IGCSE PHYSICS Section 2 Thermal Physics

Thermal PhysicsIGCSE Science

Revision Book - Section 2

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IGCSE PHYSICS Section 2 Thermal Physics

Syllabus Content_______________________________

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IGCSE PHYSICS Section 2 Thermal Physics

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IGCSE PHYSICS Section 2 Thermal Physics

Syllabus Details________________________________

2.1 Simple kinetic molecular model of matter2.1 (a) States of matterCore• State the distinguishing properties of solids, liquids and gases2.1 (b) Molecular modelCore• Describe qualitatively the molecular structure of solids, liquids and gases

Solid Liquid Gas

• Fixed volume• Fixed shape• Molecules held in position by strong bonds• Molecules vibrate about fixed position• Higher temp = higher vibrations

• Fixed volume• Shape of container• Molecules can vibrate and move but are held close together by strong bonds

• Expands to fill container• Molecules can vibrate and move around freely• Only very weak bonds exist between molecule

Increasing Kinetic Energy

• Interpret the temperature of a gas in terms of the motion of its molecules

INCREASING

TEMPERATURE

As temperature increases the particles move faster

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IGCSE PHYSICS Section 2 Thermal Physics

• Describe qualitatively the pressure of a gas in terms of the motion of its molecules

Force on wallForce on molecule

Pressure in gases

• Molecule collides with wall• Momentum changes• Force on molecule from wall• Equal and opposite force on wall from molecule• This averages over time to a constant force on the wall• The force per unit area of the wall is the pressure

• Describe qualitatively the effect of a change of temperature on the pressure of a gas at constant volume

Constant volume

Temperature / K

Pres

sure

/ Pa P/T = constant

Increasing

Temp

AS THE TEMPERATURE INCREASES • The molecules have more kinetic Energy• Faster moving molecules will hit the walls with more force• Faster moving molecules will hit the walls more often• The total force on the walls will increase• The pressure will increaseAt a constant volume the pressure

of a gas is proportional to its temperature in Kelvin.

P1 /T1 = P2 /T2

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IGCSE PHYSICS Section 2 Thermal Physics

• Show an understanding of the random motion of particles in a suspension as evidence for the kinetic molecular model of matter• Describe this motion (sometimes known as Brownian motion) in terms of random molecularBombardment

Brownian MotionSmoke (oil droplets)

Path of one droplet Smoke (oil droplets) are seen to move randomly This motion is evidence that the air particles are also moving randomly and

colliding with the smoke droplets The air particles cannot be seen but their motion can be understood by the

smoke droplets which can be seen

Supplement• Relate the properties of solids, liquids and gases to the forces and distances between molecules and to the motion of the molecules

Property Solid Liquid GasForces between particles

Strong bonds (strong forces)

Strong bonds (strong forces)

Essentially no bonding (v-weak forces)

Distances between particles

Fixed and short Short but not fixed

Long

Motion of particles

Vibrating only Vibrating and freely moving

Vibrating and freely moving

• Show an appreciation that massive particles may be moved by light, fast-moving molecules

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IGCSE PHYSICS Section 2 Thermal Physics

• Small fast moving particles collide with larger particles• The smaller particles momentum changes and exert a force on the larger particles• The larger particles motion changes as a result of these collisions

2.1 (c) EvaporationCore• Describe evaporation in terms of the escape of more-energetic molecules from the surface of a liquid• Relate evaporation to the consequent cooling

Energy

Num

ber

Enough energy to Evaporate

A. At all temperatures there will be a distribution of kinetic energy within the liquid.

B. Molecules with high kinetic energy can ‘escape’ the liquid and become a gas: Evaporation.

C. The average speed of the molecules in the liquid will decrease: Therefore, the temperature of the liquid will decrease.

Supplement• Demonstrate an understanding of how temperature, surface area and draught over a surface influence evaporation

Factor InfluenceTemperature Increases evaporation as more particles

have sufficient kinetic energy to “escape” the surface

Draught Increases evaporation as “removes” high KE particles from above the surface of the liquid

Surface area Increases evaporation as more surface

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IGCSE PHYSICS Section 2 Thermal Physics

for particles to “escape” from

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IGCSE PHYSICS Section 2 Thermal Physics

2.1 (d) Pressure changesCore• Relate the change in volume of a gas to change in pressure applied to the gas at constant temperatureSupplement• Recall and use the equation pV = constant at constant temperature

Constant Temperature

1/Volume / m-3

Pres

sure

/ Pa PV = constant

Decreasing

Volume

AS THE VOLUME DECREASES• The distance between each collision with the wall decreases •Each molecule will collide with the walls more often• The average force against the walls will increase• The pressure will increase

At a constant temperature the pressure of a gas is inversely proportional to its volume.

