12/04/23

Thermal PhysicsThermal Physics

12/04/23

Heat flows from hot to coldHeat flows from hot to cold

Net energy flow stops when their temperatures are the samei.e. They are in thermal equilibrium

12/04/23The Kelvin temperature The Kelvin temperature scalescale

• Tc = Tk - 273 • Tk = Tc +273• How many times hotter is 100ºC

than 33ºC?• How many times hotter is 10ºC

than 0ºC?• What is your temperature in 0C and

0K

12/04/23

Kinetic theoryKinetic theory

• Why is it that you can put out a candle flame with moist fingers without hurting yourself although it is at 750 oC but it is very painful to put your fingers into a cup of hot water at 80 oC?

12/04/23

Heat energy and molecular Heat energy and molecular movementmovement

• What happens to the molecules when their temperature increases?

• What is the difference between internal “heat” energy and temperature?

• What types of energy dothe molecules have?

12/04/23Heat energy and Heat energy and temperaturetemperature

100mL at 200C400mL at 200C

400mL at 800C1mL at 2000C

Which has the “most heat” energy?Which has the higher temperature?

12/04/23

12/04/23

Heat capacityHeat capacity

Click to play

Why are the temperatures different?

12/04/23

TemperatureTemperature

12/04/23

The internal energy of a substance is……

The potential energy of the molecules is….

The thermal energy of a substance is…..

12/04/23

Gas PressureGas Pressure

Gas pressure is basically due to gas molecules hitting the sides of the container they are in:

Hyperlink

12/04/23

Kinetic Theory of GasesKinetic Theory of Gases

Let’s make some assumptions:

1) Brownian motion suggests that a pure gas consists of identical molecules in constant random motion.

2) Molecules never stop so collisions between molecules are, on average, elastic (no kinetic energy is lost).

3) Because gases can be compressed easily the volume of the individual molecules is negligible compared to the volume they occupy when moving around.

4) Therefore the molecules are further apart, so we assume that there are no forces on the molecules except for when they collide.

12/04/23

Internal EnergyInternal EnergyBasically, the internal energy of a substance is the sum of the molecular kinetic and potential energies. Consider a particle:

This particle can store energy by moving in a number of ways:

1) Moving in three dimensions

2) Vibrating

3) Rotating

These are called “degrees of freedom”, and each one can store

energy.

12/04/23

The Mole and Molar massThe Mole and Molar mass

One mole of carbon

contains 6x1023 atoms

One mole of green atoms

contains 6x1023 atoms

One mole of “anything” contains 6x1023 atoms (or molecules)

One mole of carbon

weighs 12gDefine the moleDefine the mole

12/04/23

Molar massMolar mass

• One mole of carbon contains 6x1023 atoms• It weighs 12 grams• How much does one mole of iron weigh?• How many atoms are there in 2 moles of

tungsten?• How many atoms are the in 0.5 moles of

water?• Define Avagadro’s constant• Define the Mole

12/04/23

Tsokos page 161 q’s 1-6.

12/04/23

12/04/23

Specific Heat CapacitySpecific Heat CapacityThis can be thought of as “the capacity of an object to store heat”. Consider some water:

If we heat this beaker up it’s fairly clear that the amount of energy it gains depends on how much water there is and how hot it gets…

Energy α mass x temperature rise

Energy = mass x s.h.c x temp

E = mcΔT

12/04/23

Specific Heat CapacitySpecific Heat CapacityHow can we do this experimentally?

AV12

VPossible errors with this experiment:

1) Temperature throughout the liquid should be the same

Solution:

2) Heat is lost to the surroundings

Solution(s):

E = VIt and E = mcΔT

12/04/23

Another way…Another way…A

V12V

A metal

12/04/23

SHCSHC

Temperature

Time

AV12

VQ=mcΔT

Q=Pt, and P=IV

ΔT=IVt/mc

12/04/23

12/04/23

12/04/23

Thermal PhThermal Phyysicssics Questions Questions

• Specific heat capacities:• copper400Jkg-1K-1 iron460Jkg-1K-

1 water4200Jkg-1K-1 ice2100Jkg-1K-1 •  Specific latent heat of fusion of ice =

3·3×105Jkg-1

• Molar heat capacities of a diatomic ideal gas:Cv = 12·5J(molK)-1 and Cp = 20·8J(molK)-1

12/04/23

• Question 1• A piece of metal of mass 0·2kg is heated to a temperature of

200°C. It is then put into 0·2kg of water at 20°C in a container of negligible heat capacity. The "final" temperature, after stirring, is 40°C. Calculate the specific heat capacity of the metal.

