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L 17 - Thermodynamics [2]L 17 - Thermodynamics [2] Science that studies the relationships Science that studies the relationships

between heat and work and the between heat and work and the conversion of one into the otherconversion of one into the other

it applies to all living and non-living it applies to all living and non-living thingsthings

it predicts the direction of natural it predicts the direction of natural processesprocesses

why ice melts (rather than getting colder!)why ice melts (rather than getting colder!) why gases expand to fill entire volumeswhy gases expand to fill entire volumes

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350 C

But first, some practical matters Thermocouples

AMPStwo different metals

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Thermocouples- measure Temp.

• The temperature sensor you stick into your turkey is a thermocouple

• it is composed of 2 wires composed of different metals welded together - when it gets hot it makes electrical current flow

350.0 °F

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Thermocouples protect you!

• a thermocouple is used in gas heaters and dryers to protect against explosions

• a thermocouple is placed in the pilot light

• as long as the pilot light is on, the thermocouple is hot and current flows

• a circuit senses the current and allows the main gas valve to open

• if the pilot light is out, the circuit prevents the main gas valve from opening

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Thermal Expansion

• Expansion occurs in all dimensions• Expansion must be taken into account when

designing roads and bridges in climates that vary significantly from winter to summer – all materials expand, steel, concrete, asphalt . . .

Metal bar at T1

Metal bar at T2 > T1

L2

L1

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Thermal expansion

change in length = change in temperature in C

length

for Copper* 617 10 per degree C

change in Temp = 100 – 22 = 78 degrees C

6 3change in length17 10 / 78 1.3 10

length

1.3

C C m

or mm

* for concrete = 10 x 10-6 per degree C

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winter/summer expansion gaps

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expansion gaps on bridges

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Thermal Expansion problems

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Areas and volumes expand too!

cold hot coldhot

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Does the whole get bigger or smaller when heated?

cold hot

Both the inner and outer diametersincrease when the ring is heated

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H2O is Weird Stuff !

0.980

0.985

0.990

0.995

1.000

-20 0 10 20 25

Temperature in degrees C

d

ensi

ty in

g/c

m3

expansion

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Bi-Metal strips

• thermal expansion of metals is put to good use in a bi-metallic strip.

• this is two strips of different metals bonded together

metal A

metal B

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Heating a Bi-metal strip

• when heat is applied to the bi-metallic strip, both metals expand, but by different amounts!

• result:

this can be used as a switch – in your coffee maker!

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Bi-Metal strip thermal switch

when a preset temperature is reached, the bi-metallic strip opens up

ON OFF

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Internal energy, Temperature and Heat

• in a gas the molecules

have energy because

they are moving.

• the sum of all the energies of all the molecules is the system’s internal energy

• the temperature of the system is a measure of how much internal energy it has, Temperature Internal Energy

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Heat what is it ? ? ?

• Heat is the energy that flows from one system to another because of their temperature difference.

• Heat stops flowing when the two systems come to the same temperature.

• Heat was considered to be an actual fluid (caloric), but it is NOT a fluid- it is energy!

System Aat temp TA

System Bat temp TB

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Heat Flow and the laws of thermodynamics

• System A has a certain amount of internal energy and so does system B

• If energy is transferred and the internal energy of B decreases by some amount then internal energy of A must increase by the same amount.

[the first law]

• If the temperature of A is less than the temperature of B then heat flows from B to A (hot to cold). [the second law]

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1st and 2nd Laws of Thermodynamics

• the 1st law says that energy is conserved whatever energy system A gains must be accounted for by the energy that B lost. (assuming that the systems are isolated so that they do not interact with any other systems)

• The 2nd law specifies the direction of heat flow hot to cold (ice melts!)

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Reverse heat flow- why ice melts

Notice that the 1st law doesnot say that ice must melt.The first law does not prohibitthat the ice from getting colderand the water warmer!

The 2nd law specifies the direction of heat flow in natural (spontaneous) processes.

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But what about refrigerators?• A refrigerator is a device that moves heat

from something cold (inside) to something hot (the room).

needs tobe pluggedin to work![energy in]

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Refrigerators and the 2nd Law

• Does this violate the 2nd law? NO, because it is not a spontaneous process

• Refrigerators require energy input (work) (electricity) to operate.

• Heat does not flow spontaneously from cold to hot, but it can be made to flow backwards if there is an input of WORK.

• It uses electrical energy to pump heat from cold to hot.

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How does Heat flow ?

• HEAT the energy that flows from one system to another because of temperature differences.

• There are 3 processes by which heat flows• convection• conduction• radiation

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Convection• heat is carried from place to place by the

bulk movement of either liquids or gases• does not apply to solids• when water is boiled, hot liquid rises and

mixes with cooler liquid, thus the heat is transferred

• Hot air rises:• want heat into lower level of house (winter) • cooled air into upper levels (summer)

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Conduction• heat is transferred directly through a material,

with no bulk movement of stuff• only energy moves

iron is a particularlypoor conductor of heat

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heat conduction

HOT COLDHeat Flow

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Thermal Conductivity

• The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivity

• there are good thermal conductors(metals) and poor ones (insulators)

Material Thermalconductivity

Copper 400

Silver 420

Steel (stain.) 14

wood 0.15

glass 0.8

wool 0.04

Goose down 0.025

styrofoam 0.01

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Grandma’s silver spoons