P1 V1 = P2 V2

2.2 Thermal properties2.2 (a) Thermal expansion of solids, liquids and gasesCore• Describe qualitatively the thermal expansion of solids, liquids and gases

Solids

As the temperature increases the bonds in the solid vibrate more

The average separation between particles increases and so the bulk solid expands

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IGCSE PHYSICS Section 2 Thermal Physics

Liquids

As the temperature increases the motion of the particles increase and so the kinetic energy

The average separation between the particles increases so the volume of the bulk liquid increases

Gases The volume / dimensions of a gas are fixed by the container As the temperature increases the motion of the particles

increase and so the kinetic energy The particles will collide with the container with more force

and more often If the container dimensions are free to change the volume will

increase• Identify and explain some of the everyday applications and consequences of thermal expansion

State Application ConsequenceSolid Railway lines

Jam jar lids

Railway lines to space to expand at the end of the lineIf heated can be removed more easily

Liquid In thermometers As the liquid expands on heating this property can be used to measure temperature

Gas Car tyres After a long journey the pressure in your car tyres will increase as they become hot when driving

• Describe qualitatively the effect of a change of temperature on the volume of a gas at constant pressure

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IGCSE PHYSICS Section 2 Thermal Physics

Constant Pressure

Temperature / K

Volu

me

/ m3 V/T = constant

Increasing

Temp• As Temperature increases molecules have more kinetic Energy• Faster moving molecules will hit the walls with more force pushing the walls out• The volume will increase (if pressure remains constant)

At a constant pressure the volume of a gas is proportional to its temperature in Kelvin.

V1 /T1 = V2 /T2

Supplement• Show an appreciation of the relative order of magnitude of the expansion of solids, liquids and gases

State Order of magnitude of expansion

Solid SmallLiquid SmallGas Big

2.2 (b) Measurement of temperatureCore• Appreciate how a physical property that varies with temperature may be used for the measurement of temperature, and state examples of such properties

Physics property Effect of temperature ExampleVolume of a liquid Increases with increasing

temperatureMercury in glass thermometer

Resistance Increases with temperature

Resistance thermometer

• Recognise the need for and identify fixed points

Fixed Points: Used to calibrate thermometers Boiling and melting water can be used as these 2 changes occur at

fixed temperatures The thermometer can be placed in boiling water to make the 100oC

mark and then ice to make the 0oC mark

• Describe the structure and action of liquid-in-glass thermometers

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IGCSE PHYSICS Section 2 Thermal Physics

High Temp Low Temp

•As temperature rises liquid expands•At any temperature the liquid will have a fixed volume

Liquid

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IGCSE PHYSICS Section 2 Thermal Physics

Supplement• Demonstrate understanding of sensitivity, range and linearity

SensitiveNot

Sensitive

100oC

0oC

500oC

0oC

Low Range High Range

100oC

0oC

500oC

0oC

Linear Range

Non Linear Range

50oC

100oC

10oC

• Describe the structure of a thermocouple and show understanding of its use for measuring high temperatures and those that vary rapidly

V

Copper wires

Iron wire

Hot junction

Cold junction

Thermocouplethermometer

Thermocouple Thermometer Consists of two metals connected as shown When one junction between the two metal types is at a higher

temperature than the other a voltage is produced This voltage is dependent on the temperature difference between

the junctions They have a very large temperature range and can record

temperatures very quickly

2.2 (c) Thermal capacityCore• Relate a rise in the temperature of a body to an increase in internal energy

Internal Energy = Random Kinetic Energy + Potential Energy of the particles

If temperature rises this indicates that the Kinetic energy of the particles has increased

If the kinetic energy of the particles increases so does the internal energy

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IGCSE PHYSICS Section 2 Thermal Physics

• Show an understanding of the term thermal capacity

C = DQDT

(J /K)

Thermal Capacity

Specific Heat Capacity

c = DQmDT

(J /kgK)

DQ = change in energyDT = change in temperature

The energy required to raise the temperature of an object by 1K

The energy required to raise a unit mass of a substance 1K

Supplement• Describe an experiment to measure the specific heat capacity of a substance

V

A

ObjectHeater

c = ItVm(T2-T1)

Measure the temperature of a material before and after heating Measure the energy input from heating by measuring the voltage,

current and time

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IGCSE PHYSICS Section 2 Thermal Physics

Remember that the block should be insulated as energy will be lost to the surroundings

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IGCSE PHYSICS Section 2 Thermal Physics

2.2 (d) Melting and boilingCore• Describe melting and boiling in terms of energy input without a change in temperature

Tem

pera

ture

Time

Boiling

SOLID LIQUID GAS

PHA

SE C

HA

NG

E

PHA

SE C

HA

NG

E

Melting

solidification

Condensing

• State the meaning of melting point and boiling point

Melting point: The temperature at which solid and liquid phases both exist in equilibrium

Boiling point: The temperature at which a substance changes from a liquid to a gas throughout the bulk of the liquid

• Describe condensation and solidification

Condensation: As the kinetic energy of particles decreases the attractive forces between them “pull” them together into droplets. Bonds are formed.

