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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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Heat and temperature

• What is the difference between Heat and Temperature?

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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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• 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…

Heat

– The temperature rises.

– H = I2Rt

– I = current

– R= resistance

– T = time of current flow

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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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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 that area will rise.

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Temperature Scales

• Kelvin

– 0K absolute zero

– 273.15K ice point water

– 373.15K steam point of water

– Note 100 degrees between ice and steam

• Celsius

– -273.15OC absolute zero

– 0° C ice point water

– 100° C steam point of water

– Note 100 degrees between ice and steam

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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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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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Specific heat capacitySolids ( J/kg

°C )

• Iron 450

• Copper 390

• Aluminium 900

• Gold 130

• Glass 840

• NaCl 880

• Ice 2090

• Wood 1680

• Sand 820

• Diamond 500

• Concrete 880

Liquids ( J/kg°C )• Water 4180 • Methanol 2550 • Ethanol 2480 • Antifreeze 2380 • Benzene 1720 • Human body 3470Gases ( J/kg°C )• Steam 1970• Oxygen 910• Nitrogen 1040• Dry air ~1000• Hydrogen 14300• Freon11 870

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• 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

Calculations

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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)

• 1.6 x 4180 x 65

• 434.72kJ

• QPOT = mc(t2-t1)

• 0.6x900x65

• 35.1kJ

• Answer = 434.72 + 35.1 = 469.82kJ

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ProductLatent Heat of Melting

(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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Substance Latent heat of fusion kJ.kg-1 ºC

Latent heat of vaporization

kJ.kg-1ºC

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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• 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

Heat Transfer

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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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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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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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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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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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• 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.

Thermal conductivity

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• Many electrical items work better if kept cool:

–Transformers

–Motors

–Generators

Thermal conductivity

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