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PPT – Temperature and Heat PowerPoint presentation | free to download - id: 71e32e-NjEwN

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Temperature and Heat

- What's the difference?
- Write a definition of each and then compare with

your neighbor.

Temperature vs. Heat

- Temperature
- Measurement of average kinetic energy of

molecules in a substance - Measurement of internal thermal energy

- Heat
- Thermal energy that is transmitted from one

object to another - Energy in transit

Objects can't contain heat- they can contain

thermal energy (measured in temperature) which is

transmitted as heat.

Temperature Scales

- Celsius
- Water freezes at
- 0 degrees
- Boils at
- 100 degrees
- To convert from Fahrenheit F9/5(C)32

- Kelvin
- Absolute zero
- Water freezes at 273
- To convert from celsius KC273

Physical Changes due to heat transfer

- When a substance absorbs or gives off heat it can
- Change temperature
- Change phase
- BUT NOT BOTH at same time
- Describe what is happening on this graph

Temperature Changes

- ?T depends on
- ID of substance
- Mass of substance
- For example, you could add 10J to a textbook and

a metal bar and their temp change would be

different- thus the metal pots and pans

Specific Heat

- Specific heat capacity of a substance to store

heat - Depends on chemical composition
- Technically the amount of heat (Joules) required

to raise the temp of a unit mass of the substance

by 1 degree - Kind of like thermal inertia
- SO which do you think has a higher specific heat

water or aluminum?

Specific Heat (c)

- Specific heat measured in J/gC but sometimes

also see it in calories/gC - Calorie is a unit of heat based on water 1

calorie is heat to raise temp of 1g of water by

1C - So specific heat of water is 1cal/gC which is

equivalent to 4.186J/gC - Compare this to aluminum which has a specific

heat of 0.215cal/gC or 0.900J/gC - Aluminum transfers heat much more easily!

Specific Heat Charts

Temperature Changes

- The amount of heat added relates to the change in

temperature - Qmc?T
- Where Q amount of heat coming in or going out (

for increase in temp, - for decrease in temp) - c is specific heat of substance
- Note relationships- more mass means less ?T
- Higher specific heat means less ?T for same Q

Heat Transfer and Phase Changes

- If you add or subtract heat at a phase change,

the phase change will occur instead of

temperature change- remember that these cannot

occur together! Once the phase change is

complete, the temperature can change again - QmL
- Q amount of heat
- Llatent heat of fusion or vaporization

Problem Solving Heat Transfer

- In a half hour, a 65kg jogger can generate

8.0x105 J of heat. The heat is removed from the

joggers body through natural mechanisms. If this

heat were not removed, how much would the body

temperature increase? Specific heat capacity of

the human body is 3500J/kgC. - Is heat transfer involved in phase change or

temperature change? Which equation is used?

Problem Solution

- Qcm?T
- ?TQ/cm8.0x105 J /(3500J/kgC)(65kg)3.5C

Problem solving with phase changes

- Phase changes are reversible
- Note which phase change- are you using latent

heat of vaporization or fusion? - Are you increasing temperature outside of phase

change? If so, you need to solve both parts

separately - Try 10 on HW

Heat Transfer and Thermal Expansion

- When substance changes temperature it also

changes size - Most things expand as they increase temperature

but not - Water between4C-0C
- Each material has a coefficient of linear

expansiona and change in length equals - ?LaLi?T
- Using this formula, what is the unit then for the

coefficient of thermal expansion, a? - 1/C

Problem Solving Thermal Expansion

- A metal ball has a diameter that is slightly

greater than the diameter of a hole that has been

cut into a metal plate. The coefficient of

linear expansion for the metal in the ball is

greater than that for the plate. Which one (or

more) of the following procedures can be used to

make the ball pass through the hole? - A raise the temperatures of the ball and plate

by the same amount - B lower the temperatures of the ball and plate

by the same amount - C heat the ball and cool the plate
- D cool the ball and heat the plate

Solution thermal expansion

- B and D
- Since the coefficient of linear expansion of the

ball is greater than the plate, it will shrink

more per change in temperature as the temperature

of both is lowered. Also, by cooling the ball

you will decrease its size and by heating the

plate you will increase the size of the hole.

Thermal Expansion

- Thermal expansion is a property of the material
- different materials expand differently
- Engineers need to take this into account in their

designs expansion joints in bridges - bi-metal strip demo

17

Thermal expansion

- This is how thermostats work- bimetallic strips

in refrigerators, ovens, etc. open and close a

switch as the bimetallic strip bends one way or

the other due to temp changes

- If you have a metal lid on a glass jar that is

stuck on too tight, how can you use this to get

the lid off?

