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Electric Basics
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

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