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. 
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 Kilowatthours (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 
= 
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. 

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 (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 


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) 
10^{0} 
1 
kilo 
K 
10^{3} 
1000 
mega 
M 
10^{6} 
1000000 
giga 
G 
10^{9} 
1000000000 
tera 
T 
10^{12} 
1000000000000 
peta 
P 
10^{15} 
1000000000000000 