The Laws of Thermodynamics

The Zeroth Law !If Object 1 is in thermal equilibrium with Object 2 and

Object 2 is in thermal equilibrium with Object 3, then

Object 1 is in thermal equilibrium with Object 3.

Well, duh…

The First Law of Thermodynamics

The change in the internal energy of a substance is the sum of the heat transferred plus the work done.

“internal energy” is associated with the random motion of all of the molecules that make up an object- both the kinetic energy and the potential energy. The more molecules there are, the more internal energy is possible.

Which has more internal energy: a hot coal or a frozen lake???

1st Law: The change in the internal energy of a substance is the sum of the heat transferred plus the work done

D Internal Energy = Q (in or out) + W (in or out)

1st Law: The change in the internal energy of a substance is the sum of the heat transferred plus the work done

The First Law of Thermodynamics is yet another way to state that energy is neither created nor destroyed, but

may be transferred or transformed-

The Law of Conservation of Energy

Heat EnginesA “heat engine” takes in heat and with that heat energy performs work. It then gives off a lesser amount of heat at a lower temperature.

QH = QC + Wout

A heat engine is an example of the first law of thermodynamics:D Internal Energy = Q + W

If we rearrange things we get:

Q (in) = D Internal Energy + W (out)

Heat EnginesEven the very best engines are not able to transform all the heat energy into work. Some of it is ALWAYS “wasted”.

The “ideal” engine is called a “Carnot Engine”- a perfect, theoretical (but not physically possible) design.

Even the Carnot Engine would not have an efficiency of 100%.

Examples of Heat Engines

Steam Engines produce motion which can be used to do work or generate electricity.

What is another well-known “heat engine”??? Think of something most of us use every day that converts heat into motion (with some waste heat).

four-stroke internal combustion engine Internal Combustion Engine Simulator

intake stroke: intake valve is open, allowing gas and air to enter the cylinder. The piston moves downward

compression stroke: valves are closed as the piston moves upward, creating high pressure in the fuel-air mixture. At the top of the stroke, the spark plug sparks, which ignites the fuel-air mixture.

power stroke: the explosion pushes the piston down. The piston’s rod turns the crankshaft, which provides the torque to turn the wheels.

exhaust stroke: the piston moves back upward. The exhaust valve opens to allow exhaust gases to leave the cylinder.

closed

closed

closedopen

Of course, much of the energy released by the combustion of fuel does not produce the mechanical energy of motion in the

car.

In what form does much of that released chemical potential energy appear???

HEAT !

The 2nd Law of Thermodynamics

The Law of Entropy:

Natural processes always increase entropy.

Entropy: disorder

What does “entropy” have to do with heat??

… Go back to the Kinetic-Molecular Theory:

The faster atoms are moving, the more “disorder” or ENTROPY they have.

So, if you increase the temperature of a substance, you also increase its ENTROPY.

Absolute Zero is the temperature at which entropy would reach its minimum value.

2nd Law: Natural processes always increase entropy.

Alternative statement of the 2nd Law:

Heat flows spontaneously, “naturally”, from a hotter substance to a cooler substance

…the “heat” in the cooler substance will NOT flow out of that cooler substance into the warmer substance to make it even more warm!!

That nice “orderly” cooler substance is naturally going to become more “disorderly”

The Second Law of Thermodynamics

Heat won't pass from a cooler to a hotter.

You can try it if you like, But you far better notta,

cause the cold in the coolerWill get hotter as a ruler,

And that's a physical law! [Michael Flanders and Donald Swan]

Just like water flows naturally from high elevation to low elevation, heat flows naturally from warmer temperature to cooler temperature.

You can make water move from low elevation to high elevation if you use a water pump, which requires an input of energy.

You can make heat flow from a cooler object to a warmer object if you use a heat pump, which requires an input of energy.

Heat pumps are used in your refrigerator and in the heating/air conditioning system in houses!

Different kinds of heat pumps:

Cooler inside air

Warmer outside air

Direction of heat flow with the spontaneous, “natural”process

Cooler inside air

Warmer outside air

Direction of heat flow with a heat pump

What is “Absolute Zero”?

For ALL gases, as the temperature drops, the pressure within the gas drops in a direct relationship.Graphing pressure vs temperature for many gases and then EXTRAPOLATING the graphs to a pressure of ZERO (which is impossible) yields the same temperature for every gas:

-273 C = 0 Kelvins = Absolute Zero

3rd Law: It is not possible to lower the temperature to absolute zero.

Since absolute zero is the temperature at which a gas would exert zero pressure- which is impossible, reaching 0 Kelvins is impossible also.

c) (6) E,F,G, TAKS I.6B

Other changes due to heat flow…Gases: the relationship between temperature, pressure and volume:If the volume of a gas is held constant, as the temperature increases, the gas pressure increases as well.

(Gay-Lussac’s Law)

If the pressure is held constant, as the temperature increases, the volume will increase.

(Charles’ Law)

Thermal ExpansionGenerally, as temperature increases, the lengths and/or volumes of substances, including liquids and solids, also increase.

Examples:

concrete sidewalks expand and crack in hot weather,

The air in a “hot air” balloon expands and fills out the balloon

Metals expand when heated and contract when cooled.

Gasoline expands in your gas tank on a hot summer day.

Bimetallic strips:

used in thermostats

– How much will the length or volume change?

DL = LoaDT

a - the coefficient of linear expansion

DV = VobDT

b- the coefficient of volume expansion

Example1. An overpass bridge is about 300 m long.

How much will it expand if the temperature changes by 30º C (asteel = 12 x 10-6 )

Lo = 300

DT = 30

asteel = 12 x 10-6

D L = Loa DT = 300 (12 x 10-6) 30 =

0.108 m

One exception to the rule of thermal expansion is water

As the temperature of water drops its volume decreases until 4°C is reached.

Then its volume expands as the temperature continues to drop to 0°C.

Vo

lum

e

Temperature

Result: ice is less dense than water so that ice floats. This protects underwater life in cold climates