Guide
to Electricity, Work, Power, and Energy
A quick, electrifying tutorial pertaining to renewable
energy:
Work
Work, W = force x distance, ft-lbs (gm-cm); W = F ·
D
Example: Lift
100 lbs 10 feet; W = F · D = 100 lb · 10 ft = 1000 ft-lb (time to
raise it doesn’t matter)
Power
Power is the
rate of doing work; P = work/time
P = 1000 ft-lb/ 10 seconds = 100 ft-lb/second
1 horsepower
(HP) = 550 ft-pounds/second = 33,000 ft-lbs/minute
HP = W/(Tsec · 550) = W/(Tmin ·
33,000)
P = E ·
I, P = I2 · R; Example:
P = 120 V · 5A = 600 watts (W) (toaster ~900W)
746W electricity = 1 HP; 3413 BTU/kW
Energy
Energy is the ability or capacity to do work; energy
and work have the same units
The work
accomplished is equal to the energy expended.
Energy (Whr)
= watts x hours; home electrical usage meters indicate kWh (kWhr)
Energy =
power times time = P • t, where P is
power and t is time
Electricity
Ohm's Law: I =
E/R, R = E/I, E = I · R (I is current, E is electromotive force, R is
resistance)
Voltage Drop: V
= I · R (V is voltage drop across a load)
Resistance in
ohms = 1/conductance (conductance is measured in siemens)
Series R: Rtotal
= R1 + R2 + ... + Rn
Parallel R: Rtotal
= 1/(1/R1 + 1/R2 + 1/... + 1/Rn)
Electrolysis: H2SO4
(sulfuric acid) H+ SO4--,
negatively charged cathode (-) attracts H+ ions --> H2;
SO4-- ions attracted to anode (+); negative charge lost
--> H2SO4 + O2
Lead-Acid
Batteries
Lead peroxide,
sulfuric acid, sheet lead; 2.2 V/cell discharged at specific gravity of 1.1,
charged at specific gravity of 1.3
Joule's
Law: Hcalories = 0.24 I2 · R x t
Transformer
A transformer converts one voltage to a different
voltage. Since the power is constant (except for losses), Pin = Pout
= Vin · Iin = Vout
· Iout
The ratio of
wire turns on the input side, Nin, to turns on the output side, Nout
determines the voltage change: Vin/Nin = Vout/Nout.
For example:
120V/1000 turns = Vout/2000 turns;
Vout
= 2000 · (120V)/1000 = 120V · 2000/1000 = 240V because there were
twice as many turns on the output (secondary) side.
The currents are
Iin · Nin = Iout · Nout
For example: 2A ·
1000 turns= Iout · 2000 turns,
Iout
= 2A · 1000 turns/2000 turns = 1A. The current went down since the
voltage went up.
Pin = 120V ·
2A = 240W, and Pout = 240V · 1A = 240W (there would be some additional 1
to 5% loss in wire internal heating)
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RenewableEnergy/GuideElectricity.htm
updated 100111 by FRL
