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Home , Heat capacity, Specific heat capacity, Thermal expansion

Lecture 17 Chapter 15 Capacity Chapter 16

3-Oct-10

Internal

Internal energy of an Internal object depends on: Energy • Temperature 300 K 120,000 J 1 kg 200 K 80,000 J • 1000 100 K 40,000 J • Material 0 K 0

Temperature Internal Temperature Internal Energy Energy Iron 300 K 1,200,000 J 300 K 120 J

1 kg 200 K 800,000 J 1 200 K 80 J 100 K 400,000 J 100 K 40 J

3-Oct-10 0 K 0 Joules 0 K 0 Joules Specific is the amount of heat energy required to raise the temperature of one unit mass of a material by one degree. SI Unit: J/(kg•K) or J/(kg•°C) Other Units: cal/(g •°C)

Heat energy needed to raise temperature of material by ∆T is: (Specific heat cap. of material)•(mass)•∆T

3-Oct-10

Some specific heat capacity values. Specific of are complicated. Example

• How much heat energy is needed to raise the temperature of 2 kg of (s.h.c. = 387 J/kg-K) from 10° to 30°C? • Q = (s.h.c.)•m•∆T = (387 J/kg-K)(2kg)(20K) = 1.55 x 104 J • How long would it take for a 1000 W heater to do this? = (Energy provided)/∆t

∆t = (Energy needed)/(Power) = 15500J/(1000 J/s) = 15.5s

Check Yourself

Why does a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a cooler on a hot day?

Why is it that the climate in the is so hot during the day yet so cold at night?

3-Oct-10 Thermal Expansion

Due to increased molecular motion, most materials expand as temperature increases.

Space allows for expansion

Sidewalk buckles and cracks due to expansion on a hot summer day

3-Oct-10

Demo: Expansion of a Ring

Metal ball barely fits past the metal ring. Not surprising that heated ball won’t pass through cold ring. Will cold ball pass through heated (expanded) ring?

3-Oct-10 Coefficients of Linear Expansion

Thermal Expansion Differences A has two metals of different coefficients of thermal expansion, A and B in the figure. It will bend when heated or cooled. Demo: Heat, Cool, Break expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass. Pyrex glass expands much less than regular glass.

GLASS

Cracks form

COOL (quickly)

3-Oct-10 HEAT

Thermal Expansion The amount of thermal expansion of length L is: ∆L = (expansion coefficient)•L•∆T An area gets linear expansion in both directions. Holes expand as well: You have a (glass) jar and you can’t get the metal lid off. What should you do:

a) ask your friend b) run the jar & lid under cold water c) run the jar & lid under hot water

You have a (glass) jar and you can’t get the metal lid off. What should you do: a) ask your friend b) run the jar & lid under cold water c) run the jar & lid under hot water Because the metal has a substantially higher coefficient of thermal expansion than the glass, heating them will make both of them bigger, but the metal will be ‘more bigger’. Water vs. Temperature

This explains why lakes freeze from the top.

Heat Transfer (Flow of Heat Energy) Three Methods •Conduction - Thermal passed from particle-to-particle along a length of material. • - carried by moving fluid. •Radiation - Thermal energy carried by electromagnetic waves. Heat Transfer: Conduction Heat conduction can be visualized as occurring through molecular collisions. Thermal kinetic energy is passed along as “hotter” particles collide with “colder” ones.

Cross- sectional area

Q

L

Conduction

Conduction is heat flow by direct contact. Tile floor feels colder than wood floor Some materials are 98º good thermal 98º conductors, 75º 75º others are Wood is an Tile is a insulators. insulator conductor

3-Oct-10 Conduction Experimentally, it is found that the amount of heat Q that flows through a piece of material: • Increases proportionally to the cross- sectional area A • Increases proportionally to the temperature difference ∆T from one end to the other • Increases steadily with time t • Decreases with the length L of the piece • Depends on the “” of the material. More conductive → more heat flows

Thermal Conductivity

Some typical thermal conductivities:

Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators. Vacuum has a thermal conductivity = 0. Convection Convection is flow of fluid due to difference in , such as warm air rising. Fluid “carries” heat with it as it moves.

“Natural” convection: Warm fluid will rise because it is less dense then cold fluid.

Heat Transfer: Convection Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced (fans); both these examples are . Convection

Heat transfer in a fluid often occurs mostly by convection. causes warm air to rise, which carries thermal energy directly by its motion.

3-Oct-10

Convection Oven

Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.

3-Oct-10 Fiberglass Insulation

Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the disrupting the convection flow.

3-Oct-10

Radiation All objects give off energy in the form of radiation, as electromagnetic waves – infrared, visible light, ultraviolet – which, unlike conduction and convection, can transport heat through a vacuum. Objects that are hot enough will glow visibly – first red, then yellow, white, and blue as temperature increases. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars. Radiation Radiation has many different wavelengths, most of which are not visible to the eye. All radiation carries energy, and thus transfers heat. Heat Lamp

3-Oct-10 Physics 1 (Garcia) SJSU

Emission of

All objects radiate; higher 75º the temperature, the higher the frequency. 98º At room temperature, the radiated light is at frequencies too low for Attics in this house were kept our eyes to see. warm for growing marijuana. Special cameras are sensitive to this infrared radiation. 70 3-Oct-10 Reflection of Radiant Energy White and “” objects reflect light, black objects and “holes” don’t.

Hole in a box with white interior looks White tubes look black because almost none of the light black inside. entering the hole reflects back out. Black objects are also the best emitters of radiation. White objects emit less radiation, and perfectly reflective objects don’t emit at all. (Space blanket.) 3-Oct-10

Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having double- walled sides with vacuum. Silvered interior walls minimize heat transfer by radiation.

3-Oct-10 Radiation If you are in sunlight, ’s radiation will warm you. The intensity of solar radiation is 1000 W/m2. In general, you will not be perfectly perpendicular to the Sun’s rays, and will absorb energy at a rate that depends on your angle to the sun’s rays.

Seasons This angle effect is also responsible for the seasons. Greenhouse Effect

Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.

3-Oct-10 Physics 1 (Garcia) SJSU

Earth’s Greenhouse Effect

Earth’s atmosphere acts as a greenhouse, trapping . Most of the trapping is due to dioxide and , which is why they’re called “greenhouse gasses.”

3-Oct-10 KeyKey PointsPoints ofof LectureLecture 1717

• Specific Heat Capacity • Thermal Expansion • Transfer of Heat by Conduction • Transfer of Heat by Convection • Transfer of Heat by Radiation z Before Friday, read Hewitt Chap. 16. z Homework Assignment #12 is due before 11:00 PM on Friday, Oct. 8.