Volts, Amps, Coulombs, Ohms and Watts
Here are some of the things you should know volts, amperes, coulombs, ohms, and watts:
When a charge is at high voltage, it means it has lots of potential energy. High voltage is necessary to make a spark, or lightning. If the charge is small (e.g. as in our van de Graaff generator), then the high voltage is not necessarily dangerous. But usually when electricity is dangerous, it is because of high voltage. High voltage is analogous to water that is at a high location, and therefore has high potential energy; this energy can be utilized if the water falls over a dam, and runs a turbine.
The amount of charge depends on the number of extra electrons or protons in a material. We say we have one coulomb when we have 6x10^18 excess charges. That sounds like a lot, but only because the charge on one electron is so small. One gram of hydrogen contains 6x10^23 electrons. The energy E stored in a charge Q is equal to the volts x coulombs:
E = Q V
Ampere (current) = 1 coulomb per second
Amps measures the current, which is the amount of charge flowing per second. (This kind of current is analogous to the amount of water flowing per second in a river.) One ampere is one coulomb per second. So, for example, if a current I = 2 amperes of current flows onto a van de Graaff generator for a time t = 10 seconds, then the total charge on the van de Graaff will be (2)x(10) = 20 coulombs:
Q = I t
Watt (power) = 1 joule per second
The watt is a unit of power. One watt is equal to one joule per second. The power P it takes to send current I through a wire is given by:
P = V I
A bright lightbulb consumes about 100 watts, i.e. about 100 joules every second. One thousand watts is called a kilowatt, abbreviated as 1 kW. Since 1 horsepower = 746 watts, which is almost one kilowatt, is is worth remembering the approximate relationship:
1 kW = 1 hp
Ohms don't measure electricity, but rather the resistance of an object to the flow of electricity. If a voltage V is placed across an object that has resistance R, then the current that will flow is given by the equation known as "Ohm's Law":
V = I R
The power is the number of watts, and the energy is the number of watts times the time. If one watt flows out of a battery for one second, the energy expended is one joule. If it flows for one hour (3600 seconds), then the energy is 3600 joules. If one kilowatt flows for one hour, the energy is 3,600,000 joules. This is called one "kilowatt hour" (kwhr), and we pay our electricity bills for the number of kilowatt hours that we use. Typically one kwhr costs between 10 and 20 cents. That seems like a lot of energy for only a little money, but we are used to it, so most people think it is expensive.
One kilowatt hour = 1 kwhr = 3600000/4.18 calories = 860000 calories = 860 Calories => 860 gm of TNT. Since 860 is close to 1000, we can approximate this by 1 kwhr = 1000 gm of TNT. This is worth memorizing:
1 kWhr = 1 kg TNT
The amount of current that you can use in your house wires is limited by the heat that it generated when electricity flows through them. That heat is given by the equation for power, P = V I. Putting V = IR in this gives the equation:
P = R I^2
Big thick wires have low resistance, and can carry more current, but they cost more. Most houses have medium thickness wires installed, and must be kept to currents lower than 15 amperes to avoid excessive heating.
Sometimes the W of the word Watt is capitalized, even in the middle of the word, as in kW and kWhr. Sometimes it is not capitalized. For this course, you are welcome to do it either way.