Topic 19a Thermal physics (revision) 19.1 Temperature 19.1 - - PowerPoint PPT Presentation

Thermal physics (revision)

19.1 Temperature 19.1 Kinetic model of matter 20.1 Transfer of thermal energy

Topic 19a

2 0 .1 1 Tem perature

• Temperature and Heat Flow
• Temperature Scales
• Thermometers
• Chapter Review

Measuring temperature by sensation is very

• imprecise. That is why we need a temperature

scale and a thermometer to measure temperature more accurately.

tem perature and heat flow

• is measured using a thermometer

tem perature

The energy that is transferred from one region to another region due to a difference in temperature.

heat

A measure of the degree of hotness of a body. lower temperature higher temperature

hot cold heat flow

therm om eters

The choice of which thermometer to use depends on

• range of temperatures to be measured

therm om eters

• accuracy required
• physical characteristics of the substances

being examined

A thermometer must have a physical property that changes with temperature (thermometric property) Examples include:

• the volume of a column of liquid in a capillary tube
• the electrical resistance of a platinum wire
• the e.m.f. of a thermocouple
• the curvature of a bimetallic strip
• the pressure of a gas at constant volume

therm om eters

therm om eters

therm om eters

desirable features of a therm om eter

• an easy-to-read scale
• safe to use
• sensitive to temperature changes
• ability to measure a wide range of

temperatures

therm om eters

1 . linearity

The temperature reading changes linearly or proportionately with the length of the mercury column.

• temperature against length graph – a straight line

graph

2 . responsiveness

The speed with which a reading can be obtained.

• depends on the thickness of the glass bulb used

to contain the mercury

• responds faster (shorter time) through a thinner

glass wall

therm om eters

3 . sensitivity

The change in length of the liquid column per unit degree change in temperature, e.g. 2 cm/ °C

• increases if the thermometer has a larger mercury

bulb or a narrower capillary tube

• can detect a rise or fall of 0.2 °C is more sensitive than
• ne that can only measure a change of 1 °C

4 . range

The interval between the minimum and the maximum temperatures that it can measure.

• a sensitive thermometer requires a longer

capillary tube to cover the same range found on less sensitive thermometers.

To obtain a standard scale on a thermometer, two fixed points to be marked are chosen.

tem perature scales

• for the purpose of standardisation
• always the same under given conditions

These two fixed points are

• Ice point and
• Steam point.

tem perature scales

The two fixed points for the celsius scale: I ce Point Steam Point

Temperature of pure melting ice at standard atmospheric pressure Temperature at which boiling water changes into steam at standard atmospheric pressure Assigned value of 0°C Assigned value of 100°C

the Celsius scale

tem perature scales

In Celsius scale (centigrade scale), the interval between the fixed points is divided into 100 equal divisions for easy reading. Each reading is 1 degree Celsius ( °C)

ice point melting ice funnel steam point flask steam boiling water

finding the ice point finding the steam point

the Celsius scale

tem perature scales

• 1. Note the position of the

mercury in the tube at the ice point and the steam point

• 2. For any height xθ of the

mercury, the corresponding Celsius temperature θ is given by θ =

x 100°C

x100 – x0 xθ - x0

xθ - x0 x100 – x0 xθ x0 x100 θ

corresponding Celsius temperature height of column 0°C

ice point

100°C steam point

the Celsius scale

The Kelvin scale has its zero at absolute zero, which is the lowest temperature that any substance can reach.

the Kelvin or absolute scale

• has a SI unit of kelvin (K)
• absolute zero corresponds with –273°C on the

Celsius scale T (kelvin) = θ (celsius) + 273 The magnitude of a unit

• n both scales is equal

Kelvin scale Celsius scale

473 K 373 K 273 K 173 K 73 K 0 K 200 0C 100 0C 0 0C

• 100 0C
• 200 0C
• 273 0C

100 0C 100 K

m ercury therm om eters

A common type of thermometer in everyday use.

mercury ice point glass stem with thickened wall vacuum in space capillary tube safety chamber steam point bulb with thin strengthened wall

therm om eters

Properties Mercury Alcohol Uniform expansion Yes No ( irregular expansion but rather linearly over tem p. range encountered in science labs) Stick to glass No ( visible m eniscus) Yes ( transparent, m eniscus difficult to see, needs to be dyed) Reaction to tem perature changes Quick Slow Range Boiling point ( upper lim it) 3 5 7 °C 7 8 °C Freezing point ( low er lim it)

