentially the same.
Conductors & Insulators
Conductors are materials that allow electricity to flow through them. Examples of conductors are metals, carbon, and saltwater. Copper and silver are good conductors. Pure water is not a good conductor, but pure water is rarely found in nature. Salt, sediments, and minerals are found in most water and these materials greatly improve the conductivity of water. Conduction occurs because the conducting materials have free electrons that can transfer from one atom to the next all the way across the material.
Insulators, on the contrary, are materials that impede the flow of electricity. If a free electron, hence a charge, is transferred to the insulating material, the charge will remain in that location and will not transfer. Some examples of insulators are plastic, wood, and Styrofoam.
Static Electricity
Static electricity is caused by an imbalance on an insulated material. It can result in crackling, a discharge, the attraction or repelling of materials. An example is if I rub a balloon on my head I can stick it to a wall. My head should have an equal number of protons and electrons so it is balanced. The balloon should as well. When the balloon rubs on my head, friction is created, and electrons jump from my head to the balloon. The balloon then is negatively charged. If it is positioned near a wall, the negative force of the balloon will push the negative electrons in the wall away. Thus the wall has a positive charge, and the negatively-charged balloon is attracted to the positively-charged wall. Therefore, the balloon will stick to a wall. Static electricity occurs in nature in the form of lightning. Benjamin Franklin’s kite experiment determined that lightning was a form of static electricity. In the sky, clouds rub together creating friction, which leads to the discharge. Electricity does not only occur in a stationary form.
Electric Current and Magnetism
An electric current occurs when electrons move or flow through a conductive material. In class, we will make a circuit and the current will flow through the wire. If a wire is wrapped around a rod or nail, and a magnet is moved over the top, electrons will move. This is because a moving magnetic field creates electricity. In this case, the magnet is moving the magnetic field.
Michael Faraday’s work led to the discovery that electric current creates a magnetic field. Wrapping a wire around an iron nail or rod, and connecting it to a battery demonstrates this principle, and forms an electromagnet.
Early scientists came to this conclusion when a movement of a magnet caused a compass needle to jump. Wrapping an iron rod with wire, moving the magnet, and measuring the DC current with an ammeter is a method to demonstrate this idea.
Transformations of Electricity
Electrical energy is flexible because it can be turned into radiant, thermal, and mechanical energy. Radiant energy is light energy. With Thomas Edison’s invention of the filament light bulb, radiant energy could light up homes and businesses. Electricity can also turn into thermal energy or heat energy. An example of heat energy is a toaster or a burner on an electric stove. Mechanical energy, or the energy of motion, is also the result of the transformation of electricity. Once electricity is turned into mechanical energy, the object has the ability to do work. When electricity creates a motor, which will be explained below, the result is a mechanism that turns. A motor can spin a turbine and run an engine or propel a fan.
Because electricity is so flexible, the modern world depends on it for so many of our everyday conveniences. Imagine our world without electricity: no lights, no household appliances, and no internet. In addition we would not have many of the products that we consume daily since many are made in factories that use electricity for mass production.
DC Current
Direct Current, also known as DC current, is current that flows in the same direction. It has a constant polarity. A battery is an example of DC current because the battery has definite positive and negative terminals.
AC Current
Alternating current is current that rapidly changes direction. It has both voltage that switches polarity and current that switches directions. Moving loops of wire on a turbine in a magnetic field generates alternating current. From the perspective of the electrons in the wire, the magnetic field is constantly changing its direction. To fight against this, the electrons move back and forth, causing the direction of the current to alternate.
Circuits
A circuit is a path for the electricity to follow. A simple circuit is made by connecting a battery, wire, and a light bulb to form a path extending from the negative terminal of the battery to the positive terminal. Circuits can be open or closed, but only closed circuits will conduct electricity. An open circuit has a gap, or break, and therefore the electricity is unable to complete the route. When a light switch is turned off, it creates a gap and the lights will not illuminate. When the switch is turned on, the gap is closed and the lights will work.
Circuits are characterized as either series or parallel. A series circuit has one path for the electricity to follow, but a parallel circuit has more than one path for the electricity to follow. If there is a break in a series circuit then no electricity will be conducted. In a parallel circuit, only the path with the break will not conduct electricity. For this reason, homes and buildings typically use parallel circuitry. An example of this is your electrical box at your house. If one fuse goes out, only the lights and appliances on that circuit will not work. The electrical devices on the other circuits will still work. Now that we have learned how to get electricity to flow through a circuit, the electricity can be transformed into different types of energy.
