Some basic definitions, equations and analogies of electricity.

## Definitions

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

## Equations

P=IV
Power=Current*Voltage
look familiar, see P=Q*H*e/k from microhydro power
V=IR
Volts=Current*Resistance
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)+…

## Analogies

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

• 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

• This is the test case (datum).

### 1 Tank 2 Series Blenders

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

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

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

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).

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