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Electrical Current
This page: http://pierce.wesleyancollege.edu/faculty/MSP/orientex/units&laws.html
Electrical Current Flow
To be able to effectively teach about electricity and design engaging, hands-on exercises for students you will need to understand some basic electrical concepts. With each of these, try to think about how you might present this to a group of students, ideally in a way where the students discover the meaning and implications of each concept for themselves.
Concept 1: Electrons, conductors, insulators, and fields
1) Electricity is carried by electrons. An electron is a subatomic particle with zero mass and a fixed negative charge. Although we usually think of electrons as being parts of atoms, electrons can actually become dislodged from their parent atoms and allowed to roam the world. The very specific and limited conditions and rules for this roaming process define "electricity".
2) A conductor is any material that allows electrons to flow relatively freely through it. In order for this to happen, electrons must be passed from one atom to the next and each atom must be able to receive, temporarily hold, and then give up extra electrons. What kinds of materials are good conductors? Picture yourself holding a 20-foot pole in a lightning storm. What would you LEAST like the pole to be made out of?
3) An insulator is any material that effectively blocks the flow of electricity. An insulator may be able to receive or give up a limited number of electrons, but cannot effectively pass electrons from one atom to the next. What kinds of materials are good insulators? Picture yourself holding a 20-foot pole in a lightning storm. What would you MOST like the pole to be made out of?
4) You are probably used to the idea that "water and electricity should not be mixed". Does this mean that water is a conductor or an insulator? What about completely pure, distilled water? Later we may test this out.
5) An electric field describes the influence that electrons and other charged particles collectively exert through the open space around them. The static electricity experiments which you conducted earlier showed the effects of electric fields as electrical attraction and repulsion.
Concept 2: Quantifying electricity
1) Electrical charge measures the amount of free electrons present at a particular location at a particular instant in time. Charge is indicated by the symbol Q and is measured in units called Coulombs.
2) Electrical current measures the flow rate of free electrons past a particular location at a particular instant in time. Current is indicated by the symbol I and is measured in units called Amperes (generally abbreviated to Amps or just A). An amp equals one coulomb of charge flowing past a particular point in one second of time. 1 Amp = 1 coulomb per second; I=dQ/dt.
3) Electrical voltage or electrical potential measures the force driving electrons in an electrical current. Voltage is indicated by the symbol V and is measured in units called Volts (abbreviated to V).
4) Even in the best conductors, electrons do not flow completely freely. Electrical resistance is the simply the resistance to current flowing through a conductor. Resistance is indicated by the symbol R and is measured in units called Ohms (abbreviated to the Greek letter omega W).
Understanding Current Flow
A simple way to understand some aspects and properties of current flow is by analogies to the flow of water.
1) Picture a stream of water flowing through a pipe or hose. The amount of water flowing out of the end of the pipe over a period of time is the current, analogous to an electrical current. Water is pushed through the pipe by water pressure, analogous to electrical voltage. The pipe is a restricted area which resists the flow of water, analogous to electrical resistance. NOTE: in this analogy it is important to clearly distinguish between the rate of water flow and the velocity of water flow - current describes the former, not the latter.
2) Picture a bottle of water with a small hole near the bottom. The height and weight of the water column above the hole determines the water pressure forcing water out of the hole, analogous to electrical . The rate of water draining out of the bottle is analogous to electrical . The size of the hole restricts the flow of water, analogous to electrical .
3) Picture a fountain. Water is projected into the air under pressure, analogous to electrical . The rate (not velocity!) at which water leaves the fountain head is analogous to electrical . Finally, the diameter of the fountain riser pipe and fountain head determine the resistance to water flow, analogous to electrical .
Note that water does not accurately represent all properties of electricity. Which properties does it not represent well?
Laws for Electrical Circuits
One set of laws describe how electricity flows through conductors and resistors. There are two of these laws and they are called Kirchoff's Current Laws:
1) Current flow into a node must equal current flow out of a node. This means simply that electrons cannot pile up at any point in a resistive circuit. English translation: "What goes in must come out".
2) Current always and only flows in complete loops. This means that in order to get current to flow from any source, it must have a path back to that source. English translation "What goes around comes around".
The other law describes the relationship between current, voltage, and resistance. This is Ohm's Law. Simply stated, at any point in a resistive circuit voltage in volts equals current in amps times resistance in ohms, or V = I x R.
Exercises:
1) Based on Ohm's Law, if you hold the voltage constant and increase the resistance, what happens to the current? If you hold the resistance constant and increase the voltage, what happens to the current?
2) Try to relate each of these laws to the three water analogies presented earlier. What works well to illustrate each law? What doesn't work so well?
More About Current
Current flow follows one of two patterns for cars, household appliances, and such.
1) In direct current (DC) electrons flow in a constant direction, under a constant voltage. This is the kind of current you typically find in battery-driven devices. DC electrical supplies are characterized by the constant voltage they provide. Typical voltages would be 1.5 volts for dry cells, 6 volts for lantern batteries, 9 volts for transistor batteries, and 12 volts for car batteries.
2) In alternating current (AC) electrons flow rapidly back-and-forth, so there is no long-term transfer of electrons along the wires. This is the kind of current you typically find in power grids, such as that supplying your house. AC currents are characterized by the voltage and the frequency at which the current (and voltage) oscillates back and forth. In the United States household AC current is supplied at a frequency of 60 cycles per second (60Hz) and at voltages of 220 and 110 volts.
There is a final annoying thing about electrical current for which we have Benjamin Franklin to thank.
1) Electrons are negatively charged. In a DC circuit electrons flow from the negative battery pole, around the circuit, to the positive pole.
2) However, conventional current, by definition, is the flow of positive charges, e.g. from the positive pole of a battery, around a circuit, to the negative pole.
3) Thus electrical current flows in one way around a circuit, while the actual carriers of that current flow the opposite way around the circuit. Awkward!!
So even though we ignored Ben about the crazy turkey idea, we are nevertheless permanently stuck with his boo boo on electrical polarity.
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