PHY205H1F Summer Physics of Everyday Life

Class 5: Temperature, Heat

• Temperature• Heat• Specific Heat Capacity• Thermal Expansion• Thermal Expansion of

Water and Ice

• Conduction• Convection• Radiation• Newton’s Law of

Cooling• Global Warming and

Greenhouse Effect

• It’s one of the first things we think about in the morning: What’s the temperature like outside?

Temperature

• We can measure temperature with a• Temperature has something to do with

of the atoms in a material

• A cold cup of coffee has a low amount of

• If you put it in the microwave, the microwave radiation transfers energy to the coffee, increasing its

, which increases its temperature.

Temperature

• Gas—how fast the gas particles are bouncing around

• Liquid—how fast particles slide and jiggle past one another

• Solid—how fast particles move as they vibrate and jiggle in place

Absolute temperature (in degrees Kelvin) is proportional to the average translational per particle in a substance.

[animated gif downloaded Jan.25 2013 from http://www.deanza.edu/faculty/mccauley/6a_site_images/Translational_motion-250.gif ]

There is twice as much molecular kinetic energy in 2 liters of boiling water as in 1 liter of boiling water. Which will be the same for both?

A. Temperature

B. Thermal energy

C. Both A and B.

D. Neither A nor B.

TemperatureCHECK YOUR NEIGHBOR

To say that body A has a higher temperature than body B is to say that body A has more

A. internal energy.

B. mass.

C. kinetic energy per particle.

D. potential energy.

TemperatureCHECK YOUR NEIGHBOR

Temperature Scales• To convert TF in Fahrenheit to TC in Celsius:

• To convert TC in Celsius to TF in Fahrenheit:

• Though Canada officially switched to Celsius in 1970, Fahrenheit is still sometimes used, especially on kitchen appliances and in hospitals.

“Absolute” Temperature

• To convert TC in Celsius to T in Kelvin:

• To convert T in Kelvin to TC in Celsius:

• Note that temperature is due to the microscopic vibrations of the atoms in a material (internal thermal energy).

• The coldest possible temperature would be when this thermal energy dropped to zero: atoms STOPPED vibrating.

• For all materials, this happens at −273 °C• The Kelvin temperature scale is the same as Celsius, but

with a different zero point. Absolute zero has T = 0 K.

Temperature Conversion Example• The commonly accepted average core body temperature

for a human (taken internally) is 98.6 degrees Fahrenheit.• What is this in Celsius?

• What is this in Kelvin?

Is it possible for an object to have a negative Kelvin temperature?

A. Yes

B. No

TemperatureCHECK YOUR NEIGHBOR

Temperature scales summary• Celsius

– 0C for point of water – 100C for point of water

• Fahrenheit– 32F for point of water– 212F for point of water

• Kelvin – 273 K for point of water – 373 K for point of water– 0 at absolute zero, when atoms have

STOPPED vibrating!– same size degrees as Celsius scale

Heat is a form of energy

• Heat is when internal energy is transferred from one thing to another due to a temperature difference

• Heat is internal energy in .

Flow of internal energy• Heat flows from a substance to a

substance until thermal equilibrium is reached

• Heat never flows unassisted from a low-temperature to a high-temperature substance

[image downloaded Jan.25 2013 from http://summitjourneytowellness.blogspot.ca/2010/02/journey-to-wellness_5148.html ]

If a red-hot thumbtack is immersed in warm water, the direction of heat flow will be from the

A. warm water to the red-hot thumbtack.

B. red-hot thumbtack to the warm water.

C. There will be no heat flow.

D. Not enough information.

HeatCHECK YOUR NEIGHBOR

Quantity of heat• Measured in joules or calories• 4.18 joules of heat are required to change the

temperature of 1 gram of water by 1 Celsius degree

• 4.18 = 1

[photo by Scott Wallace, downloaded Jan.25 2013 from http://www.compadre.org/informal/index.cfm?Issue=11 ]

Quantity of Heat

Energy ratings of foods and fuels are determined from energy released when they are burned.

Unit of energy, the Calorie, is common for foods.• kilocalorie or 1000 calories called a

Calorie• heat needed to change the temperature

of 1 kg of water by 1C

Specific heat capacity• Defined as the quantity of heat required to

change the temperature of 1 kg of the substance by 1 degree Celsius

• Like thermal —resistance of a substance to a change in temperature

[image downloaded Jan.25 2013 from http://video.planetgreen.discovery.com/home-garden/hot-water-bottle-bed.html ]

The same quantity of heat is added to different amounts of water in two equal-size containers. The temperature of the smaller amount of water

A. decreases more.

B. increases more.

C. does not change.

D. Not enough information.

Quantity of HeatCHECK YOUR NEIGHBOR

You heat a half-cup of tea and its temperature rises by 4C. How much will the temperature rise if you add the same amount of heat to a full cup of tea?