• 3 9 °C
• 1 1 5 °C

Cost Expensive Cheap Poisonous Yes No

com paring m ercury and alcohol therm om eters

measure a lower temperature range than mercury

clinical therm om eters

A type of maximum thermometer specially designed for measuring the temperature of the human body.

constriction normal body temperature curved glass stem bulb space holds mercury if thermometer is

• verheated

therm om eters

clinical therm om eters

• short temperature range from about 35°C to 42°C
• constriction in capillary tube located just above

the bulb of the thermometer

• Constriction prevents the mercury from flowing

back into the bulb when the thermometer is taken

• ut of the patient’s mouth.
• After taking the reading, it is given a good shake

to force the mercury back into the bulb.

therm om eters

• consists of two wires of different metals joined

together at the end to form two junctions

• if the junctions are at different temperatures, a

voltage is produced

• the larger the temperature difference, the larger the

voltage produced

w ire A w ire B w ire A sensitive voltm eter hot junction cold junction

therm ocouple

• the temperature range depends on the two metals

used for the wires

• can operate over a very wide range of temperatures

from –200°C to 1700°C

• Advantages: suitable for measuring
• 1. wide temperature differences,
• 2. Temperature which vary rapidly due to its quick

response and

• 3. the temperature at a point as the wire junctions are

very small

• To produce a larger voltage, several thermocouples are connected in

series (thermopiles) to increase the sensitivity of the instrument

therm ocouple

Temperature Thermometer Physical properties that change with temperature measured by Fixed points: ice point steam point requires Volume of fixed mass of liquid Thermocouple Scales: Celsius scale Kelvin scale like Mercury-in-glass thermometer

Structure Sensitivity Range Linearity Responsiveness

has the advantages of e.m.f. (voltage) used in used in Resistance thermometer resistance used in

2 0 .1 2 Kinetic Model of Matter

• States of Matter
• Brownian Motion
• Molecular Model of the Three States of Matter
• Effects of Temperature
• Pressure Exerted by a Gas

Matter is made up of tiny particles called atoms and

• molecules. These particles are much too small to be

seen by naked human eyes. How do we prove their existence?

The kinetic theory of m atter states that all matter is made up of a large number of tiny atoms or molecules which are in continuous motion. kinetic m olecular m odel of m atter

brow nian m otion

dotted lines represent the path of smoke particles between collision Using a microscope, smoke particles can be seen moving continuously and haphazardly, as a result of being hit by unseen fast-moving air molecules. Brownian motion provides evidence for the kinetic molecular model of matter (kinetic theory of matter).

Dem onstrations

• Gas molecules colliding with a smoke particle

can cause the particle to trace out a zig-zag path.

• Suspension of fat droplets (milk) in water
• Pollen grains sprinkled on water

brow nian m otion

brow nian m otion

If heat is supplied, the motion of the smoke particles becomes more vigorous. The smaller the smoke particles, the more rapid is their motion. Diffusion is a result of Brownian motion.

smoke particles moving continuously and haphazardly (irregular or random motion)

diffusion

Diffusion is the spreading of molecules of their own accord without any external aid. Gas molecules move randomly all the time and they move into any available space. Hence they stay mixed and do not separate out.

brow nian m otion

diffusion

Rate of diffusion depends on

• temperature of gases:

higher temperature leads to faster diffusion

• density of gases: greater

density leads to slower diffusion

diffusion in air

brow nian m otion

If bromine vapour is released into a similar space full of air

• bromine molecules keep hitting air molecules which get

in the way

• bromine vapour spreads quickly throughout the space

but much slower than in vacuum

bromine vapour contained bromine vapour diffused vacuum

diffusion in liquid

Diffusion also takes place in liquids, at a very much slower rate.

