# heat transfer

## Thermal Energy

The quantity that indicates how warm or cold an object is called temperature.

Temperature is not "heat," but "how hot (or cold)."

We measure temperature with a thermometer, and there isn't really an upper limit on temperature.  There is a lower limit!

## whaT does temperature mean?

For a gas, temperature is a measure of how fast the gas particles are bouncing back and forth.

For a liquid, temperature is a measure of how fast particles slide and jiggle past each other.

For a solid, temperature is a measure of how fast the particles move as they vibrate and jiggle in place.

In physical science class, we'll measure temperature in Celsius or in Kelvin

Kelvin is an "absolute scale" -- 0K is absolute zero.

## Absolute zero

Absolute zero, or zero K,
is the lowest limit of temperature at –273ºC.  At this temperature, atoms or molecules have lost all available kinetic energy. A substance cannot get any colder.

## thermometers

How does a thermometer work?

• Measures temperature by expansion or contraction of a liquid (mercury or colored alcohol)
• Reading occurs when the thermometer and the object reach thermal equilibrium (having the same average kinetic energy per particle)
• Infrared thermometers operate by sensing IR radiation

## temperature -vs- thermal energy

The temperature of an object does not depend on how much of the substance you have.  The thermal energy (or, even better, the internal energy) of a substance does depend on the amount of the substance.

Example: If a 95C glass of water contains 8000J of internal energy, and then half is poured out.

What is the temperature of the remaining water?
What is amount of internal energy of the remaining water?

## temperature -vs- thermal energy

Example: If a 95C glass of water contains 8000J of internal energy, and then half is poured out.

What is the temperature of the remaining water?
95C

What is amount of internal energy of the remaining water?
4000J

Which has more internal energy, an iceberg or a glass of boiling water?

## thermal energy

Thermal energy in a sparkler
• Temperature of sparks very high (2000oC)
• Lot of energy per molecule of spark
• Total energy is small: relatively few molecules per spark
• Low transfer of energy

What is heat?

## heat

Heat is defined as a flow of thermal energy due to a temperature difference.

The natural direction of heat flow is from a higher-temperature substance to a lower-temperature substance.

## heat

• 1 liter of water in left pot. 3 liters in right pot.
• both pots absorb the same quantity of heat
• temperature increases three times as much in the pot with the smaller amount of water.

## quantity of heat

Heat is energy in transit, measured in units of energy — joules or calories.

One calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1 Celsius degree.

4.19 joules = 1 calorie

so 4.19 joules of heat will change that temperature of
1 gram of water by 1 Celsius degree.

## Quantity of heat

• Heat is energy in transit.
• Heat is measured in joules, calories, or Calories.
• 1 food Calorie equals 1000 calories.To the weight watcher, the peanut contains 10 Calories.
• To the scientist, the peanut releases 10,000 calories.
• (41,900 joules) of energy when burned or digested.

## check question

You heat a half-cup of tea and its temperature rises by 8ºC. How much will the temperature rise if you add the same amount of heat to a full cup of tea?

1. 0ºC.
2. 2ºC.
3. 4ºC.
4. 8ºC.

## check question

You heat a half-cup of tea and its temperature rises by 8ºC. How much will the temperature rise if you add the same amount of heat to a full cup of tea?

1. 0ºC.
2. 2ºC.
3. 4ºC.
4. 8ºC.

## the laws of thermodynamics

Thermodynamics is simply the movement of heat.

First law of thermodynamics:
When heat flows to or from a system, the system gains or loses an amount of heat equal to the amount of heat transferred.

Simply:
heat added = increase internal energy + external work done by the system

Remember, we cannot create or destroy energy!

## the laws of thermodynamics

Second law of thermodynamics:
Heat never spontaneously flows from a cold substance to a hot substance
Examples:
• in summer, heat flows from the hot air outside into the cooler interior of a dwelling
• in winter, heat flows from the warm inside to the cold exterior

Heat can flow from cold to hot only when work is done on the system or by adding energy from another source (as in heat pumps and air conditioners, where the direction of heat flow isn’t spontaneous)

## the laws of thermodynamics

Third law of thermodynamics:
No system can reach absolute zero.

