thermal insulators and conductors thermal conductivity: the ability to conduct heat. thermal...

Thermal Insulators and Conductors

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.Because metals have more “free electrons,” they

are better thermal conductors than materials that do not have this property such as wood, styrofoam, and gases.

http://www.youtube.com/watch?annotation_id=annotation_348266&feature=iv&src_vid=yXT012us9ng&v=vqDbMEdLiCs

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

Best conductorsBoth heat & electricalIn order:SilverCopperGoldAluminum

Kinetic-Molecular TheoryAs a substance gets hotter, its molecules move faster!Faster molecules have higher kinetic energy.A higher kinetic energy results in 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.

Microscopic:Cannot be seenby 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 TheoryAs 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.

Since the molecules in aluminum can store more potential energy than the molecules in gold, the temperature of gold rises much faster than the temperature of the same mass of aluminum.

Specific Heat Capacity• 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• The higher the heat capacity, the more heat the substance

can “hold” or “give off” with minimal temperature change.• .

For example, you put 1 kg piece of steel- about 2 ¼ pounds) 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 steel or in the water?

The steel 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.

http://www.youtube.com/watch?v=yXT012us9ng&list=PL908547EAA7E4AE74

– 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.

Why are our ocean’s so important?

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.

• In contrast, in a desert there's a wide daily range of temperature because the temperature of the land goes up a lot more than the ocean for the same amount of radiation,

• and at night the temperature goes down a lot as the land loses infrared radiation to space.

The average specific heat capacity of a human body is approximately 3500.

Substance Specific Heat Capacity(J.kg-1.K-1)

water 4200

ice 2100

ethanol 2400

copper 390

aluminium 900

glass 840

mercury 140

wood 1700

lead 130

Heat transfer and 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 = 4180 J/kgK)

Q = mcDTWhat is DT? Final – initial =

100° – 15° = 85°Q = 3 (4180) 85 =Q = 1,065,900 J

Example: Q = mcDT

Sand has a specific heat of 0.5 cal/gram C. How ̊many calories does it take to raise the temperature of 100 g of sand from 60 C to 70 C ? ̊̊ ̊̊

Q = mcDTQ = (100) (0.5) (70 – 60)Q = 500 cal

Example:

• How much would this amount of heat energy raise the temperature of 100 grams of water?

From the previous problem, Q = 500 cal. We need to know the specific heat capacity, c, of water that involves calories. By definition, it is 1 cal/g C.Q = mcDT 500 = (100)(1) (ΔT) 5 = ΔT The temperature changes 5 K or ˚C (they both have equal increments).

• What would the final temperature of the water be if its initial temperature was 5 C? ̊̊

Q = mcDTA 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/kgK