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Electricity Basics

This is a very simple set of definitions to understand electrical terms and notation. For a more complete list, see Electricity.(Note: the Wikipedia links provided on this page are often much more technical than the treatment here.)  Many of the properties of electricity can be introduced by analogy to water (charge) flowing in pipes (conductors)under pressure (potential) from high points to low points(from higher to lower voltages). Small diameter pipes(resistors) resist the flow of water (current) and cause pressure drops (voltage drops). Reservoirs (capacitors) store water (charge). The amount of water (charge)stored in a reservoir (capacitor) is the product of its surface area (capacitance) and the height of the water(voltage / potential). The amount of work that can be done by emptying a reservoir (capacitor) by letting its water flow out (current) depends on the quantity of water(charge) and its height (voltage). For both electricity and hydraulics the product of these quantities is the energy stored therein. The amount of energy released per unit time is power. Releasing the energy slowly (low power) takes longer, but the same work is generally accomplished as releasing the energy quickly(high power). Power is the product of water flow (current) and its pressure (voltage).

Capacitance farad (F) C A measure of the amount of electric charge stored for a given voltage.
Concept Capacitance = Charge ÷ Voltage
Equation C = Q ÷ V
Units farads = coulombs ÷ volts
Water analogy A capacitor stores charge much as a reservoir stores water. If a reservoir feeds a pipe, the higher the water level, the more pressure on the pipe. Similarly, the more charge on a capacitor, the more voltage the capacitor presents to the system. Capacitance is then similar to the surface area of the reservoir.

Charge coulomb (C) Q A measure of the excess of positively charged particles over the negatively charged particles.
Concept Charge = Current × Time
Equation Q = I × T
Units coulombs = amperes × seconds
Concept Charge = Capacitance × Voltage
Equation Q = C × V
Units coulombs = farads × volts
Water analogy Electrical charge is analogous to the amount of water stored in a reservoir.

Current ampere amp (A) I A measure of how much charge is moving through a conductor per time.
Concept Current = Charge ÷ Time
Equation I = Q ÷ T
Units amps = coulombs ÷ seconds
Ohm's Law
Concept Current = Voltage ÷ Resistance
Equation I = V ÷ R
Units amps = volts ÷ ohms
Water analogy Electrical current is analogous to water current. The larger the current, the more water (charge) that moves past a given point over time.

Energy joule (J) E Energy effects changes in a system. The change may happen quickly by expending a lot of energy in a short period of time (high power), or it may happen more slowly by using proportionally lower energy for a longer period of time (low power). For many systems, the energy required to effect change is independent of whether it is done quickly or slowly (i.e. by high power or low power), and so energy just measures the capacity for change in the system.
In Physics, the SI unit for energy is the joule. In electrical calculations a joule is a watt·second (Ws). More common units in electrical work are watt·hours (Wh), which are 3600 Ws, and the Kilowatt-hours (KWh), which are 3,600,000 (Ws).
Concept Energy = Power × Time
Equation E = P × T
Units joules = watts × seconds
Concept Energy = Charge × Voltage
Equation E = Q × V
Units joules = coulomb × volts
Water analogy Imagine a quantity of water at a given height (a given potential). The higher it is, the more work it can do running downhill. The more water there is, the more it can do. The work it can do is represents its energy, which is the product of the quantity and potential.

Inductance henry (H) L A measure of the amount of magnetic energy stored for a given current.
Concept Inductance = Magnetic flux ÷ Current
Equation L = F ÷ I
Units henrys = webers ÷ amps
Water analogy An inductor stores energy much as a water wheel does. When the water speeds up, the wheel resists, but eventually speeds up to match the new speed. When the water slows down, the water wheel transfers some of its energy back to the water.

Water analogy Electric potential is analogous to water pressure. Where electric potential (pressure) is uniform, there is no force or push, just as we do not feel the tremendous atmospheric pressure at sea level. However, in places where potential (pressure) varies, it produces a force that can push charged objects to different locations (i.e. create a current). A potential difference is called voltage, and is measured in volts.

Power watt (W) P Energy per time
Concept Power = Energy ÷ Time
Equation P = E ÷ T
Units watts = joules ÷ seconds
Concept Power = Voltage × Current
Equation P = V × I
Units watts = volts × amps

Resistance ohms (O) R A measure of how hard it is to move current through a conductor.
Ohm's Law
Concept Resistance = Voltage ÷ Current
Equation R = V ÷ I
Units ohms = volts ÷ amps
Water analogy It is harder to move water through a narrow pipe than a wide one. Similarly it requires more effort to move current through a high resistance conductor than a low resistance one.

Voltage volts (V) V An electric potential difference—voltage—may exist between two points. (Often the earth is used as one point, and is considered to be 0V.) The voltage between two points measures the work it is to move charge between them.
Ohm's Law
Concept Voltage = Current × Resistance
Equation V = I × R
Units volts = amps × ohms
Water analogy Voltage is analogous to pressure differences.

Most common scientific prefixes used in electrical calculations
This is not a complete list. For a complete list see SI prefix.

Common Prefixes
Prefix Abbreviation Scientific Decimal equivalent
femto f 10-15 0.000000000000001
pico p 10-12 0.000000000001
nano n 10-9 0.000000001
micro µ 10-6 0.000001
milli m 10-3 0.001
(none) (none) 100 1
kilo K 103 1000
mega M 106 1000000
giga G 109 1000000000
tera T 1012 1000000000000
peta P 1015 1000000000000000