• copper(II) sulphate solution and water become

uniformly mixed after a while due to diffusion

lid water copper (II) sulphate solution copper (II) sulphate solution

diffusion of copper (II) sulphate solution in water

single uniform layer

brow nian m otion

the m olecular m odels of the three states of m atter

molecular structure of a solid molecular structure

• f a liquid

molecular structure of a gas

Solid Liquid Gas Forces betw een m olecules

• balanced

forces which hold molecules in fixed positions

• forces as strong

as those in solid

• molecules not

held in fixed position, move among one another throughout liquid

• negligible
• only at moments
• f collision, the

intermolecular forces act Distances betw een m olecules

• arranged close

together in a regular pattern

• not arranged in a

regular pattern

• slightly further

apart than in solid

• far apart
• mainly empty

space between molecules

Com parison of properties due to the m olecular structure of solids, liquids and gases

Solid Liquid Gas Motion of m olecules

• vibrate about

fixed positions

• alternately

attracting and repelling one another

• free to move

about

• alternately

attracting and repelling one another

• move randomly

with high speed, colliding with one another and with the walls of the containers Com press- ion

• cannot be

compressed

• molecules are

arranged close together

• little space

between them

• cannot be

compressed

• molecules are

still close together

• little space

between them

• can be easily

compressed

• far apart
• mainly empty

space between molecules

Solid Liquid Gas W hen heated

• molecules gain

energy and vibrate more

• separation

between molecules increases slightly (solid expands)

• molecules

vibrate and move about more vigorously

• separation

between molecules increases slightly (liquid expands)

• move even more

randomly with higher speed, colliding with one another and with the walls of the containers (gas expands a lot)

Solid Liquid Gas Fixed shape No fixed shape

• can flow
• take the shape of

container No fixed shape

• can flow
• spread easily to fill

any vessel

• take the shape of

vessel Fixed volume Fixed volume No fixed volume

• take the volume of

vessel Not compressible Not compressible Highly compressible Hard and rigid

• large force required

to change its shape Definite surface No surface

states of m atter

Boyle’s law

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.

pressure-volum e relationship

• f a gas

effects of tem perature

As temperature increases,

relationship betw een the m otion of m olecules and tem perature

• surrounding air particles move faster and hit the

particles move frequently and harder

• Brownian motion of the smoke particles in the air

becomes more vigorous

• thermal energy is transferred to the molecules and

the molecules gain kinetic energy causing molecules to move faster

pressure exerted by a gas

Pressure is defined as force per unit area, the force acting on the container within is the gas pressure. Pressure exerted by gas molecules increases due to:

relationship betw een the m otion of m olecules and pressure

• a decrease in volume of a container, or (and)
• an increase in temperature

pressure exerted by a gas

pressure of a gas in term s of m otion of its m olecules – effect of volum e

• 1. Number of gas molecules per cm 3 doubles

half the volume of container

• 2. Number of collisions of molecules with the

wall in one second doubles

• 3. Pressure double

pressure exerted by a gas

pressure of a gas in term s of m otion of its m olecules - effect of tem perature

• 1. Molecules move faster

temperature of gas in container increases

• 2. Molecules hit the walls

more frequently and more violently (with greater force)

• 3. Pressure increases

consists of

Matter

Solid Gas

(a) Properties of solids, liquids and gases (b) Relationship between the motion of molecules and temperature (c) Pressure of a gas

Liquid

used to explain

(a) Brownian motion (b) diffusion Kinetic theory of matter

is based on evidence like

are discussed in

2 0 .1 3 Transfer of therm al energy

• Conduction
• Convection
• Radiation

A bird can reduce the heat loss from its body during cold weather by fluffing up its feathers. What are the processes of heat transfer?

transfer of therm al energy

Heat travels from a place of higher temperature to a place of lower temperature. Three processes by which heat may be transmitted

• conduction
• convection
• radiation

simple demonstration of the three processes: conduction, convection and radiation

conduction

Conduction is the process by which heat is transmitted through a medium from one particle to another. Heat is transmitted from the hot to the cold end.

how conduction w orks

heat travels from the hot end

• f the rod to the cooler end

high temperature low temperature

direction of heat transfer along the rod

conduction

conduction in solids

• atoms are strongly held in fixed positions; they

vibrate about their fixed positions

• atoms are quite close together

solid particles

conduction

conduction in solids

• by the vibration of particles like atoms and molecules
• by the movement of free electrons through the metal

Metals are different from other kinds of solids because they also have electrons that move about

• freely. Most of them are good thermal conductors

because heat can be conducted in two ways:

conduction in good thermal thermometers

conduction

conduction in solids

Poor thermal conductors have electrons that are held firmly to the atoms. Heat is transmitted only through the vibration of atoms and molecules.

heat is transmitted very slowly from particle to particle

plastics

conduction

conduction in liquids and gases

Liquids are poor thermal conductors. Generally, substances (except for mercury) which are liquids at room temperatures are poor conductors.

