LAWS OF THERMODYNAMICS
1st Law – Conservation of Energy
2nd Law – As energy is converted into another form, some of it is lost
to heat.
There are no 100% efficient machines; Entropy is always increasing;
Perpetual motion machines are impossible.
Energy tends to go from more usable (higher quality) to less usable
(lower quality). When you use energy, you lower its quality.
3rd Law – Entropy at absolute zero is exactly equal to zero.
Which Law of Thermodynamics?
a) The amount of energy in an isolated system stays constant.
b) An energy transformation occurs and results in increased disorder.
WORK & ENERGY
Energy = “ability to do work” but what is “work”?
Work = force x distance that object moves
Thus, we get the idea that energy allows one to move
objects from one place to another, thereby
accomplishing some physical labor or “work.”
Energy can be in many forms (solar, electrical, chemical, thermal,
nuclear) but ALL FORMS CAN BE USED TO DO WORK.
System
Length
Mass
Time
SI
Meter
Kilogram
second
USCS
foot
slug
second
System
Force =
Mass x
Acceleration
SI
newton
kg
m/s2
USCS
lb
Slug
ft/s2
System
Energy =
Force x
Distance
SI
joule
=
newton
meter
USCS
ft-lb
=
lb
ft
newton (N)
Unit of force; 1 N = 1 kg-ms2
joule (J)
Unit of energy. The joule is a relatively small amount of energy;
used a lot in science. The energy content of a large donut is about 100,000 joules.
calorie (c)
1 calorie = 4.186 joules
1 calorie is the heat required to raise
the temp. of one gram of water by one Celsius degree.
The kilocalorie is known as the “BIG” calorie and written C or Calorie. Food
calories are always big Calories. Thus, when one speaks of 100 Calories in a slice
of bread, it is actually means that 100 kilocalories or 4.186 x 105 J would be
released through burning the dried biomass.
Btu (British thermal unit) – amount of heat required to raise the temp.
of one pound of water by one degree Fahrenheit.
1 Btu = 252 cal = 1055
One Btu is approx. the amount of heat released by burning one match.
Btus are common in water heaters, furnaces, & air conditioners.
Water heater could be rated at 40,000 Btu/h
Btu/h gives the rate at which heat can be produced.
Heating values for fuels are often stated in terms of Btus per unit weight.
Ex) Coal – has typical value of 25 million Btu/ton
Ex) Petroleum – 37 million Btu/ton
Therm - Gas companies in the U.S. often measure sales in “thermal
units” or therms.
1 therm = 100,000 Btu
Power
Power is used to describe energy flow. Power is defined as “the time rate of doing
work” and measured in joules/second.
SI unit = watt (W) in honor of James Watt inventor of steam engine.
1 watt = 1 joule/second
USCS unit = horsepower (hp) where 1 hp = 550 ft-lb/s = 746 W
Power vs. Energy
Most electrical appliances are rated in terms of their power. But just like when
filling the tank of a car at the gas station, you must pay for the total number of
gallons pumped, not the rate at which you pumped it.
So with electricity, we pay for the total number of joules of electricity consumed,
not the power or rate it was delivered.
kilowatt-hours (kWh) - U.S. electrical energy is measured in kWh
1 kWh = energy to power TEN 100-watt lightbulbs for an hour.
1 kWh = 1,000J/s x 3,600s = 3.6 x 106 J
Electric Power Plants - Rated in terms of their capacity to
deliver electric power.
Ex) Large coal power plant could be rated at 1,000 MW
(megawatts). Sometimes MWe, where the “e” on the W stands for
“electric” and signals that the rating is for the output capacity of the
plant, and not the energy input.
Energy Efficiency = Energy produced x 100
Energy consumed
PRACTICE
(a) A light bulb has an efficiency rating of 5.2 percent. How much
energy is produced for every 1.00 joule of electrical energy?
(b) Is the energy produced light or heat energy?
PRACTICE QUESTIONS
1. Given that 1 kcal of heat is required to increase the temperature of 1
kg of water by 1°C:
• a. How many kcals would be required to heat 100 kg of water by 20°C
for a bath?
• b. How many joules is this?
• c. How many Btus?
• d. If your water heater can supply 40 kBtu/h, how long will it take to heat
this water?