Most of us are used to single measurements of everyday items. A ruler can give us linear measures in units of inches or centimeters; a measuring cup can give us volume measures in units of cups or fluid ounces. But when you’re measuring electrons in a circuit your measuring something really small!
For example, your standard car speakers will require the radio to pump 104,000,000,000,000,000,000 electrons per second…well that’s just silly! Even if it were helpful to know how many electrons it took to power my sub-woofer it would be easier to write as “104-followed-by-18-zeros” or in scientific notation as 104 x 1018. But there are many cases where we need a unit of measure that counts a lot of electrons at once. Using some very clever experiments, people decided that the count of 6.241×1018 electrons could be called “one coulomb” and that moving that many electrons past a point in one second would be called “one ampere”.
1 Ampere = 1 coulomb / 1 second
Whenever we come across a measurement that includes the word “per” or “rate” we should use a division sign to symbolize it. Thus “miles per gallon” becomes miles/gallon and “miles per hour” becomes mi./hr. Since ampere’s or amps are measured in units per second, it is a measure of the speed at which the electrons are flowing, not the number of electrons that are present. Actually the number of electrons present won’t matter in electronics. Every helium atom has two electrons but I guarantee you won’t charge your smartphone by plugging it into a helium balloon. The only way to get those electrons to flow is if there are more electrons on one side of the circuit than the other. The difference between the two is measured in volts. This is a very different measure than the one for current (amperes) which measured the velocity of electrons in a wire. Volts are a measure of force (you can think of it as pressure) on the electrons to move. This force or energy that results from having more electrons on one pole than the other is measured in a unit called Joules. The number of joules per coulomb (energy per number of electrons) is what we refer to as volts.
1 volt = 1 joule/1 coulomb
We now have two measurements of electricty; the ampere as the unit of measure of current (symbolized as I) and volts as the measure of voltage (symbolized as V). But the only time anything interesting happens is when we pass the current and the voltage through something (e.g. a light bulb, a radio or a computer) What we’re changing is the amount of each that travels through a conductor. Whenever we change the ability of a conductor we call it the conductor’s “resistance”. We measure resistance in units called “ohms” (symbolized as Ω) and an ohm is measured in terms of volts per amp or
1 Ohm = 1 Volt / 1 Ampere
Resistance is the third leg of the fundamental law for all electronics. Just like the example we gave earlier about π , the combination of resistance, current and voltage can describe everything interesting that’s happening in a circuit.
Besides what’s happening in a circuit, FIRST robotics will be concerned with how much the robot weighs, how long the battery can last and how much things cost. We’ll have to constantly be measuring, inventorying, estimating and reporting on measured quantities.
When we’re figuring out what to buy for our robot we’ll have to stick to the budget that FIRST has given us (I think we can add up to $400 to the $5,000 in parts that FIRST will give us). Budgeting requires that we estimate how much each part will cost and make sure we don’t use all of the money before we’ve purchased everything we need. How might the notion of budgeting apply to a circuit? Can you use up electricity?
Want to know more
http://www.chem.tamu.edu/class/fyp/mathrev/mr-scnot.html
http://www.youtube.com/watch?v=kQamA_mRhqA
jzahir 
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