• particles are further

apart than those in solids

• particles have freedom

to move about a little more than solid particles; little energy transfer

• conduction is limited

liquid particles

conduction

conduction in liquids and gases

Gases are also poor thermal conductors. Thermal conductivity of gases is even lower than that

• f liquids.
• particles have much less

contact with each other compared to liquids and solids

• particles that absorb heat

energy move about quite independently and freely; very little energy transfer

• conduction is very limited

gas particles

uses of good conductors

conduction

Good conductors are used in situations where heat has to be transmitted quickly.

soldering solder pan kettle

conduction

uses of poor conductors ( insulators)

Poor conductors (insulators) are used in situations where unwanted heat has to be kept away or to prevent heat loss.

insulated cavity wall double glazing ceiling tiles contain air bubbles wallpaper contains air bricks contain air

convection

• occurs in fluids (liquids and

gases), but not in solids

• density changes cause

convection currents; hot fluids rise and cold fluids sink A process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.

cool (higher density) hot (lower density) liquid or gas transfer of heat by convection

convection in liquids

convection

The circulation of a liquid in this manner is called a convection current.

small flame water potassium permanganate crystals purple streak flask

hot w ater system

convection

boiler hot water storage tank

A hot water system makes use of the principle of convection.

expansion pipe ball valve cold tank cold water drawn down hot water rising

cooling system in a car – the radiator

convection

When a car engine is running for a long time, a lot of heat energy is produced. It is necessary to cool the engine so that it does not overheat.

hot water air flowing into radiator copper tubes with cooling fins cool water pump engine cool water absorbs heat from engine hot water rises and enters cooling fins

convection

Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises.

convection in gases

smouldering paper smoke glass window glass cylinders box lighted candle

convection currents in air

convection

convection in gases

bottle is deformed due to the effects of convection

hot water hot water

slightly rigid plastic bottle convection current

convection

Air-conditioners are best positioned high, near the ceiling of a room.

air-conditioners

cold air warm air

air- conditioner

• cold air which is denser sinks
• warm air which is less dense

rises

• cycle repeats until room air

temperature is the same as the temperature set on the thermostat of the air- conditioner

sea breeze

convection

sea breeze

• land heats up faster than the sea
• air above land heats up and rises
• cool air above sea rushes in

cooler warmer

land breeze

convection

land breeze

• sea cools slower than the land
• warm air above sea rises
• cool air above land moves to the sea

warmer cooler

• does not require any medium
• can take place in a vacuum

radiation

Radiation is a method of heat transfer by which a heat source transmits electromagnetic waves. (in the form of infra-red radiation).

em ission of radiation

radiation

• the hotter an object is, the more energy it radiates
• dull black surfaces are good emitters or radiators than

shiny ones

• the greater the surface area and temperature of the
• bject, the faster is the rate of heat transfer from it

boiling water

dull black shiny

Which side feels hotter?

uses of good and poor em itters

radiation

Good emitters are used in situations where heat has to be quickly emitted.

Good Em itters Poor Em itters Cooling fins at the back of a refrigerator painted dull black A shiny metal teapot refrigerator cooling fins at the back of a refrigerator painted dull black

absorption of radiation

radiation

dull black polished cork held by wax cork falls off wax melts first

• dull black surfaces are good absorbers of radiation

than shiny ones

• in general, good emitters or also good absorbers

uses of good and poor absorbers

radiation

Good Absorbers Poor Absorbers Solar heating panels are painted in dull black paint Houses in hot countries and factory roofs are painted in white, light-coloured paint, aluminium paint Light colours are chosen for clothes and cars in hot weather

Good absorbers are used in situations where heat has to be quickly absorbed.

solar panels

• keeps hot liquids hot and keeps cold liquids cold

vacuum flask

application

plastic cap vacuum hot liquid glass silvered surfaces foam plastic support

• uter case

Parts of vacuum flask

Conduction Convection Radiation

Vacuum (between walls)

Stops conduction Stops convection

Silvered surface

• f walls

(on side of vacuum)

Reduces or minimizes radiation

Plastic cap

Reduces or Minimizes conduction Stops convection

Foam plastic support

Reduces or Minimizes Conduction

by Heat transfer Conduction Radiation A medium Convection requires A medium requires Particles in a solid, liquid or gas Electromagnetic waves by

energy transmitted by emission of

by by

energy transmitted by movement energy transmitted through vibration of

Of a heated gas

• r a liquid

particle

does not require a medium