Some basic definitions, equations and analogies of electricity.


Electrical Parameter (symbol) Measuring Unit (symbol) Description Water Analog Elec. Units Base Units
Voltage (V) volt (V) Pressure (Potential) difference due to charge difference. V=I*R Head: Pressure (Potential) difference due to height difference J/C kg•m²/(s³•A)
Current (I) amp (A) Flow of charge in charge/time or coulombs/sec. I=V/R Flow: Flow of water in volume per time such as liters/sec C/s or W/V A
Resistance (R) ohm (Ω) Opposition to the flow of charge. R=V/I Friction: Opposition to the flow of water V/A kg•m²/(s³•A²)
Power (P) watt (W) Rate or work or energy transfer, storage, etc. Power=Energy/Time also P=I*V Power: Power=Flow (Q) * Pressure (H) J/s or A•V kg•m²/s³
Energy (E) watt-hour (Wh) The ability to do work. Often E=P*t (where t is time) Energy: The ability to do work 3600 J kg•m²/s²


look familiar, see P=Q*H*e/k from microhydro power
I=V/R might be more edifying since current is usually the result of pressure acting on resistance.
This only applies to ohmic circuits, those circuits which display a linear relationship between current and voltage (i.e. the resistance does not change based upon current or voltage).
Series Parallel
VT=V1+V2+… V stays same
I stays same IT=I1+I2+…
RT=R1+R2+… 1/RT=(1/R1)+(1/R2)+…


The following animated analogy illustrates the operation of direct current (DC) circuits.

Water Tank - Electricity Analogy
Component Analog
Tank Battery
Tank Vertical Difference Battery Voltage Difference
Water Flow Electrical Current
Mechanical Energy Appliance (Blender) Electrical Energy Appliance
Power=Head*Flow Power=Voltage*Current

For each example, ask yourself:

  • How fast will the battery run out?
  • How fast will the virgin margaritas be made?
  • And most importantly why?

If you would like to do math to support these analogies, use:

  • Feet = volts
  • GPM = amps
  • Each blender has a resistance of 6 Feet/GPM = 6 ohms

1 Tank 1 Blender

Series 1T1B.gif

  • This is the test case (datum).

1 Tank 2 Series Blenders

Series 1T2B.gif

Notice that:

  • The flow is 1/2 the speed of our test case.
  • The two blenders in series are each going 1/4th the speed of our test case.

1 Tank 2 Parallel Blenders

Parallel 1T2B.gif

Notice that:

  • Each blender is at the same speed as our test case.
  • The flow from the tank is twice as fast as our test case.

2 Parallel Tanks 1 Blender

Parallel 2T1B.gif

Notice that:

  • The blender is the same speed as our test case.
  • The flow from each tank is half as fast as our test case.

2 Series Tanks 1 Blender

Series 2T1B.gif

Notice that:

  • The blender is 4 times the speed as our test case.
  • The total flow is twice the speed as our test case.

PS The second tank has a lid that keeps it closed.

Background Essentials

See Rural Electrification Systems for more background information (these page should be integrated together).

External links

Everything you every wanted to know about Lead Acid batteries.
Car and Deep Cycle Battery FAQ
Fantastic site on physics in general. Easy to understand, but accurate information on DC Circuits.
HyperPhysics - DC Circuits
Some easy to follow basic theory.
Lesson 3: Electricity - Colegio Franklin Delano Roosevelt
Some more information on electricity. Not wrot with error like at least one of their other pages.
How Stuff Works - Electricity
Understanding current

See also

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