Unit 61: Engineering Thermodynamics

Lesson 6: Heat

Objective

• The purpose of this lesson is to consider the concepts of work and heat.

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Heat

• Energy can be transferred macroscopically to and from a system

• Energy can also be transferred microscopically to and from a system by means of interactions between molecules that form the surface of the system and those that form the surface of its surroundings.

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Heat

• If the molecules that form the system boundary are more active that those of the boundary of the surroundings they will transfer energy from the system to the surroundings – i.e. the faster molecules transferring energy to the slower molecules. The problem is that this microscopic behaviour is not observable

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Heat

• Temperature is a macroscopic quantity that increases with increasing molecular activity.

• The macroscopic transfer of energy that results in an increase in temperature we refer to as heat.

• Thus Heat is the energy transferred across a boundary of a system due to the difference in temperature between the system and its surroundings.

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Heat

• Note: A system does not contain heat – it only contains energy – heat is the energy in transit! We often refer to this as heat transfer.

• Consider a hot block and a cold block of equal mass. The hot block contains more energy than the cold block due to its greater molecular activity. When the blocks are brought together into contact with each other energy flow from the hot block to the cold one by means of heat transfer.

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Heat

• Eventually the blocks will attain thermal equilibrium with the blocks arriving at the same temperature. Heat transfer ceases the hot block has lost energy – the cold block as gained energy.

• Heat, like work is something that crosses a boundary

• Because a system does not contain heat, heat is not a property – its differential is inexact

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Heat

• For a particular process between states 1 and 2 the heat transfer could be written as Q1-2 but it is generally written as Q. The rate of heat transfer being Q.

• By convention if the heat transfer is to a system it is considered as positive; from a system its is considered negative. Note this is the opposite convention for work!

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Heat

• Thus positive heat transfer increases the energy of a system whereas positive work transfer decreases the energy of a system.

• A process in which there is zero heat transfer is called an adiabatic process. (such a system is approximated experimentally by insulating the system so that negligible heat is transferred.

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Heat

• Note: energy contained in a system may be transferred to the surroundings with by work done or heat transfer.

• Thus an equivalent reduction of energy of a system is achieved if either 100 J say of heat transfer is done of 100 J of work done.

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Heat

• Note: energy contained in a system may be transferred to the surroundings with by work done or heat transfer.

• Thus an equivalent reduction of energy of a system is achieved if either 100 J say of heat transfer is done of 100 J of work done.

• There are three modes of heat transfer: conduction, convection and radiation. In engineering it is often the case that all three modes need to be considered.

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Heat

• Note it is sometimes convenient to refer to heat transfer per unit mass.

q = Q/m

• There are three modes of heat transfer: conduction, convection and radiation. Often engineering designs need to consider all three

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Heat

• Conduction heat transfer in a material exists due to the presences of temperature differences within the material.

• It can exist in all substances but is most often associated with solids.

• It can be expressed using Fourier’s LawQ = kAΔt/Δx

Where k is the thermal conductivity.

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Heat

• Convection heat exists when energy is transferred from a solid to a fluid in motion.

• It is a combination of energy transferred by both conduction and advection (energy transfer due to bulk motion of a fluid).

• It can be expresses using Newton’s Law of cooling…Q = hcA(Ts - T∞)

Where hc is the heat transfer coefficient

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Heat

• Radiation is energy transferred as photons.• It can be transferred through a perfect

vacuum or transparent substances such as air.• It can be expressed using the Stefan-

Boltzmann Law…Q = εσA(T4 – T4

surr)

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