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4.1 Understanding Thermal Equilibrium

Thermometric Properties

Heat Temperature

Thermal equilibrium

Application

Oven RefrigeratorThermometer

CalibrationsTypes of thermometers

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Relationship between Temperature and Heat

1. Temperature is a measure of the degree of hotness of a body.2. A hot body has a high temperature whereas a cold body has a low

temperature.

3. Heat is a form of energy being transferred from a hot body to a coldbody.

4. Heat and temperature are two different physical quantities.Temperature Heat

The degree of hotness of a body A form of energy

Base quantity Derived quantity

Unit: Kelvin (K) or degree Celsius

(C)

Unit: joule (J)

Can be measured using a

thermometer

No specific measuring equipment

Table 4.1 The difference between heat and temperature

Mechanism Thermal Equilibrium

Energy is transferred at a faster

rate from the hotter object to the

colder object. Energy is also

transferred from the colder object

to the hotter one, but at a slower

rate. There is a net flow of energy

from the hotter object to the

colder object.

The hotter object cools down while

the colder object warms up. After

some time, energy is transferred at

the same rate between the two

objects. There is no net heat

transfer between the objects. The

two objects are said to be in

thermal equilibrium.

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Microwave oven RefrigeratorAn oven can maintain Fruits are normally kept

its temperature by the in refrigerator to

use of thermostat maintain their freshness

Thermometer Cooling drinksA doctor use a thermometer A hot drink can be

to measure the body cooled by adding a few

temperature of the patient. ice cubes to the drink

Thermal Equilibrium

When two objects are in thermal equilibrium,there is no net flow of heat between them.

Two objects in thermal equilibrium have thesame temperature.

Applications of Thermal

Equilibrium

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Liquid-in-glass Thermometer

The characteristic of

the liquid used in

liquid-in-glassthermometer

Be easily seen Expand and contract rapidly over a wide range of

temperature No stick to the glass walls of the capillary tube

How a liquid-in-glass

thermometer works

?

The bulb of the thermometer contains a fixed massof mercury. The volume of the mercury increases

when it absorbs heat.

The mercury expands and rises in the capillary tube.The length of the mercury column in the capillary

tube indicates the magnitude of the temperature.

How can a

thermometer be

calibrated ?

A temperature scale is obtained by choosing twotemperatures, called the fixed point.

Definition of ice point and steam point.Fixed point Definition Value

Lower point:

Ice point

The temperature of

pure melting ice

0C

Upper point:

steam point

The temperature of

steam from water

that is boiling understandard

atmospheric

pressure

100C

When the two fixed points have been marked on the

stem of the thermometer, the range between them is

divided equally into 100 divisions or degrees. The

thermometer now has a scale.

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Explain the working

principle of a

thermometer

When the thermometer is placed in contact with hotwater, heat is transferred from hot water to the

thermometer.

Thermal equilibrium between the thermometer and

hot water is reached when the net rate of heat transfer

is zero.

The thermometer and the water are at the same

temperature. At this point, the thermometer reading

shows the thermometer of the water.

What are

characteristics of

mercury that makes

it suitable as a

liquid-in-glass

thermometer ?

It is a good conductor of heat It has a high boiling point, 357c It expands uniformly when heated It is opaque (does not allow light to pass through)

and it can be seen easily.

Mercury freezes at a temperature of -39C and it is

therefore not suitable for measuring temperatures

below this temperature, such at the north pole.How to increase the

sensitivity of a

mercury

thermometer ?

Thin capillary tube A glass bulb with thinner wall Large bulb

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4.2 Understanding Specific Heat Capacity

Q = mc

Heat Capacity

Idea Increase in temperature

of material

Specific Heat Capacity

Determine the

specific heat capacity

of aluminiumApplication

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Heat Capacity

The amount of heat required to change

its temperature by one degree

Concept of Heat Capacity

The total amount of heat in a body is affected

by:

The temperature of the body (heatsupplied to the body)

The mass of the bodyThe type of material

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Example:

What does specific heat of aluminium

900 J kg C mean ?

900 j of heat needs to be supplied to 1

kg of aluminium to produce a 1 C

temperature increase.

What does specific heat of water 4

200 J kg C mean ?

4 200 J of heat needs to be supplied to

1 kg of water to produce a 1 C

temperature increase.

