1. Learning outcome 1
• Demonstrate knowledge of the concepts of heat and
temperature, heat capacity and heat transfer.
• 1.1 Describe the difference between heat and temperature
and define the various units for each.
• 1.2 Calculate the heat energy needed to cause a
temperature rise in a simple process.
• 1.3 Describe methods of heat transfer.
• 1.4 Define thermal conductivity and its dependent factors.
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2. Heat and temperature
• What is the difference between Heat and
Temperature?
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3. Heat
• Heat (page 47 Elect Principles)
– is a measure of the total kinetic energy of the
molecules or atoms in a body.
– The quantity of energy stored is measured in
Joules
– Symbol – J
– Heat transfer is the process of a hot body giving off
heat to a cold body.
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4. Heat
• Note that heat is a quantity. Cold isn’t!
• Cold is the absence of heat.
• If all heat is removed…
– The body is at absolute zero (-273.15°C).
• If heat is squashed into a smaller volume…
– The temperature rises.
– R= resistance
– H = I2Rt
– T = time of current flow
– I = current
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5. Temperature
• Temperature is a measure of the degree of movement of
the random oscillations of the molecules.
• Alternatively, it can be defined as a measure of the
hotness of a body.
• No movement = No temperature.
• (ie. Absolute Zero)
• If a body is not storing heat its temperature is absolute
zero.
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6. Temperature
• While heat and temperature are related, they are not the
same thing.
• As a body stores additional thermal energy (heat) its
temperature will raise.
• The rise in temperature is governed by:
–the mass of the body
–the type of material
• If heat is squashed into a smaller area, the temperature in
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that area will rise.

7. Temperature Scales
• Kelvin • Celsius
– 0K absolute zero – -273.15OC absolute zero
– 273.15K ice point – 0° C ice point
water water
– 373.15K steam point – 100° C steam point of
of water water
– Note 100 degrees – Note 100 degrees
between ice and steam between ice and steam
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8. Heat Capacity
• The ability of a substance to store heat.
• If equal masses absorb equal amounts
of thermal energy (heat), different
substances show a different
temperature increase.
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9. Specific Heat Capacity
• Symbol - c
• The amount of heat in Joules needed to raise the
temperature of one kg of the substance by 1°K
• water 4180J/kg/K
• lead 130J/kg/K
• copper 390J/kg/K
• see page 48 Elect Principles
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10. Specific heat capacity
Solids ( J/kg Liquids ( J/kg°C )
°C ) • Water 4180
• Methanol 2550
• Iron 450 • Ice 2090 • Ethanol 2480
• Antifreeze 2380
• Copper 390 • Wood 1680 • Benzene 1720
• Human body 3470
• Aluminium • Sand 820 Gases ( J/kg°C )
900 • Steam 1970
• Diamond 500 • Oxygen 910
• Gold 130 • Nitrogen 1040
• Concrete 880 • Dry air ~1000
• Glass 840 • Hydrogen 14300
• Freon11 870
• NaCl 880
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11. Calculations
• Q = m x c x (t2-t1)
– Where:
– Q = Quantity of heat
– m = mass in kg
– c = specific heat capacity (tables)
– t2 - t1 change in temperature
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12. Calculations
• eg. An aluminium pot (mass 0.6kg) contains 1.6 litres
(1.6kg) of water. The pot and water is heated from 30C to
95C. Calculate the heat received by the pot and water.
Assume no losses
• QH20 = mc(t2-t1)
• 0.6x900x65
• 1.6 x 4180 x 65 • 35.1kJ
• 434.72kJ • Answer = 434.72 + 35.1
= 469.82kJ
• QPOT = mc(t2-t1)
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13. Latent Heat of Melting
Product
(kJ/kg) (Btu/lb)
Aluminum 321 138.2
Ammonia 339 146
Aniline 113.5 48.8
Carbon dioxide 184 79
Copper 176 75.6
Glycerin 176 75.6
Iron, gray cast 96 41.4
Iron, white cast 138 59.4
Iron, slag 209 90.0
Lead 22.4 9.65
Mercury 11.8 5.08
Nickel 19.4 8.35
Silver 88.0 37.9
Sulphur 39.2 16.87
Tin 58.5 25.2
Water 334 144
Zinc 118 5.63
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14. Latent heat of
Latent heat of fusion
Substance ºC vaporization ºC
kJ.kg-1
kJ.kg-1
Water 334 0 2258 100
Ethanol 109 -114 838 78
Ethanoic acid 192 17 395 118
Chloroform 74 -64 254 62
Mercury 11 -39 294 357
Sulphur 54 115 1406 445
Hydrogen 60 -259 449 -253
Oxygen 14 -219 213 -183
Nitrogen 25 -210 199 -196
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15. Heat Transfer
• Heat moves from high to low temperature levels.
The rate of heat transfer is partly dependant on the
difference between the two temperature levels.
• 3 types of heat transfer
• Conduction
• Convection
• Radiation
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16. Heat transfer
• Conduction
–the heat is transferred through a material
by means of collisions between atoms.
• Q: Why does the steel of the chair you are
sitting on feel cold to the touch?
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17. Heat transfer
• Convection
–The heat is transferred by the actual
motion of the material (liquids and
gases).
• Q: What heats up a HWS?
Conduction? Convection? Both?
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18. Heat transfer
• Radiation
–Heat is radiated out as Infra Red energy.
–Radiant energy is similar to light/radio waves.
•Sun
•Standing near a fire
• Q: What heats up the earth?
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19. Thermal conductivity
• Thermal conductivity is the material’s ability to
transmit heat by conduction.
• Depends on four factors:
–Type of material
–Length of transfer path
–Cross-sectional area of path
–Temperature difference
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20. Thermal conductivity
• As a general rule
– Metals tend to have good thermal
conductivity
– Electrical insulators tend to have poor
thermal conductivity
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21. Thermal conductivity
• The frame of a motor is designed to conduct
the heat from the windings (centre of
motor) to the surface and then dissipate the
heat to the environment.
• The frame of a Hot Water Service is
designed to ensure the heat is trapped in
the centre of the Service.
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22. Thermal conductivity
• Many electrical items work better if kept
cool:
–Transformers
–Motors
–Generators
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