Heat can be transferred between objects or locations in three ways: conduction, convection, and radiation. 1) Conduction involves the transfer of heat through direct contact of objects. It occurs from high-temperature particles to neighboring lower-temperature particles. 2) Convection relies on the movement of fluids like gases and liquids to transfer heat. Warm fluids that are less dense rise and circulate, carrying heat energy with them. 3) Radiation transfers heat through electromagnetic waves like infrared and does not require contact or the movement of matter. All objects radiate heat based on their temperature through photons.

1. Heat Transfer
Ms Nadia Junaid

2. Temperature & Heat
• Difference between Heat and Temperature
• What is Heat?
• Heat is a form of energy.
• Energy is capacity of a body to do work.
• How to quantify energy?

3. • Heat Energy is quantified (measured) in
• Calories, kilo Calories, BTU (British thermal
Unit) or in other energy units.
• What is a Calorie
• Calorie is the amount of heat which is
required to raise the temperature of unit mass
of water (1gm) upto a unit degree (1
centigrade)

4. Temperature ?
• Temperature is indication of heat energy.
• There are units for indicating temperature like
• Centigrade, Fahren heit, Kelvin, Rankine.
• Temperature scales and their inetrconversion.
• Mercury filled thermometer : Its location in a
capillry at temperature of freezing point of
water is marked as zero, and at boiling
temperature its expansion (location in
capillary marked as 100)

5. Heat Transfer
• Heat can be transferred from one
location/substance to an other.
• How heat is transferred?
• 1- There must be a difference in temperature
between the location/substances which is
called as temperature gradient
• Temperature gradient is the basic driving force
for transfer of heat.
• Temperature difference is denoted by ΔT

6. Quantity of heat Transfer and
Rate of heat Transfer
• Quantity of Heat transfer is denoted by Q and
units are Calories, or BTUs.
• Rate of heat transfer is amount of heat
transferred per unit time
• Calories/sec, BTU/hr etc

7. Question?
• Tell me any other
unit of
Heat/energy?

8. Answer
• Watt,
• Kilowatt
• Horsepower
• etc.

9. Conductivity
• Heat Conductivity is the measure of ease of
flow of energy from a substance.

10. How Heat is Transferred?
• Heat is transferred by three different Modes.
• Conduction
• Convection
• Radiation

11. Heat Transfer

12. Modes of Heat Transfer

14. CONDUCTION
• Heat is transferred from one particle of matter
to another in an object without the
movement of the object.
• Conduction = CONTACT

15. Conduction
• Stir your hot soup with a metal spoon
• Pretty soon you need a pot holder because
the end of the spoon you are holding gets hot
• This is heat transfer by conduction
• Energy travels up the spoon from the end in
the hot soup to the end in your hand

16. Have you ever…
• Touched a metal spoon sitting in a pan of boiling
water only to be surprised by HOW hot it is??
Think back to what you know about metals and
nonmetals. What conducts heat better, metal or
nonmetal? Why?

17. Example of Conduction• Think of a metal spoon in a pot of
water being heated.
• The fast-moving particles of the
fire collide with the slow-moving
particles of the cool pot.
• Because of these collisions, the
slower particles move faster and
heat is transferred.
• Then the particles of the pot
collide with the particles in the
water, which collide with the
particles at one end of the spoon.
• As the particles move faster, the
metal spoon gets hotter. This
process of conduction is
repeated all along the metal until
the entire spoon is hot.

18. 18
Heat Conduction
Conduction is heat transfer by means of
molecular agitation within a material
without any motion of the material as a
whole.
If one end of a metal rod is at a higher
temperature, then energy will be
transferred down the rod toward the colder
end because the higher speed particles will
collide with the slower ones with a net
transfer of energy to the slower ones. TH
TC
conduction though glass
Q
Q

19. Formula for heat transfer by
conduction
Q = K.A. ΔT
• Where K= Thermal conductivity (
• A= Area
• T = Temperature difference

20. Thermal Conductivity

21. 21
Material Thermal
conductivity k
(W.m-1.K-1)
diamond 2450
Cu 385
Al 205
Brick 0.2
Glass 0.8
Body fat 0.2
Water 0.6
Wood 0.2
Styrofoam 0.01
Thermal conductivity, k
property of the material
kdiamond very high: perfect heat sink, e.g.
for high power laser diodes
khuman low: core temp relatively
constant (37 oC)
kair very low: good insulator
* home insulation
* woolen clothing
* windows double
glazing
Metals – good conductors:
electrons transfer energy from
hot to cold

22. 22
For conduction between two plane surfaces (eg heat loss through
the wall of a house) the rate of heat transfer is
energy
transferred
through
slab
Q
TH
TC
L
H CQ T T
k A
t L
 


dQ dT
k A
dt dx
 
Thermal conductivity k (W.m-1.K-1)
steady-state
heat current H = dQ/dt
Q QA

23. Convection
• Energy transfer through movement of fluids at
different temperatures (fluids - liquids and
gases)
• Examples
– Camp fire
• See soot, embers rise & swirl
• Rise upwards – warm air expands
– Why?