Volume Thermal Expansion

- Similar to linear expansion, the expansion of

volume with an increase in temp is related to the

initial volume, the change in temperature, and a

proportionality constant - This is primarily useful with liquids since they

dont expand linearly - The coefficient of volume expansion ß
- ?VßVo?T

Problem solving- thermal expansion

- A concrete sidewalk is constructed between two

buildings on a day when the temperature is 25

degrees celsius. The sidewalk consists of 2

slabs, each 3m in length and negligible

thickness. As the temperature rises to 38

degrees, the slabs expand but no space is

provided for thermal expansion. Concrete has a

coefficient of linear expansion of 12 x 10-6 The

buildings dont move so the slabs buckle upward.

Determine the vertical distance (y) that the

slaps stick up after buckling. - Hint- DRAW!

20

Solution

- The slabs expand linearly according to ?LaLi?T
- ?L(12 x 10-6)(3m)(13)0.00047m
- The new length of the slab is then 3.00047 which

gives us the hypotenuse of the triangle. The

height, y, is found through the pythagorean

theorem. The x is the original length, 3m. - Ysqrt (3.00047)2(3)2 0.053m

21

How does heat transfer?

- Youve heard this before
- Conduction, convection, radiation
- Remember them?

Convection

- Think convection currents!
- The different temp molecules actually move
- Thus the hot air rises phenomenon

Conduction

- Heat transfer through collisions of molecules-

energy transferred from higher temp to lower temp - Thermal conductors transfer heat well
- Thermal insulators do not

Factors Affecting Conduction

- Amount of heat (Q) conducted along a bar depends

on what factors?

t Time T Temperature difference AThickness or

cross section area LLength (inversely

proportional) The material involved (thermal

conductivity, k, of that material)

Conduction of Heat through a material

- Q(kA?T)t/L
- So try one When excessive heat is produced in

the body, it must be transferred to the skin and

dispersed to maintain a constant 37.0C. One

possible mechanism for transfer is through body

fat which has a thermal conductivity of

0.20J/smC. Suppose the heat travels through

0.030m of fat to reach the skin which has a

surface area of 1.7m2 and a temperature of 34.0

C. Find the amount of heat that reaches the

skin in a half hour.

Problem Solving- Conduction

- How much heat would flow through an 8-m2 area of

an uninsulated concrete block (k 0.8 W/Km)

house in 24 hours if the temperature is 37ºC on

the one side and 22ºC on the other. The block is

25 cm thick.

Solution- Conduction

- ?Q kA(T2-T1)t/ L
- ?Q (0.8)(8)(37-22)(24)(3600)/0.25
- ?Q 3.32 x 107 J

Radiation

- Energy transfer without a medium- through

electromagnetic waves - We will do lots more on radiation later!

1st Law of Thermodynamics

- Energy Conservation ?U?K?Q0
- When you add energy to a system it can do 2

things - Increase the internal energy of system if it

stays in the system (measured by increased temp) - Do work if it leaves the system

1st Law of Thermodynamics

- ?U Q W
- ?U represents the net change in the internal

energy of the gas - Q represents the net heat added () or removed

(-) from a confined gas - W is work done by the confined gas (-) or on the

confined gas ()

Problem

- A 5-kg aluminum block slides from rest down a

1-meter long, 37º incline. When it arrives at the

base of the incline it's speed is only 3 m/sec. - How much energy is lost to frictional heat?

Solution

- PEtop mgh 5(10)(1 sin 37º) 5(10)(.6) 30 J
- KEbottom ½mv2 ½(5)(3)2 22.5 J
- Energy lost 7.5 J

One step further

- If 880 J of heat are needed to raise the

temperature of 1 kg of aluminum by 1 Cº - that

is, aluminum has a specific heat of 880 J/kg Cº -

how much did the temperature of our block

increase after sliding down the incline?

solution

- 7.5 mc ?T
- 7.5 (5)(880) ?T
- ?T 0.0017 Cº

2nd and 3rd Laws of Thermodynamics

- Heat flows from hot to cold
- No system can reach absolute zero

Heat Engines

- Transfer internal energy into mechanical work
- Heat flows from hot reservoir (burning fuel) to

cold reservoir (exhaust) and some of that energy

is removed from system as work (drives piston)

Carnot Efficiency

- Heat Engines CANNOT be 100 efficient always

some heat exhaust - Ideal efficiency depends on T differences
- Thot-Tcold
- Thot

- Heat engines are limited by this- they must

strive fro huge differences in temp - In cars, fuel cells and electric motors are NOT

heat engines so they can achieve higher eficiency

Four Gas Processes

- Isothermal

- Isobaric

- Constant Temp
- P1V1P2V2

- Constant Pressure
- V1/T1V2/T2

Four Gas Processes

- Adiabatic

- Isochroic

- Constant Heat- accomplish this by performing

rapidly (pumping up a tire) or isolating the

system - When you pump up a tire, you compress the gas

causing the temperature to rise even though you

have not added heat energy! - Opposite true for expansion- air expanding

rapidly drops in temp

- Constant Volume
- P1/T1P2/T2