Batteries
Did you know that a frog led to the invention of the first battery? It is true. Luigi Galvani, a doctor and a professor of anatomy, was cleaning a frog, presumably for an experiment, and he noticed that frogs’ legs twitched wildly when touched by wires made of two different metals. Galvani thought he discovered some type of “animal electricity.” However, the work of Alessandro Volta showed differently. Volta thought it was some kind of reaction between two different kinds of metals and a fluid. Volta was correct and with further experimentation he invented the voltaic pile in 1800. It was the first electric battery and could produce an electric current when needed. The design was a stack of copper and zinc disks separated by a piece of cloth or cardboard soaked in brine or saltwater.16
Alessandro Volta did not understand the science behind his discovery. However, the science behind the voltaic pile is understood today. Two different metals, or electrodes are separated by a conducting fluid called the electrolyte. Charged atoms or molecules called ions move through the electrolyte to balance charges at the metal surfaces. Now, an electric current can flow through a wire from the metal electrode with the higher energy to the metal electrode with the lower energy in a similar way as water flows from a higher place to a lower place.
Figure 1: Dry cell battery.
Electricity is generated when wires are attached to the voltaic pile and the circuit is closed. Batteries then evolved into various forms of wet cells, which used the same general principle of moving charged particles, or ions, to generate electricity. As the name implies, wet cells use chemicals in liquid form.
In 1812, the first dry-cell battery was invented called the Zamboni pile. In the late 1800’s, improvements were made to dry cell batteries. Despite the name, dry cells do not use dry chemicals, but chemicals in a paste-like form. Zinc and carbon were used in Zamboni’s pile. The batteries became known as zinc-carbon batteries and are still produced today.
Motors and Generators
A motor is a machine that turns electricity into mechanical energy. When electricity runs through a coil it becomes an electromagnet. So if the coil is suspended and placed in a magnetic field the forces of the poles will cause the coil to spin. The spinning action is mechanical energy.
Michael Faraday created the first motor, which is now called a homopolar motor. Faraday also determined that if a moving electric current creates magnetism, then the opposite must also be true. So Faraday used moving magnetic fields to generate electricity, thus inventing the Dynamo, or a generator. Faraday’s discoveries really could not do any mechanical work for industrial purposes, but laid the groundwork for the future.
Important discoveries were made when Thomas Edison, using DC current, and Nikolai Tesla, using AC current, were competing to find a way to bring light to houses and the workplace. Both men were trying to generate electricity in order to light up an area. In the end, Tesla won out with his AC current because AC current can travel further without losing much current. The high voltage used to transmit AC current’s power is reduced as it enters homes and buildings for safety reasons. With Edison’s DC current, the electricity could only travel about a mile without a voltage drop. So a power station would be needed in the center of every square mile.17
Electrolysis
William Nicholson and Anthony Carlisle built the first Voltaic Pile in England, and began conducting experiments with it. While experimenting, Nicholson and Carlisle noticed the production of bubbles when two wires attached to the battery were put in water. Intrigued, they conducted further experiments with a larger amount of water. Then they took a tube full of water and inserted the two wires. They were surprised when bubbles emerged from both wires. Nicholson and Carlisle discovered electrolysis, or the decomposition of water into hydrogen and oxygen. They also embarked on a new field: electrochemistry, which would change the field of chemistry.
Fuel Cells
William Grove, a friend of Faraday and a professor of experimental philosophy, experimented with electrolysis and thought that process could be reversed. He reasoned that by reversing the process, he could create electricity. He was correct and, in 1839, he built the first fuel cell.
Francis T. Bacon developed modern-day fuel cell technology. Bacon’s technological improvements paved the way for the use of fuel cells on space shuttles. One big improvement was using nickel instead of platinum as a catalyst to remove the electrons from the hydrogen atoms. Platinum was prohibitive because of its cost. Bacon pressurized the cell to keep the liquid from boiling, but the pressurization also improved the efficiency of the cell. Bacon made single- cell, six-cell, and forty-cell models. In 1959, Allis-Chalmers, a tractor manufacturer “demonstrated the first fuel-cell powered vehicle.”18
Fuel cells are similar to batteries in some ways. A fuel cell uses two different gases, hydrogen and oxygen, to create electricity. Hydrogen enters one side of a panel and electrons are stripped away, turning the hydrogen into a positively-charged ion. The electrons flow, creating electricity. Oxygen is pumped into the fuel cell, the opposite side and through another panel. The hydrogen ions flow through a membrane and bond with the oxygen to produce water as exhaust. Fuel cells are a clean source of energy, but it is difficult to build practical ones.
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