• Question 2• A piece of metal of mass 100g, has a temperature of 100°C. It

is put into 100g of water at 20°C in a container of negligible heat capacity. After stirring, the maximum temperature of the "mixture" (metal and water) is 27·5°C. Calculate the specific heat capacity of the metal.

• Question 3• The specific heat capacity of water is very high. What effect

does this have on the weather conditions experienced by people living on islands?

• Question 4• How long will it take to change the temperature of 200kg of

water from 15°C to 40°C, using a heater of power 3kW. Assume that all the thermal energy remains in the water.

12/04/23

3 Phases of matter3 Phases of matter

Solids –fixed positions, many strong bonds (potential energy)

Liquids – Changing positions, many temporary bonds

Gases – Free moving, no bonds

12/04/23

Changing stateChanging stateHyperlink

12/04/23Change of state (Melting and Change of state (Melting and boiling) boiling)

Heat energy is going to internal EK

Heat energy is going to potential energy only, therefore there is no rise in temperature.

12/04/23

Heating iceHeating ice

150

100

50

0

-50

Temp/OC

Time/sThis flat line shows where energy is being used to break bonds – this has to be done during melting. The amount of energy needed to turn 1kg of a solid into a liquid is called the Specific Latent Heat of Fusion L.

This flat line shows where energy is being used to break the temporary bonds for boiling. The amount of energy needed to turn 1kg of a liquid into a gas is called the Specific Latent Heat of Vaporisation L.

12/04/23

Latent heat (changes in Latent heat (changes in potential energy)potential energy)

12/04/23

Melting Q’sMelting Q’s

• Question 1• A piece of ice at -20°C is put into a copper calorimeter

of mass 0·2kg which contains 0·15kg of water at 20°C. The water is stirred until all the ice has melted. At this time the temperature of the water (and calorimeter) is 15°C. Calculate the mass of the piece of ice. (8 grammes).

• Question 2• A refrigerator is capable of removing 50J of heat per

second from a container of water. How long will it take to change 2kg of water at 10°C into ice at -5°C? Assume that the rate of removal of heat remains constant and that the container has negligible heat capacity. Are these assumptions likely to be valid in practice?

12/04/23

Latent Heat of FusionLatent Heat of Fusion

Energy = mass x specific latent heat of fusion

From the previous slide we can say that the energy needed to melt water is given by…

To work out L experimentally you could…

AV12

V

VIt = mL

Tsokos page 171 q’s 1-11.

12/04/23

12/04/23

Evaporation Evaporation

•High energy molecules escape when they reach the surface.

•The average kinetic energy of the remaining particles drops

•The temperature of the remaining liquid is lowered

Average speed decreases

12/04/23

BoilingBoilingHyperlink

12/04/23

Evaporation and BoilingEvaporation and Boiling

• Boiling occurs at a fixed temperature

• Evaporation occurs at any temperature

• Boiling happens throughout the body of the liquid

• Evaporation only happens at the surface of the liquid.

12/04/23

PressurePressurePressure depends on two things:

1) How much force is applied, and

2) How big (or small) the area on which this force is applied is.

Pressure can be calculated using the equation:

Pressure (in N/m2) = Force (in N)

Area (in m2)

F

APOR in cm2 and

N/cm2

12/04/23

Some pressure questionsSome pressure questions

1) Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 1m2.

2) A brick is rested on a surface. The brick has an area of 20cm2. Its weight is 10N. Calculate the pressure.

3) A woman exerts a pressure of 100N/cm2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on?

4) (Hard!) The pressure due to the atmosphere is 100,000N/m2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body?

Pressure = force

area

P = F

A

12/04/23

Kinetic theoryKinetic theory

The collisions of the molecules with the container (not with each other) create the pressure.

•Particles collide with the walls of the container

•They change their momentum

•They exert a force on the wall

•This creates “pressure”.

12/04/23

12/04/23Assumptions of the kinetic Assumptions of the kinetic theorytheory

1. Molecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic)

2. Molecules are in continuous rapid, random motion.

3. The average kinetic energy of the molecules is proportional to the absolute temperature of the gas.

4. The molecules do not exert any appreciable attraction on each other.

5. The volume of the molecules is infinitesimal when compared with the volume of the gas.

6. The time spent in collisions is small compared with the time between collisions.

12/04/23Macroscopic behaviour of a Macroscopic behaviour of a gasgas

• Why does the volume of a gas expand when heated?

• Why does the pressure of a gas increase when heated?

• How does the temperature relate to the molecular behaviour?

Tsokos page 171 q’s 1-10.

12/04/23