## check question

When work is done on a system, compressing air in a tire pump for example, the temperature of the system

1. increases.
2. decreases.
3. remains unchanged.
4. is no longer evident.

## check question

When work is done on a system, compressing air in a tire pump for example, the temperature of the system

1. increases.
2. decreases.
3. remains unchanged.
4. is no longer evident.

Explanation:
In accord with the first law of thermodynamics, work input increases the energy of the system.

## check question

When a hot cup is filled with cold water, the direction of heat flow is
1. from the cup to the water.
2. from the water to the cup.
3. random, in no particular direction.
4. nonexistent.

## check question

When a hot cup is filled with cold water, the direction of heat flow is

1. from the cup to the water.
2. from the water to the cup.
3. random, in no particular direction.
4. nonexistent.

## entropy

Entropy is a measure of the disorder in a system.

Whenever energy freely transforms from one form to another, the direction of transformation is toward a state of greater disorder and, therefore, toward one of greater entropy.

## entropy

Let's restate the second law of thermodynamics:
Natural systems tend to disperse from concentrated and organized-energy states toward diffuse and disorganized states.

Energy tends to degrade and disperse with time.

The total amount of entropy in any system tends to increase with time.

## check question

Your garage gets messier each week. In this case, the entropy of your garage is

1. increasing.
2. decreasing.
4. nonexistent.

## check question

Your garage gets messier each week. In this case, the entropy of your garage is

1. increasing.
2. decreasing.
4. nonexistent.

Explanation:
If your garage became more organized each week, then entropy would decrease in proportion to the effort expended.

## specific heat capacity

Specific heat capacity is defined as the quantity of heat required to change the temperature of 1 unit mass of a substance by 1 degree.

• thermal inertia that indicates the resistance of a substance to a change in temperature.
• sometimes simply called specific heat.

Example1 : the filling in a pie has a greater specific heat capacity than the pie crust (don't burn your mouth!)

Example #2: Chris tries to make french fries...

## specific heat capacity

Water has one of the highest specific heat capacities of any substance.
Cool uses for this property of water:
1. Steam engines
2. Moderation of coastal region temperatures
3. Moderation of Europe from the warm Caribbean water

## thermal expansion

• Due to rise in temperature of a substance; molecules jiggle faster and move farther apart
• Most substances expand when heated, and contract when cooled
Examples:

• railroad tracks closely laid on winter days expand and buckle in hot summer

• warming metal lids on glass jars under hot water loosens the lid by greater expansion of the lid than the jar

## thermal expansion

This gap in the roadway of a bridge is called an expansion joint; it allows the bridge to expand and contract.

## check question

When stringing telephone lines between poles in the summer, it is advisable to allow the lines to

1. sag.
2. be taut.
3. be close to the ground.
4. allow ample space for birds.

## check question

When stringing telephone lines between poles in the summer, it is advisable to allow the lines to

1. sag.
2. be taut.
3. be close to the ground.
4. allow ample space for birds.

Explanation:
Telephone lines are longer in a warm summer and shorter in a cold winter. Hence, they sag more on hot summer days than in winter. If the lines are not strung with enough sag in summer, they might contract too much and snap during the winter—especially when carrying ice.

## check question

Anette heats a metal ring that has the same inner diameter as the diameter of the metal ball. When the ring is hot, the room-temperature ball
1. fits into the hole as before.      2. no longer fits into the hole.
3. fits into the hole with more room to spare.  4. None of these.

## check question

Anette heats a metal ring that has the same inner diameter as the diameter of the metal ball. When the ring is hot, the room-temperature ball
1. fits into the hole as before.      2. no longer fits into the hole.
3. fits into the hole with more room to spare.  4. None of these.

Explanation:

When the ring is heated, ALL parts expand. This means expansion of the inner and outer circumference, inner and outer diameter, width, you name it! The hole expands just as if it were solid metal.

