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)
P = 1000 ft-lb/ 10 seconds = 100
ft-lb/second
Power
Power is the rate of doing work; P =
work/time
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 = 110 V · 5A
= 550 watts (W)
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
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
Voltage Drop: V = I · R
Resistance = 1/conductance
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 turns 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 because the voltage went up.
Pin = 120V · 2A = 240W, and Pout =
240V · 1A = 240W (there would be some additional 2 to 10% loss in
heating)
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RenewableEnergy/GuideElectricity.htm
updated 090226 by FRL