A. 0CB. 2C

C. 4CD. 8C

Quantity of HeatCHECK YOUR NEIGHBOR

MaterialSpecific Heat(kJ/kg °C)

Alcohol 2.3Aluminum 0.87Ammonia 1.1Brass 0.38Brick 0.9Cement 1.55Chloroform 1.05Citron Oil 1.84Diamond 0.52Ether 2.21Freon 0.91Gasoline 2.22Glass 0.84Glycerine 2.43

MaterialSpecific Heat(kJ/kg °C)

Gold 0.13Hydrochloric acid 3.14Iodine 2.15Lava 0.84Mercury 0.14Milk 3.93Olive oil 1.97Paper 1.34Plastic (hard) 1.67Salt 0.88Steel 0.49Water 4.18Wood 2

[solids data from http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html ]

[fluids data from http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html ]

Specific Heat Capacities of Common Materials

The high specific heat capacity of water• has higher capacity for storing energy than almost any

other substance.• involves various ways that energy can be absorbed.

– increases the jiggling motion of molecules, which raises the temperature

– increases the amount of or within the molecules, which becomes

potential energy and doesn’t raise temperature– water molecules can absorb energy without

increasing translational kinetic energy

A. Water

B. Land

C. Both of the above are the same.

D. None of the above.

Which has the higher specific heat capacity, water or land?

Specific Heat CapacityCHECK YOUR NEIGHBOR

Thermal expansion• Due to rise in temperature of a substance, molecules

jiggle faster and .• Most substances when heated and

when cooled.– Railroad tracks laid on

winter days expand and can buckle in hot summer.

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– Warming metal lids on glass jars under hot water loosens the lid by more expansion of the lid than the jar.

Thermal expansion Different substances expand at different rates.Example:

• When the temperature of a bimetallic strip of and is increased, greater expansion

occurs for the strip, which bends to turn a pointer, to regulate a valve, or to close a switch.

Bimetallic strips are used in heaters, oven thermometers, refrigerators, and electric toasters.

Thermal Expansion• The metal in a metal ring expands.• So the ring will get thicker and longer – more

circumference, so larger.• All linear dimensions of an object expand if the

temperature : outer diameter, and inner diameter of ring.

• Also the inner diameter of a metal jar lid when it is heated!

Expansion of water to ice• When water becomes ice, it ! Ice has

open-structured crystals resulting from strong bonds at certain angles that increase its volume. This make ice than water.

Thermal Expansion: The exception to the rule

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

A. expands.

B. contracts.

C. remains unchanged.

D. Not enough information.

Thermal ExpansionCHECK YOUR NEIGHBOR

Conduction

• Transfer of by electron and molecular collisions within a substance, especially a solid

If you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand?

A. Yes

B. In some cases, yes

C. No

D. In some cases, no

ConductionCHECK YOUR NEIGHBOR

Insulation• Doesn’t prevent the flow of internal energy• the rate at which internal 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.

[image downloaded Feb.1 2013 from http://owenscorning.eu/en/products/residential-insulation/pink044.aspx ]

Liquid Nitrogen• Molecular Nitrogen, N2,

composes 70% of the air we breath

• Below -196°C ( Kelvin), Nitrogen is liquid

• We have a big tank of liquid nitrogen (LN2) at the North end of this building

• We use it to cool things to study materials – Condensed Matter Physics research

• It is also good at parties to quickly cool beer

Convection

• Transfer of involving only bulk motion of fluids

Examples:• Visible shimmer of air above a

hot stove or above asphalt on a hot day

• Visible shimmers in water due to temperature difference

ConvectionReason warm air rises• Warm air expands, becomes , and

is buoyed upward.• It rises until its density equals that of the

surrounding air.

Example: Smoke from a fire rises and blends with the surrounding cool air.

[animation from http://bmsscience8209.edublogs.org/files/2010/10/Convection-1zb8331.gif ]

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

A. Warm air cools when rising.

B. There is a thick insulating blanket of air above valleys.

C. Both A and B.

D. None of the above.

ConvectionCHECK YOUR NEIGHBOR

[image of Mt. Kilamanjaro from http://www.bmycharity.com/beckysclimb ]

Cooling by • Opposite to the warming that occurs when air is

compressed

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).

Careful, the part at the nozzle that you can’t see is steam—ouch!

