Electricity Basics

This is a very simple set of definitions to understand electricalterms and notation. For a more complete list, seeElectricity.(Note: the Wikipedia links providedon this page are often much more technical than the treatment here.)Many of the properties of electricity can be introduced byanalogyto 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) andcause pressure drops (voltage drops). Reservoirs (capacitors)store water (charge). The amount of water (charge)stored in a reservoir (capacitor) is the product of itssurface area (capacitance) and the height of the water(voltage / potential). The amount of work that can be done byemptying a reservoir (capacitor) by letting its water flowout (current) depends on the quantity of water(charge) and its height (voltage). For bothelectricity and hydraulics the product of these quantities is theenergy stored therein. The amount of energy released per unit time ispower. Releasing the energy slowly (low power) takes longer, but thesame work is generally accomplished as releasing the energy quickly(high power). Power is the product of water flow (current)and its pressure (voltage).

Quantity / Concept	Units	Symbol	Definition
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.
	Definition Concept Charge = Current × Time Equation Q = I × T Units coulombs = amperes × seconds
	Capacitors 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.
	Definition 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
	Capacitors 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 = Φ ÷ 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.
Potential	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 (Ω)	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 seeSI 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