## expansion of metal

• Bimetallic strip (brass and iron welded together).
• When the strip is heated, brass expands more than iron.
• When cooled, brass contracts more than iron.
• Due to this behavior, the strip bends as shown.

## expansion of water

Water is strange -- it expands as it cools to ice.

Ice has open-structured crystals resulting from strong bonds at certain angles that increase its volume.

This make ice less dense than water.

## expansion of water

Water between 0ºC and 4ºC
• does not expand with temperature
• as temperature of 0ºC water rises, contraction occurs due to melting of ice crystals in water
• contraction of water continues until 4ºC

Water at 4ºC
• smallest volume and greatest density

When 0ºC water freezes to become ice
• largest volume and lowest density.

## check question

When a sample of 0ºC water is heated, it first

1. expands.
2. contracts.
3. remains unchanged.
4. not enough information.

## check question

When a sample of 0ºC water is heated, it first

1. expands.
2. contracts.
3. remains unchanged.
4. not enough information.

Explanation:
Ice water contracts due to the melting of microscopic slush crystals. Water continues to contract until it reaches a temperature of 4ºC. With further increase in temperature beyond 4ºC, water then expands.

## the fact that water expands as it cools could be the reason life on earth exists!

• As water cools in winter, it becomes more dense and sinks. Sinking continues until the entire pond is at 4ºC.
• Then, as water at the surface is cooled further, it floats on top and can freeze.
• Once ice is formed, temperatures lower than 4ºC can extend down into the pond.

## water and ice

Cooling of water winter
• becomes more dense and sinks
• continues sinking until entire pond is 4ºC

Further cooling of water at the surface
• water floats on top and can freeze

Ice formation
• temperatures lower than 4ºC can extend down into pond

and

## Change of Phase

There are three ways that heat is transferred between objects:

1. Conduction
2. Convection

## conduction

Conduction is the transfer of internal energy by electron and molecular collisions within a substance, and primarily in solids.

In the figure, does heat flow from the nail to the ice or the other way around?

## conduction

How does insulation affect conduction?
Does it prevent the flow of internal energy?

Insulation slows the rate at which energy flows.

Example: Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to a cool exterior in winter, and the reverse in summer

## conduction

Wood coals, even if they are red-hot, are poor conductors, so you can walk across them if you walk swiftly enough.

## convection

Convection is the transfer of heat involving only bulk motion of fluids.

## cooling by convection

Convection is heat transfer, so there is a cooling effect for the object that is losing heat.
Example: The “cloudy” region above hot steam issuing from the nozzle of a pressure cooker is cool  to the touch (a combination of air expansion and mixing with cooler surrounding air). The part at the nozzle that you can’t see is steam.

## convection currents

• Convection currents produced by unequal heating of land and water.
• During the day, warm air above the land rises, and cooler air over the water moves in to replace it.
• At night, the direction of air flow is reversed.

## why does warm air rise?

Just like we discussed in the lesson on fluid dynamics, it is buoyed because it is less dense.

• Warm air expands, becomes less dense, and is buoyed upward
• Air rises until its density equals that of the surrounding air
Example: Smoke from a campfire rises and blends with the surrounding cool air.

## check question

Although warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green?

1. Warm air cools when rising.
2. There is a thick insulating blanket of air above valleys.
3. Both of the above.
4. None of the above.

## check question

Although warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green?

1. Warm air cools when rising.
2. There is a thick insulating blanket of air above valleys.
3. Both of the above.
4. None of the above.

Explanation:
Earth’s atmosphere acts as a blanket, which for one important thing, keeps Earth from freezing at nighttime.

Radiation is the transfer of energy via electromagnetic waves that can travel through empty space

• Every object above absolute zero radiates
• From the Sun’s surface comes light, or solar radiation
• From the Earth’s surface is terrestrial radiation in the form of infrared waves below our threshold of sight
• You radiate infrared waves as well

## emission and absorption

• The surface of any material both absorbs and emits radiant energy.
• When a surface absorbs more energy than it emits, it is a net absorber, and temperature tends to rise.
• When a surface emits more energy than it absorbs, it is a net emitter, and temperature tends to fall.