Solidification: As the kinetic energy of particles decreases permanent bonds form between the particles so they are no longer able to move freely.

Supplement• Distinguish between boiling and evaporation

Property Boiling EvaporationTemperature Only at the boiling point At all temperaturesLocation Throughout the liquid Only at the surfaceParticles All particles Only the particles with

sufficient kinetic energyTemperature of liquid Remains the same Reduces

• Use the terms latent heat of vaporisation and latent heat of fusion and give a molecular interpretation of latent heat

LATENT HEAT OF VAPORISATION: Energy change associated with a substance

Boiling or Condensing

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IGCSE PHYSICS Section 2 Thermal Physics

LATENT HEAT OF FUSION: Energy change associated with a substance

Melting or Solidifying

l = DQm

(J /kg)

Specific Latent HeatThe amount of energy per unit mass absorbed or released during a change of phase

Molecular Interpretation: At a phase transition bonds are being broken or formed. This gives an associated intake or release of energy.

• Describe an experiment to measure specific latent heats for steam and for ice

SPECIFIC LATENT HEAT OF STEAM

V

A

Heater

l = ItVm1 – m2

00250.0g

Take a volume of liquid to its boiling point Measure the mass of the liquid Boil the water for a fixed period and calculate the energy input by

measuring the voltage and current for the heater Record the mass of the liquid after heating and calculate the mass turned

to a gas

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IGCSE PHYSICS Section 2 Thermal Physics

SPECIFIC LATENT HEAT OF ICE

V

A

Heater

l = ItVm1 – m2

00250.0g

Ice

Melted Water

Heat the ice until it starts to melt Capture the melted ice in the beaker on the balance Heat for a fixed length of time and calculate the energy input by

measuring the voltage and current supplied to the heater Measure the mass of water melted in that period

2.3 Transfer of thermal energy2.3 (a) ConductionCore• Describe experiments to demonstrate the properties of good and bad conductors of heat

Good Conductor

Poor Conductor

Medium Conductor

Heat Source

Wax

Coin

Coins or other objects can be attached to a object to be tested One end of the object is then heated The ability of the object to conduct heat can be judged by how

quickly the wax melts and so coins are released

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IGCSE PHYSICS Section 2 Thermal Physics

Supplement• Give a simple molecular account of heat transfer in solids

Transfer of Thermal Energy

HOT COLD

Conduction

2.3 (b) ConvectionCore• Relate convection in fluids to density changes and describe experiments to illustrate convection

Convection

Hot air is less dense and so is forced up

Cold air is less dense and so sinks down

ConvectionCurrents

2.3 (c) RadiationCore• Identify infra-red radiation as part of the electromagnetic spectrum

Electromagnetic SpectrumWavelength

3 x 104 m 3 x 10-4 m 3 x 10-8 m 3 x 10-12 m3 m

104 Hz 1020 Hz108 Hz 1012 Hz 1016 Hz

Frequency

Radio waves

Microwaves

InfraredUltraviolet

X-rays

Gamma rays

7.5 x 10-7 m 4 x 10-7 m

4 x 1014 Hz 7.5 x 1014 HzVisible Light

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IGCSE PHYSICS Section 2 Thermal Physics

Supplement• Describe experiments to show the properties of good and bad emitters and good and bad absorbers of infra-red radiation

PROPERTIES OF ABSORBERS PROPERTIES OF EMMITERS

Hea

t Sou

rce

Bla

ck su

rfac

e

Shin

y su

rfac

e Wax

Coin

Hea

t Sou

rce

Bla

ck su

rfac

e

Shin

y su

rfac

e2.3 (d) Consequences of energy transferCore• Identify and explain some of the everyday applications and consequences of conduction, convection and radiation

Energy Transfer Applications ConsequencesConduction Saucepan or wok Made of copper or other

good conductorsConvection Air conditioner Normally placed on the

ceiling as cold air dropsRadiation Paint In hot climates houses

are painted white

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