Gas Cooling By Expansion

• Molecules in a region of expanding gas collide more often with receding molecules than with approaching ones

• Their rebound speeds therefore tend to decrease, and, as a result, the expanding gas cools.

• This phenomenon is used in , which use cooled coils to remove heat from a low temperature environment

Radiation

• Transfer of energy via such as light or infrared.

The surface of Earth loses energy to outer space due mostly to

A. conduction.

B. convection.

C. radiation.

D. radioactivity.

RadiationCHECK YOUR NEIGHBOR

Radiation• Transferred energy• Exists as electromagnetic waves ranging from

long (radio waves) to short wavelengths (X-rays)• In region, ranges from long

waves (red) to short waves (violet)

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Wavelength and Frequency

Which body glows with electromagnetic waves?

A. Sun

B. Earth

C. Both A and B.

D. None of the above.

RadiationCHECK YOUR NEIGHBOR

Radiation• Every object radiates.• From the Sun’s surface comes light, or

solar radiation, which we can see.• From the Earth’s surface comes terrestrial radiation

in the form of waves below our threshold of sight.

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Image in reflected,

visible light

Image in emitted, infrared radiation

Blackbody Radiation• Frequency of radiation is proportional to the

absolute of the source ( ).Tf ~

Which is the better statement?

A. A black object absorbs energy well.

B. An object that absorbs energy well is black.

RadiationCHECK YOUR NEIGHBOR

Which melts faster in sunshine—dirty snow or clean snow?

A. Dirty snow

B. Clean snow

C. Both A and B.

D. None of the above.

RadiationCHECK YOUR NEIGHBOR

Reflection of radiant energy• Any surface that reflects very little or no radiant

energy looks dark

Examples of dark objects: eye pupils, open ends of pipes in a stack, open doorways or windows of distant houses in the daytime

• Good reflectors are poor .

• Poor absorbers are poor .

• A white container will radiate heat

than a black container

A hot pizza placed in the snow is a net

A. absorber.

B. emitter.

D. neither

RadiationCHECK YOUR NEIGHBOR

Newton’s law of cooling:Rate of cooling ~ T

• Rate is proportional to the temperature difference, T, between the object and its

• Also applies to rate of warmingExamples:• Warmer house leaks internal energy to

the outside than a house that is less warm.• Frozen food will warm in a warm room

than in a cold room.

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When to add cream to coffee…• Psy likes to drink his coffee hot, and he likes cream in his

coffee. He buys it at Starbucks but does not want to drink it until he gets to his home, which is a 5 minute walk. To keep the coffee as hot as possible, should he add the cream at Starbucks or at home?

A. Starbucks, before the 5-minute walk

B. At home, just before drinking

C. It doesn’t matter

When to add cream to coffee…• My reasoning:• When you add the cream to the coffee, it is going to

decrease its temperature by some amount; this is about the same whether you do it at home or at Starbucks.

• When you carry the coffee through the environment for 5 minutes, it will be losing heat, since the coffee temperature is higher than the environment.

• The rate of heat loss over this 5 minutes is proportional to

• If you added the cream first, is less while you carry it, so it is going to lose heat en route.

• In the end, this leads to coffee when you eventually drink it.

It is commonly thought that a can of beverage will cool faster in the coldest part of a refrigerator. Knowledge of Newton’s law of cooling

A. supports this knowledge.

B. shows this knowledge is false.

C. may or may not support this knowledge.

D. may or may not contradict this knowledge.

Newton’s Law of CoolingCHECK YOUR NEIGHBOR

Greenhouse effect• Named for a similar effect

in florists’ greenhouses

Greenhouse Gases• The Earth’s atmosphere contains mostly

Nitrogen and Oxygen, both of which are (non-absorbing) of both visible and

infrared radiation• Certain gases are for visible

radiation, but for infrared radiation

• These are called “ ”:– Carbon Dioxide– Water vapour– Methane– Nitrous oxide

[image from http://en.wikipedia.org/wiki/File:Exhaust_pipe_muffler.JPG ]

Greenhouse Effect on Earth• Energy absorbed as light from the Sun• Part reradiated by Earth as longer-wavelength

radiation• Terrestrial radiation absorbed by atmospheric

greenhouse gases and re-emitted back to Earth.

• Equilibrium temperature determined by

of greenhouse gases in the atmosphere

• More greenhouse gases means higher

earth

Before Class 6 on Wednesday• Please read Chapters 19 and 20, or at least watch

the 20-minute pre-class video for class 5• Pre-class reading quiz on chapters 19 and 20 is due

Wednesday June 5 by 10:00am