24. CONVECTION
• Convection is the movement that transfers heat within
fluids and air (gas)
• Heat is transferred by currents within the fluid or gas
• Convection = VENTS (through air and liquid particles)
• Convection moves in
a circular pattern

25. Examples of Convection:
Have you ever noticed that the air near the ceiling is
warmer than the air near the floor? Or that water in a
pool is cooler at the deep end?
Examples: air movement in a home, pot of heating
water.
Pick one of these examples and draw the circular pattern
in your notes.

26. 26
Heat Convection
Convection is heat transfer by mass motion of a fluid such
as air or water when the heated fluid is caused to move
away from the source of heat, carrying energy with it.
Convection above a hot surface occurs because hot air
expands, becomes less dense and rises (natural or free).
Convection assisted by breeze, pump or fan – forced
convection.
Hot water is likewise less dense than cold water and rises,
causing convection currents which transport energy.
Convection coefficient, h
T between surface and air way from surface2 1~ ( )
dQ
h A T T
dt


27. 27

28. 28
Sea & Land Breezes, Monsoons
35 oC 20 oC 17 oC11 oC
What is the role of
heat capacity, c of water and soil?

29. Radiation
Key Question:
How does heat from
the sun get to Earth?

30. Radiation
• Transfer of Energy by electromagnetic waves
• No movement of matter – will work in vacuum
or outer space
• No contact or movement of fluid
• Infrared, visible light, UV rays
• Example
– Fire emits radiation (infrared), skin absorbs E,
increase in temperature

31. RADIATION
• Radiation is the transfer of energy by
electromagnetic waves
• Radiation does NOT require matter to transfer
thermal energy
• Radiation = Radiates (heat escaping the sun)

32. Radiation
• Radiation can be absorbed and emitted
• Color
– Summer day: wear light or dark clothes?
– Winter day: wear light or dark clothes?
– Why?

33. Examples of RADIATION
1. Fire
2. Heat Lamps
3. Sun

34. 34
RADIATION
Energy transferred by electromagnetic waves
All materials radiate thermal energy in amounts
determined by their temperature, where the energy is
carried by photons of light in the infrared and visible
portions of the electromagnetic spectrum.
Thermal radiation wavelength ranges:
IR ~ 100 - 0. 8 m
Visible ~ 0.8 - 0.4 m 800 – 400 nm
UV ~0.4 - 0.1 m
For exam: more detail than in the textbook

35. 35
A blackbody absorbs all the radiation incident upon it and emits the max possible
radiation at all wavelengths
(e = a = 1)
A graybody is a surface that absorbs a certain proportion of the energy of a
blackbody, the constant being constant over the entire band of wavelengths
(0  e = a < 1)
emissivity e
absorption coefficient (absorptivity) a
Stefan-Boltzmann constant  = 5.6710-8 W.m-2.K-4

36. Stefan-Boltzmann formula
P =  AT4
Surface area (m2)
Stefan-Boltzmann constant
5.67 x 10-8 watts/m2K4)
Power
(watts)
Absolute temperature
(K)

37. Power radiated from the surface of an object
A, e, T
Qrad
4rad
rad
dQ
P Ae T
dt
 
net rad absPP P 
Emission & Stefan-Boltzmann Law
Pnet > 0 net heat transfer out of system
• Surface Area, A
• Emissivity, e = 0 to 1
• Stefan-Boltzmann constant
σ = 5.67 x 10-8 W.m-2.K-4

38. Problem 1: Estimate the Sun’s temperature
Assume e = 1
Distance from Sun to Earth: RSE = 1.5 x 1011 m
Radius of the Sun: RS = 6.9 x 108 m
Solar radiation at Earth’s surface: I = 103 W.m-2
 = 5.67 x 10-8 W.m-2.K-4
Solution
Power radiated by Sun
Prad = I A = I 4pRSE
2 = (103)(4p)(1.5x1011)2 W = 2.83 x 1026 W
Surface area of the Sun, ASun = 4pRS
2 = 5.98 x 1018 m2
T 4 = Prad / (ASun e  )  T = 5.4 x 103 K
RSE
RS
4
radP Ae T

39. Problem .2 Estimate the Earth’s surface Temperature TE
(assume NO atmosphere)
Solar constant Io= 1360 W.m-2
AE = 4pRE
2
Pabs = Prad  TE = 255 K = -18 oCe = 1
Earth
Earth albedo (reflectivity) aE = 0.3
Adisk = pRE
2
What is natural greenhouse effect?
Power absorbed by Earth:
Pabs = (1-aE) AdiskIo
Power radiated by Earth:
Prad = AE  TE
4
RSE
radiation emitted from
the surface of a sphere