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Temperature and Heat

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Title: Heat Transfer and Phase Changes Author: Millard Brown Last modified by: Damien Created Date: 1/5/2011 4:55:09 PM Document presentation format – PowerPoint PPT presentation

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Title: Temperature and Heat


1
Temperature and Heat
  • What's the difference?
  • Write a definition of each and then compare with
    your neighbor.

2
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.
3
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

4
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

5
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

6
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?

7
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!

8
Specific Heat Charts
9
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

10
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

11
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?

12
Problem Solution
  • Qcm?T
  • ?TQ/cm8.0x105 J /(3500J/kgC)(65kg)3.5C

13
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

14
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

15
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

16
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.

17
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
18
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?

19
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

20
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
21
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
22
How does heat transfer?
  • Youve heard this before
  • Conduction, convection, radiation
  • Remember them?

23
Convection
  • Think convection currents!
  • The different temp molecules actually move
  • Thus the hot air rises phenomenon

24
Conduction
  • Heat transfer through collisions of molecules-
    energy transferred from higher temp to lower temp
  • Thermal conductors transfer heat well
  • Thermal insulators do not

25
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)
26
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.

27
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.

28
Solution- Conduction
  • ?Q kA(T2-T1)t/ L
  • ?Q (0.8)(8)(37-22)(24)(3600)/0.25
  • ?Q 3.32 x 107 J

29
Radiation
  • Energy transfer without a medium- through
    electromagnetic waves
  • We will do lots more on radiation later!

30
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

31
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 ()

32
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?

33
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

34
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?

35
solution
  •  7.5 mc ?T
  • 7.5 (5)(880) ?T
  • ?T 0.0017 Cº

36
2nd and 3rd Laws of Thermodynamics
  • Heat flows from hot to cold
  • No system can reach absolute zero

37
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)

38
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

39
Four Gas Processes
  • Isothermal
  • Isobaric
  • Constant Temp
  • P1V1P2V2
  • Constant Pressure
  • V1/T1V2/T2

40
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
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