Specific Heat Capacity

The amount of heat that must be supplied to

increase the temperature by 1 C for a mass of 1 kg

of the substance

Specific heat capacity, c = Q SI unit: J kg C

Q = heat absorbed / released, unit J

m= mass of the substance, unit kg

= change in temperature, unit C

Q = mc

Quantity of heat

absorbed or lost by a

substance

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Physical meaning of specific heat capacity, c

When two objects of equal mass are heated at equal rates, the object withthe smaller specific heat capacity will have a faster temperature.

When two objects of equal mass are left to cool down, the temperature ofthe object with smaller heat capacity will drop faster.

A substance with a small value of

specific heat capacity

Heats up and cools at a faster rateFor example, metal like iron, steel,

copper and aluminium is used as

pots and pans because they can be

quickly heated up when there isonly small heat absorption.

Sensitive to temperature changesA thermometer has low specific

heat capacities so it enables heat

to be easily absorbed and released

even when small quantities of heat

are involved.

A substance with a high value of

specific heat capacity

Heats up and cools at slower rate.Require more heat to raise its

temperature by a specific amount.

Poor conductor of heat handle ofpot

Can absorb a great amount of heatwithout a high increase in

temperature

For example, water acts a heat

reservoir as it can absorb a greatamount of heat before it boils. Water

is used as cooling agent in a car

radiator.

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Cooking Pot

Copper Base Low specific heat capacity. The pot becomes hot very quickly. This

enables quick cooking of the food in the pot.

High density. The heavier base ensures that the pot is stable andwill not topple over easily.

Wooden Handle Large specific heat capacity. The handle will not become too hot

when heat is absorbed.

Poor conductor of heat. Alumni body

Relatively low specific heat capacity. The pot becomes hot quickly. Low density so it will be lighter Does not react with the food in the pot.

Applications of Specific Heat Capacity

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Sea Breeze

Land has a smaller specific heat capacity than sea. Faster increase intemperature, ie; land is warmer than the sea

Air above the land is heated up and rises. Cooler air from the sea moves from towards the land as sea breeze.

Land Breeze

At night, heat is lost from the land and sea. Sea has a larger specific heat capacity so sea is warmer than land. Warmer air above the sea rises.

Cooler air from the land moves towards the sea as land breeze.

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The cooling system of a car engine

Water has a high specific heat capacity and lower cost. So water can be auseful a cooling agent.

A water pump circulates the water. Heat produced by the engine is absorbed by the water that flows along the

space in engine walls.

The hot water flows to the radiator where heat is lost to the cooler air thatflows through the cooling fins.

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4.3 Understanding Specific Latent Heat

Q = mL Q = mL

melting boiling

freezing condensation

Specific Latent Heat

Specific latent heat

of fusionSpecific latent heat

of vaporisation

Application

Li uid GasSolid

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Latent Heat

When a solid melts, latent heat of fusion isabsorbed but the temperature remains constant

at its melting point.

For a liquid to solidify at its freezing point, latentheat of fusion has to be removed.

When a liquid is boiling, latent heat ofvaporization is absorbed but the temperature

remains constant at its boiling point.

When vapour condenses back into the liquidphase, latent heat of vaporization is released.

The heat absorbed or the heat released at constant

temperature during change of phase.

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Change of State of Matter Explained by Using the Kinetic Theory

AB : The heat absorbed by the solid AB : The heat absorbed by the

raises its temperature to its melting liquid raises its temperature to

point. its boiling point.

BC : Heat is absorbed by the solid as BC : Heat is absorbed by the

it melts, but the temperature remains liquid as it boils, but the

constant. The solid melts to become a temperature remain constant.

liquid at the same time. The liquid boils to become a

gas at the same temperature.

PQ : Heat is given out by the liquid as PQ : Heat is given out by the

it cools to its freezing point. gas as it cools to the boiling point

QR : Heat is given out by the liquid as QR : Heat is given out by the

it solidifies but the temperature remains gas as it condenses but the

constant. The liquid solidifies to become temperature remain constant.

a solid at the same temperature. The gas condenses to becomea liquid at the same temperature.

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Common characteristics of the four processes in the change of phase:

A substance undergoes a charge of phase at a particular temperature. Heat energy is transferred during change of phase. During change of phase, the temperature remains constant even though

there is transfer of heat.

Notes: The temperature of a substance is proportional to the average kinetic

energy of its particles.

Temperature increases when the average kinetic energy og the particlesincrease.

Temperature decreases when the average kinetic energy of the particlesdecreases.

Temperature remains constant when the average kinetic energy doesnot change.

Why does the temperature remains

constant during change of phase?

During change of phase, thetransfer of heat does not cause a

change in the kinetic energy of

the molecules.

