Thermodynamics: the study of thermal energy

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# Thermodynamics: the study of thermal energy - PowerPoint PPT Presentation

Thermodynamics: the study of thermal energy. Heat, Q. Heat is NOT temperature. You do NOT measure heat with a thermometer! Heat is the flow of energy . Heat: Transfer of “disordered” energy at the microscopic level as a result of temperature differences

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### Thermodynamics: the study of thermal energy

Heat, Q

Heat is NOT temperature. You do NOT measure heat with a thermometer!

Heat is the flow of energy.

Heat: Transfer of “disordered” energy at the microscopic level as a result of temperature differences

Unit: Joules (just like all forms of energy)

James Joule first determined the relationship between mechanical energy and heat- (motion transformed into heat)

An older unit for measuring heat was the “calorie”.

One food Calorie = 4186 Joules

I wonder how many joules of energy are in this strawberry shortcake?

What is “Cold”?

Just as darkness is an absence of light,

Cold is a absence of heat.

Heat flow is a measurable quantity. As more and more heat flows out of something, it gets colder and colder.

When two substances of different temperatures are in contact, heat will flow until they are at the same temperature, “thermal equilibrium”
Transfer of Thermal Energy

Convection- the bulk flow of fluids, (gases or liquids, NOT solids) “convection currents”

Conduction- direct contact

Conductivity

A block of wood and a block of metal sit on a table. You touch each.

Which one feels colder?

Is it really colder?

Why does it feel that way?

Thermal Conductivity: The ability to conduct heat.

Thermal Insulators do not conduct heat readily.

Generally, metals have high thermal conductivity and gases have low thermal conductivity.

For metals, the thermal conductivity is quite high, and those metals which are the best electrical conductors are also the best thermal conductors.

Best conductors

Both heat & electrical

In order:

Silver

Copper

Gold

Aluminum

Temperature scales

Celsius C = 5/9(F° – 32)

Fahrenheit F = 9/5C° + 32

Kelvin K = C° + 273

Water (at normal Earth atmospheric pressure)

freezes at 0°C = 32°F

boils at 100°C = 212°F

Is there a HOTTEST possible temperature?

Temperatures do not appear to have an upper limit.

Is there a COLDEST possible temperature?

The lower limit on temperature is called “absolute zero”, which equals 0 Kelvins. However, it is impossible for a substance to be at absolute zero.

The Kelvin scale, which has its zero at ABSOLUTE ZERO, was named for William Thomson, “Lord Kelvin”, who found the value for absolute zero using fundamental laws of thermodynamics.
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

### Heat (energy) Transfer

HEAT

Potential Energy

Stored in the vibrations of the molecules

Limited by the “degrees of freedom” available to the molecule

• Kinetic Energy
• - Motion of atoms and molecules
• Reflected in the TEMPERATURE of the substance
• Faster = higher temperature

If the molecules have many “degrees of freedom”, they can store more potential energy, with less change in the kinetic energy of the molecules. Therefore, the temperature will change slowly.

If the molecules have few “degrees of freedom”, they can store little potential energy, with more change in the kinetic energy of the molecules. Therefore, the temperature will change more rapidly.

HEAT

Potential Energy

Kinetic Energy

Microscopic:

Cannot be seen

by eyes alone- usually you can’t “measure” the kinetic energy of the molecules

Macroscopic:

Can be seen

with eyes alone- you can measure the temperature!

Kinetic-Molecular Theory

As a substance gets hotter, its molecules move faster!

Faster molecules have higher kinetic energy.

A higher kinetic energy is reflected by a higher temperature!

You may not be able to SEE molecules moving fast without a microscope, but you can see an increase in temperature on a thermometer.

Kinetic-Molecular Theory

As a substance gets hotter, its molecules move faster!

Faster molecules have higher kinetic energy.

A solid: the molecules are tightly packed together and move more slowly. When you add heat…

A liquid: the molecules are not packed as tightly together and move around. When you add more heat…

A gas: the molecules are not bound together and move very fast. When you add more heat…..

A plasma: the atoms themselves are ripped apart to become ions.

Heat Transfer

Since the atoms in aluminum can store more potential energy than the atoms in gold, as heat flows, the gold atoms will gain more kinetic energy so that the temperature of gold rises much faster than the temperature of the same mass of aluminum.

In one episode of the original TV series, “Mission Impossible”, the team utilized that fact that gold heats up so quickly compared to other materials. They drilled a hole in the bottom of a vault and inserted an electric heat rod. The gold in the vault heated up, melted and flowed through the hole in the bottom of the vault before any of the paper money or other coins got too hot.

Would that really work?? Hmmm, maybe- but don’t try it!

Specific Heat Capacity, “c”
• The specific heat capacity, “c”, of a substance is the amount of heat required per kilogram to raise the temperature by one degree.
• Different substances have different specific heat capacities. Aluminum has a higher specific heat than gold! More heat must be transferred into aluminum than into gold for the same change in temperature
• The higher the heat capacity,

the more heat the substance

can “hold” or “give off” with

minimal temperature change.

Unit: J/kg·K (or J/kg·°C)

Specific Heat Capacity of Water

For example, you put 1 kg piece of metal on a hot plate for two minutes. You also put a container of 1 kg water in on an identical hot plate.

Would you rather place your finger on the metal or in the water?

The metal will be at a much higher temperature!

Both received the same amount of heat energy.

But water has a higher specific heat capacity- it can absorb or release more heat energy with little temperature change.

Water has one of the highest specific heats of all substances. It can absorb and give off great amounts of heat energy with little temperature change.
• It takes a long time to heat water and it takes a long time for water to cool down!
• Another example: The filling on a hot apple pie burns our tongues and not the crust even though they are the same temperature because of the water content in the filling.
• The filling can give off a lot of heat and STILL be hot.
The oceans help maintain a small range of temperature on Earth that is compatible with life by absorbing heat in the day and releasing it at night with little change in the ocean’s temperature.
Weather along the Coastline
• Coastal regions do not experience a large change in temperature because the water absorbs solar radiation in the day and releases it at night
In contrast, in a desert there\'s a wide daily range of temperature because no water is available to absorb heat in the day and release it at night
Table of Specific Heat
• The average specific heat capacity of a human body is approximately 3500.
Heat transferand temperature change

As heat, Q, flows into or out of a substance, its temperature change, DT, will depend on the mass, m, of the substance and its specific heat capacity, “c”.

Q = mcDT

Example: How much heat, Q, is required to raise the temperature of a 3 kg pan of water from 15°C to boiling temperature?

(specific heat of water = 4186 J/kg·K)

Q = mcDT

What is DT? Final – original =

100° – 15° = 85°

Q = 3 (4186) 85 =

Q = 1,067,430 J

Q = mcDT

A 0.2 kg block of metal absorbs 1500 J of heat when its temperature changes from 20 to 35 degrees Celsius. What is the specific heat capacity of the metal?

c = Q ÷ (mDT)

c = 1500 ÷ (0.2 • 15)

c = 500 J/kg·K

Q = mcDT

What is the change in the temperature of 3.5 kg water if 950,000 J of heat are added? C = 4186 J/kg·K

DT = Q ÷ (mc)

DT = 950,000 ÷ (3.5 • 4186)

DT = 64.9°