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.

(Guy-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

CHANGES OF STATE

Evaporation: A change of phase from liquid to gas that takes place at the surface of a liquid.

Boiling: a change of phase from liquid to gas that takes place beneath the surface of a liquid.

Condensation: The change of phase from gas to liquid.

Freezing: the change of phase from liquid to solid.

Sublimation: the process in which a substance goes from solid to gas, bypassing the liquid state.

Precipitation: rain, sleet, snow, hail that falls to Earth when cloud particles become too heavy

Water Cycle

• Changes of State– solid to liquid OR liquid to solid

• melting point: the temperature at which a substance changes from solid to liquid OR liquid to solid

– Liquid to vapor OR vapor to liquid• Boiling point: the temperature at which a

substance changes from liquid to vapor OR vapor to liquid.

There is NO change in temperature during a change of state.

The heat required when the temperature changes is given by

Q = mcDTThe heat required during a change of state

is used to break apart those vibrating bonds, which does not affect the temperature!

There is NO change in temperature during a change of state.

The heat required during a change of state is given by

Q = mLwhere L is the latent heat of either

vaporization or fusion

Melting Ice

If a cup of ice is left out in a room, it will eventually begin to melt and the water will eventually come to thermal equilibrium with the room temperature.

While the ice is melting, the temperature remains at 0 degrees Celsius until no water remains. Once the complete change of state has taken place, the temperature of the water will start increasing.

Phase Change Graph

DT

DT

DT

Ice/waterQ = mLf

water/steamQ = mLv

Examples

How much heat must be added to 3 kg ice at -7º C till it just begins to melt?

Cice = 2100 J/kgK

Q = mcDT

Q = 3 (2100) (7)

Q = 44100 J

How much heat must be added to 3 kg of ice at 0º C to melt all of it?

Lf ice = 3.33 x 105 J/kg

Q = mLf

Q = 3 (3.33 x 105 )

Q = 999000 J

How much heat must be added to 3 kg of water at 30º C so that all of it becomes steam at 100 º C ?

Lv water = 22.6 x 105 J/kg

First, the water must reach its boiling temperature, then it must all be boiled.

Q = mcDT + mLv

Q = 3 (4186)(70) + 3(22.6 x 105 )

Q = 7659060 J

How much heat must be added to 4 kg of water at 100º C so that all of it becomes steam at 120 º C ?

Lv water = 22.6 x 105 J/kg

csteam = 2010 J/kgK

First, the water must all be boiled, then the steam must heat up.

Q = mLv + mcDT

Q = 4(22.6 x 105 ) + 4(2010)(20)

Q = 9200800 J

Water Plasma????

For water vapor to change into “water plasma” would require a temperature of approximately 12,000 degrees Celsius!

The boiling point of water

Is the boiling point of water ALWAYS 100 degrees Celsius?

Where is there less atmospheric pressure?

The boiling point of waterIn the mountains, there is less atmospheric pressure pushing down on the surface of water. Therefore, when heat is added, it is easier for liquid water molecules to “escape”- in other words, to “boil”.

Boiling water at high altitudes is NOT 100˚C! On top of Pike’s Peak, in Colorado, water will boil at only 86˚C.

You have to boil your potatoes a lot longer if you’re in the mountains!

Boiling water there is just not as hot as boiling water closer to sea level.

The effect of increased pressure on boiling temperature of water

If pressure above the surface of water increases, it is more difficult for liquid to change into gas- so the temperature of liquid water will climb higher than 100 degrees.

Pressure cookers cook food more quickly because the temperature of the water inside is HOTTER than 100˚C!

– When two substances at different temperatures are in contact, one will gain the heat, one will lose the heat and the final temperature of both will be the same.

heat gained = heat lost

m1cDT = - m2cDT

m1c(Tf – To) = - m2c(Tf – To)

and Tf = Tf

calorimeter: an insulated container used in experiments to study heat transfer.

• When placing a hot object inside a calorimeter filled with water, the heat lost by the object is gained by both the water and the calorimeter.

- heat lost = + heat gained

- Qobject = Qwater + Qcalorimeter

By FAR, the most common form of matter in the universe both by mass and volume is......

PLASMA!