During melting, the heatabsorbed is used to break up the

bonds between the particles. The

particles are feed from their fixed

positions and are able to vibrate

and move among each other.

When a liquids boils, the heatabsorbed is used to completelybreak the bonds between the

particles and also to do work

against atmospheric pressure

when the gaseous vapour

expands into the atmosphere.

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Latent Heat

Latent Heat of Fusion

The heat absorbed by amelting solid is known as

the latent heat of fusion.

During melting, the heatabsorbed is used to break

up the bonds between the

particles.

The particles are freedfrom their fixed positions

and are able to vibrate and

move among each other as

illustrated in Figure 4.10.

Latent Heat of Vaporisation

The heat absorbed duringboiling is known as the

latent heat of vaporisation.

When a liquid boils, theheat absorbed is used to

completely break the bonds

between the particles and

also to do work against

atmospheric pressure when

the gaseous vapourexpands into the

atmosphere, as illustrated

in Figure 4.11.

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Specific Latent Heat, l

The amount of heat required to change the phase of a substancedepends on the mass and the type of material of the substance.

The specific latent heat of a substance, lis the amount of heat requiredto change the phase of 1 kg of the substance at a constant temperature.

The specific latent heat, unit: J kg

Where Q = latent heat absorbed or released by the substance

M =mass of the substance

If 3 kg of ice at 0 C requires 1 008 000 J of latent heat for it to melt to become

water at 0 C, 1 kg of ice requires 1 008 000 336 000 J of latent heat.

3

This means that the specific latent heat of fusion of ice is 336 000 J kg .

The latent heat absorbed or released when a substance of mass, m changes

from one phase to another is given by

Specific latent heat of fusion The amount of heat required to

change 1 kg of the substance from

solid to liquid phase without a changein temperature

Specific latent heat of vaporisation The amount of heat required to

change 1 kg of the substance from the

liquid to gaseous phase without a

change in temperature

Q = ml

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4.4 Understanding the Gas Laws

The Gas Laws

Boyles law Pressure Law Charles Law

Absolute zero

General Gas Equation

Application

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Properties of Gases

Property of gas Explanation based on the kinetic theory

Volume The molecules move freely in random motion and fill upthe whole space in the container.

The volume of the gas is equal to the volume of thecontainer.

Temperature The molecules are in continuous random motion and havean average kinetic energy which is proportional to the

temperature.

Pressure The molecules are in continuous random motion. When a molecule collides with the wall of the container

and bounces back, there is a change in momentum and a

force is exerted on the wall.

The force per unit area is the pressure of the gas.

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Relationship between Pressure and Volume

When the volume of a gas is decreased, the number of molecules per unitvolume increases, that is, the same number of molecules will move in a

smaller space. The molecules collide more frequently with the walls of the container. This

increase in the rate of collision results in an increase in the pressure exerted

by the gas.

Boyles Law

Boyles law states that for a fixed mass of gas, the pressure of the gas isinversely proportional to its volume when the temperature is kept

constant.

The mathematical expression for Boyles Law is

Boyles Law can be shown graphically as in Figure 4.13.

that is PV = constant

or P V = P V

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Relationship between Volume and Temperature

When the temperature of a gas raised, the molecules move faster astheir kinetic energy increases.

The molecules push the piston so that the gas expands and the pressureremains constant.

The volume of a gas decrease when its temperature is lowered. Thetemperature at which the volume of the gas is expected to become zero

can be obtained by extrapolating the graph of volume against

temperature.

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The temperature -273 C is the lowest possible temperature and isknown as the absolute zero of temperature.

The corresponding temperatures in the Celsius and Kelvin scales are asfollows:

Temperature Celsius scale Kelvin scale

Absolute zero -273 C 0 K

Ice point 0 C 273 K

Steam point 100 C 373 K

Unknown temperature C ( + 273) K

Charles Law

Charles law states for a fixed mass of gas, the volume of the gas is directlyproportional to its absolute temperature when its pressure is kept constant.

The mathematical expression for Charles Law is

V T

That is = constant

Or =

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Relationship between Pressure and Temperature

When the temperature of a gas is raised, the kinetic energy of themolecules increases.

The molecules move faster and collide more frequently with the walls ofthe container. Thus, the pressure of the gas increases.

Pressure Law

Pressure law states that for a fixed mass of gas, the pressure of the gas isdirectly proportional to its absolute temperature when its volume is kept

constant. The mathematical expression for the pressure law is

P T

That is = constant

Or =