Thermodynamics - Heat and Temperature

8/4/2019 Thermodynamics - Heat and Temperature

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Thermodynamics

HEAT AND TEMPERATURE


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1.0 - Introduction

Heatis a form of energy.

It is measured like other formsof energy in J (Joules).

The above statement should

have led you to realise that heatand temperature are twodifferent things as temperatureis measured in K (Kelvin) ormore commonly in C (DegreeCelsius or Centigrade) or F(Degree Fahrenheit).

An object may contain variousdifferent types of energy.


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1.1 - Introduction Cont.

Take for example a studentthrowing a jar containing bees.The object has the potentialenergy caused by thegravitational field of the earth

and kinetic energy as it ismoving. However, the individualbees also have their own kineticenergy - we will call this therandom kinetic energyof the jar.

If we said that the jar is now a

metal ball and the bees in thejar are the atoms of the metalball, the average random kineticenergycaused by the vibrationof the atoms is the temperatureof the metal ball.


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1.2 - Introduction Cont.

In a substance, Kinetic Energy ispresent due to the masses andvelocities of its particles beingvibrated, rotated or translated, andPotential Energy is present due tothe attractive forces between each

of the particles as bonds ANDbetween separate particles.

The sum of the kinetic and potentialenergies of all theparticles is calledthe internalor thermal energyofthe substance.

The term heatis used to describethe internal energy of a substance.

The study of the transfers of thisenergy is called thermodynamics.


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2.0 - Thermal Equilibrium

Thermal Equilibrium is the state at whichtwo objects in an isolated environment gainthe same temperature after the process ofheat transfer from the body containingmore heat (TB) to the other (TA).

Note that in an isolated environment, there

is no heat lost to the surroundingstherefore, the heat lost by the hotter object(TB) is equal to the heat gained by the lesshotter object (TA).

Note that this is theoretical and in practise,heat is lost through radiation (even if theexperiment is conducted in space) all

objects that have temperatures aboveabsolute zero, radiate energy in the form ofelectro-magnetic radiation. And whenconducted on earth heat is also lostthrough conduction and convection (referto Slide 5.0 Calorimetry).


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3.0 - Measuring Temperature

There are many differentscales used to measuretemperature. Below are thethree scales that are mainlyused in the present day;

The Fahrenheit Scale

Developed by Germanphysicist, Gabriel Fahrenheit(1686 1736)

In this scale, the freezingpoint of a salt solution is 0F,

the freezing point of purewater is 32F, and the boilingpoint of pure water is 212F.

This scale is mainly used inthe US, UK and Canada.


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3.1 - Measuring Temperature Cont.

The Celsius Scale

Developed by Andres Celsius(1701 1744)

In this scale, the freezingpoint of pure water is 0C and

the boiling point of purewater is 100C.

The Kelvin/Absolute Scale

Developed by Lord Kelvin(1824 1907)

In this scale, 0 K is the

absolute zero temperature

this means that at thistemperature, there isabsolutely no particle motion.

Note; (0 K = -273.15C).


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4.0 - Specific Heat Capacity

In a room, that has a constant temperature(say 23C), all the objects have the sametemperature (23C thermal equilibrium).However, if we humans touched a metallicobject in the room it would feel much morecold than a non-metallic object in the same

room. This is due to the fact that metals aregood conductors of heat. The heat from ourbodies is conducted faster to the metalsthan to the non-metals. And because ourbody senses the rate at which heat istransferred to or away from our body, themetals feel more cold (remember that themetals still have the same temperature).

Good conductors of heat refers to

substances with a low heat capacity i.e.they require relatively less amounts ofenergy to raise its temperature (refer tonext slide for a more precise and detaileddescription of specific heat capacity).


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4.1 - Specific Heat Capacity Cont.

Specific Heat Capacityis the measure of

how much energy is required to raise the

temperature of 1 kg of a substance by 1

K or 1C (note a change of 1 K is exactly

the same as a change of 1C).

Different substances have differentspecific heat capacities. On the left side

is a table containing the specific heat

capacities of some commonly seen

substances

On the bottom left is the formula for

specific heat capacity. In this formula, Qis the heat energy required (J), m is the

mass (kg), c is the specific heat capacity,

Tis the change in temperature

(measured in either C or K refer to the

first dot point in this slide).


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5.0 - Calorimetry

When two substances areplaced together in a closedsystem, thermal equilibriumoccurs.

In practice, there is always someheat lost to the surroundings.There are two main ways inwhich such heat loss could beminimised;

Carrying out the experiment

quickly. Use calorimeters, which have

good insulation to limit the lossof heat to the surroundings. Thisprocess is called Calorimetry.

Return to Slide 2.0 Thermal Equilibrium


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6.0 - Change of State

The amount of energy required to

melt 1 kg of an object is called the

specific latent heat of fusion.

The amount of energy required to

vaporise 1 kg of an object is called the

specific latent heat of vaporisation.

On the left is the formula for the

energy required to change the state of

a substance. In this formula; Q is the

heat energy required (J), m is the mass

(kg), and L (specific latent heat) of theobject becomes;

Lffor the specific latent heat of fusion (or)

Lvfor the specific latent heat of

vaporisation.


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7.0 - Changing the melting and boilingPoints

Most substances have fixedmelting and boiling points aslong as they are in pure form.

To change the melting and

boiling points of varioussubstances, there are twomain methods which couldbe used;

Adding Impurities to thesubstance (and/or)

Changing the pressure of thesubstance and/or itsenvironment.


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8.0 - Evaporation

Liquids turn into gas without boiling.This process is called evaporation andoccurs all the time.

For a substance to change state,energy is required. But note that notall the individual particles of a

substance have exactly the sameenergy (also note temperature is themeasure of the average randomkinetic energyof the particles of asubstance).

This is the reason for evaporationindividual particles with relatively

higher energy are able to reach thesurface of the substance and escape(e.g. when you leave a bowl of waterat room temperature, it willeventually evaporate to nothing).


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9.0 - Laws of Thermodynamics

There are two laws ofthermodynamics;

The first law ofthermodynamics states thatthe total increase in thethermal energy of an isolatedsystem is equal to the sum ofthe heat added to it and thework done on it. Note thatthis is just an extension ofthe law of conservation of

energy. The second law of

thermodynamics relates heattransfer to differences intemperature.


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9.1 - Laws of Thermodynamics Cont.

The laws of thermodynamics alsohelped develop a new term inphysics, called entropy.

Entropyis the measure of thedisorder of a system the more

disorder, the more entropy. It statesthat in nature, all ordered systemshead towards becoming disordered.

An example of this (fromthermodynamics) would be when twoobjects, say water and ice, are placedin contact with each other and allowed

to reach thermal equilibrium. Afterequilibrium is reached, the orderedmolecules of the ice become lessordered; and therefore it now has alower entropy.


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FINALLY! WERE DONE!


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This presentation is only designed to help you learn

easiernot thorough. So, refer to you textbook fordetailed information on this chapter! And practicethe questions in your textbook if any!

REMEMBER!


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Thermodynamics - Heat and Temperature

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