What is it really? It is all around us yet we almost never see it. When you stop and think about how important electricity is to our daily lives it is almost scary that most of us know very little about how it works.
Electricity can be simply defined as the movement of electrons through some material that will allow that to happen.
The movement of electrons can be made to do amazing things, the computer you are reading this on is proof of that. The most common medium used for electric current is metal wire. As electrons flow through a wire, their energy can be converted to other types of energy and made to do real useful work for you and me.
The thing that doesn’t get much attention is how do those darn electrons get moving in the first place? Anything that can make them move is called an Electromotive Force, or EMF.
A select set of electrons can move freely between atoms in some materials. In fact, given the right material like some metals, they can move millions of meters per second. It is interesting to note that these free moving electrons in their natural state are in chaos, shooting in all directions at once within the outer layer of atoms in the conductor, called the valence shell. The electrons in this level of energy are called valence electrons and it is from this shell alone electrons can move between atoms. Even though they are moving about at incredible speeds, they can not produce any work in disarray. They are crashing into each other and their forces are scattered. This is why you don’t get a shock when touching a piece of metal alone. Next time you hold a piece of metal think about tiny (1.68 x 10-19) shooting stars inside, zooming in tiny bursts in all directions. To produce actual work, like run a light bulb, they must be marshaled into a single direction so their charges aggregate into real power. But how on earth do we do that?
When a potential difference in voltage is applied across a conductor an electric field is created in that conductor. Free electrons in the valance shell of a solid material react to this electric field and the majority of them will flow together in a single group against the direction of the electric field. Like Salmon swimming up stream, the electrons are free to move and are drawn forward by the electric field not unlike how flakes of metal react to a magnet, but in this case the magnetic field is repulsing the tiny electrons so they try to move away from the field and balance the force. Like how magnets when the same poles are pushed together, they will snap out ouf your hands and find the right balance automatically (positive-meets-negative) this happens between the magnetic field applied across the conductor and the tiny negative electrons. The greater the field, the faster and more of them move, i.e, the larger the voltage, the more potential current you can have. The ions (atoms the electrons came from) are fixed into the solid and unable to move. Like a fire bucket brigade, the people (ions) don’t move, only the buckets of water (electrons) do. Whereas in a solution, the ions are free to move too, so current can be from positively charged protons as well. The movement of electrical charge carriers are responsible for current in a circuit.
Electric potential difference or voltage is equal to the work which would have to be done against a static electric field to move a unit of charge between two points. Like water pressure can force a volume of water through a pipe, the higher the pressure the larger the quantity and speed the water moves –the potential difference between two points becomes a source of energy (pressure/gravity) that can cause a current to flow in an electrical circuit or device. Like a skateboard down a hill or water from a garden hose, stored energy is released and directed.
Actually, electromotive force is not really a force, but a measure of how much work would be done by moving an electric charge between two points.
Now that we have some understanding of charge and how it moves through a conductor let’s see how this is put to work.
There are five different ways to generate EMF:
- Electricity and magnetism are inseparable. Wave a magnet near a coil of wire and an electric current is induced in it. All the serious electrical power we use is generated by rotating magnets in coils of wire. The effect is reversible, if you have a current through a coil of wire with a metal core, say a nail, you will have an electromagnet. The electromagnetic effect runs motors, unlocks apartment lobby doors, turns the water on and off in your washing machine among other things.
- Chemical change
- In a strong second is chemical change. A simple voltage potential, a difference where free electrons can flow from the high to low potentials.The act of placing two different types of metal in salt water will make a battery. An example would be Magnesium and Brass. Electromotive force is generated by changes taking place in the magnesium. When it is all gone the EMF is too. In a properly designed reaction the effect is reversible. An external or electromotive force applied to the cell will undo the chemical change and replace the magnesium. Car batteries are an excellent example of a properly designed reaction.
- The conversion of electron flow to heat is a real common one. Heat is generated in any material that resists the flow of electricity. A piece of stainless steel wire, the wire that is found in heaters, toasters, stoves and water-heaters has its resistance carefully tailored to create heat continually without melting. EMF can be converted to heat and heat can be converted to EMF using a thermocouple. If two dissimilar pieces of metal are joined and heated a small current flows. This effect is used on gas appliances to signal when the pilot light is not lit.
- The piezoelectric effect is the least know of the five effects, yet it is used everywhere. Some subsistence will induce an electron flow when stressed or whacked. Bar-b-que lighters are famous examples. They have a tiny hammer that strikes a small piece of material when you click the button which produces enough current to cause a spark to jump a gap. Another version of this converts wiggles in a record grove to wiggling electric currents. The pezio electric effect works in reverse too. If a current is made to flow in the material it will flex. Tiny speakers that beep put this effect to use in greeting cards, coffee pots, computers – they are everywhere. A quartz crystal another version. It is used in electronics devices to control speed. Put in an electronic circuit, a crystal can be made to flex and relax and regular intervals which depend on the size of the crystal. It regulates watches, clocks, computers and keeps all our radio and television transmissions on the same wave length.
- The light bulb can be considered to be just a special case of resistance heating. The heater inside (filament) gets hot enough to display visible light. The Light Emitting Diode, or LED emits light directly without creating heat. An LED has made it into just about every electronic device everywhere. They are very cheap and last forever. An LED will also generate a very tiny amount of EMF when light falls on it, but it is so small that it is hard to even measure. Solar cells are much more efficient at creating EMF. They are mostly used by things orbiting our planet and some small devices. Generating electricity from the Sun has a strong appeal as a great renewable sources of power.
Five ways to generate an electromotive force and five matching ways to use that force. So the next time you are about to turn that switch on or plug that device in, think about those little atoms and their electrons moving through electric fields. They are doing the real hard work and is hard to think what life would be like without them.