## emission and absorption

• The ability of a material to absorb and radiate thermal energy is indicated by its color.
• Good absorbers and good emitters are dark in color.
• Poor absorbers and poor emitters are reflective or light in color.

## emission and absorption

• Whether a surface is a net absorber or net emitter depends on whether its temperature is above or below that of its surroundings.
• A surface hotter than its surroundings will be a net emitter and tends to cool.
• A surface colder than its surroundings will be a net absorber and tends to warm.

## check question

If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be

1. lower.
2. higher.
3. unaffected.
4. None of the above.

## check question

If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be

1. lower.
2. higher.
3. unaffected.
4. None of the above.
Explanation:
If a good absorber were not also a good emitter, there would be a net absorption of radiant energy, and the temperature of a good absorber would remain higher than the temperature of the surroundings. Nature is not so!

Darkness is often due to reflection of light back and forth many times partially absorbing with each reflection

Good reflectors are poor absorbers

## check question

Which of the following does NOT emit radiation?

1. A lit fluorescent lamp.
2. A lit incandescent lamp.
3. A burned out incandescent lamp.
4. None of the above.

## check question

Which of the following does NOT emit radiation?

1. A lit fluorescent lamp.
2. A lit incandescent lamp.
3. A burned out incandescent lamp.
4. None of the above.

Explanation:
Everything continually emits radiation—and everything continually absorbs radiation. When emission is greater than absorption, temperature of the emitter drops. When absorption is greater than emission, temperature increases. Everything is emitting and absorbing radiation continually!

## newton's law of cooling

• Approximately proportional to the temperature difference ΔT between the object and its surroundings
• In short: Rate of cooling ~ ΔT

Examples:
• Hot apple pie cools more quickly in a freezer than if left on the kitchen table
• Warmer house more quickly leaks thermal energy to the outside than a cooler house

## the greenhouse effect

The "greenhouse effect" is named for a similar temperature-raising effect in florists’ greenhouses

## the greenhouse effect

Understanding the greenhouse effect requires two concepts:

1. All things radiate at a frequency (and therefore wavelength) that depends on the temperature of the emitting object
2. transparency of things depends on the wavelength of radiation

## climate change / greenhouse effect

What causes climage change?
• Energy absorbed from the Sun

## climate change / greenhouse effect

What causes climate change? (continued)

• Terrestrial radiation absorbed by atmospheric gases and re-emitted as long-wavelength terrestrial radiation back to Earth
• Reradiated energy unable to escape, so warming of Earth occurs
• Long-term effects on climate are of present concern

## phases of matter

Matter exists in the three common phases: solid, liquid, and gas (a fourth phase of matter is plasma).

When matter changes from one phase to another, energy is transferred.

## heat transfer and change of phase

Evaporation: change from a liquid to a gas

## heat transfer and change of phase

Evaporation process
• Molecules in liquid move randomly at various speeds, continually colliding with one another
• Some molecules gain kinetic energy while others lose kinetic energy during collision
• Some energetic molecules escape from the liquid and become gas
• Average kinetic energy of the remaining molecules in the liquid decreases, resulting in cooler water

## why pigs wallow

• Pigs have no sweat glands and therefore cannot cool by the evaporation of perspiration.
• Instead, they wallow in mud to cool themselves.

## heat transfer and change of phase

Sublimation is the form of phase change directly from solid to gas
Examples:
• dry ice (solid carbon dioxide molecules)
• mothballs
• frozen water

## heat transfer and change of phase

Condensation is the opposite of evaporation
• Warming process from a gas to a liquid
• Gas molecules near a liquid surface are attracted to the liquid
• They strike the surface with increased kinetic energy, becoming part of the liquid

## check question

When a liquid changes phase to a gas, it

1. absorbs energy.
2. emits energy.
3. neither absorbs nor emits energy.
4. becomes more conducting.

## check question

When a liquid changes phase to a gas, it

1. absorbs energy.
2. emits energy.
3. neither absorbs nor emits energy.
4. becomes more conducting.

By Chris Gregg

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