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 |
---|---|
V_{T}=V_{1}+V_{2}+… | V stays same |
I stays same | I_{T}=I_{1}+I_{2}+… |
R_{T}=R_{1}+R_{2}+… | 1/R_{T}=(1/R_{1})+(1/R_{2})+… |
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 |
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
- 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).
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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