Thermodynamics

Thermodynamics
Inquiry question: How are temperature, thermal energy and particle motion related?
Students:
● explain the relationship between the temperature of an object and the kinetic energy of the particles within it
(ACSPH018)
● explain the concept of thermal equilibrium (ACSPH022)000000000000
● analyse the relationship between the change in temperature of an object and its specific heat capacity through the

equation 𝑄=𝑚𝑐Δ𝑇 (ACSPH020)
● investigate energy transfer by the process of:
– conduction
– convection
– radiation (ACSPH016)
● conduct an investigation to analyse qualitatively and quantitatively the latent heat involved in a change of state
● model and predict quantitatively energy transfer from hot objects by the process of thermal conductivity
● apply the following relationships to solve problems and make quantitative predictions in a variety of situations:

– 𝑄=𝑚𝑐Δ𝑇, where c is the specific heat capacity of a substance

– 𝑄/𝑡=𝑘𝐴Δ𝑇/𝑑 where 𝑘 is the thermal conductivity of a material

Word Cloud

what is the kinetic particle

Kinetic Particle Model

• All matter is made up of particles (called atoms).
• These particles have kinetic energy (i.e. they are always moving).
• No kinetic energy overall is lost or gained during collisions between

particles.

• There are forces of attraction and repulsion between the particles in a

material.

• All particles are moving (though more and more slowly) until they reach

the temperature known as absolute zero (approx. or 0K).

• In a solid the particles are not still, they vibrate around an average

position.

Kinetic Particle Model

• Heating is a process that always transfers thermal energy from a hotter substance to
a colder substance.

• Heat (measured in Joules, J) is the transfer of thermal energy from a hotter body to a
colder one.

• Heating is observed by the change in temperature, expansion of the substance or a

change of state.

• Temperature is related to the average kinetic energy of the particles in the
substance. The faster the particles move, the higher the temperature of the substance.

Multiple Choice

Which of the following is true of a solid?

• Particles are moving around freely.

• Particles are not moving.

• Particles are vibrating in constant motion.

• A solid is not made up of particles.

Heat and Temperature

• An increase in the total energy of the

particles in a substance results in an
increase in temperature if there is a net
gain of kinetic energy.

• Sometimes heating only results in the

change of state or expansion of a
substance, not a change in temperature.

• In these cases the total internal energy of

the particles have still increased, but only
the potential energy (not the kinetic).

Measuring Temperature

• In 1593 Galileo Galilei made one of the first thermometers.
• This was not particularly accurate, as it did not take into account changes in air

pressure, but did suggest some basic principles for determining a suitable scale
of measurement.

• He suggested 2 fixed points, the hottest day in summer and the coldest day in

winter.

• This is referred to as an arbitrary scale as the fixed points are randomly chosen.
• The 2 most common arbitrary scales for temperature are Celsius and

Fahrenheit.

• Absolute scales are different to arbitrary scales.
• To be absolute it must have no negative values, the fixed point must be

reproducible and zero must have the lowest value.

Kelvin Scale

• For water this is slightly above .
• The absolute or Kelvin temperature scale is

based on absolute zero and the triple point of
water.

• Absolute zero has been indicated by

experiments as the limit to how cold things
can get.

• Absolute zero = 0 K= -273.15°C.
• All molecular motion ceases at absolute zero.

This is the coldest temperature possible.

Multiple Choice

Which of the following temperature(s) cannot possibly exist? (More than one answer is possible)

• 1 000 000°C

• -50°C

• -50K

• -300°C

Open Ended

convert the following temperature: $$30\degree C$$ to kelvin.

Open Ended

convert the following temperature: 375K to Celcius.

Heat and Temperature

• The kinetic particle model (as discussed) can

be used to explain the idea of heat as a
transfer of energy.

• Heat (measured in Joules, J) is the transfer of

thermal energy from a hotter body to a colder
one.

• Heating is observed by the change in

temperature, expansion of the substance or a
change of state.

• When a solid substance is heated the particles

within the material either gain kinetic energy
(move faster) or potential energy (move away
from their equilibrium positions.

Laws of Thermodynamics

• There are 4 laws of thermodynamics, the zeroth, first, second and third.
• The zeroth law of thermodynamics was discovered last, but considered

the most important and needed to be place first.

• The zeroth law relates to thermal equilibrium and thermal contact and

allows temperature to be defined.

• If two objects are in thermal contact, energy can flow between them.
• Assuming that the first is warmer than the second, thermal energy will

flow from the first to the second.

• Thermal equilibrium is reached when there is no longer a flow of energy

between two objects in thermal contact.

• If two objects are in thermal equilibrium with a

third object, then they are all in thermal
equilibrium with each other.

Multiple Choice

An uncooked chicken is placed into an oven that has been preheated to 180°C. Which of the following statements describe what happens as soon as the chicken is placed in the oven? (More than one answer is possible.)

• Thermal energy flows from the chicken into the

  hot air.

• The chicken and the air in the oven are in

  thermal equilibrium.

• Thermal energy flows from the hot air into the

  chicken.

• The chicken and the air in the oven are not in thermal equilibrium.

Multiple Select

A chicken is inside an oven that has been preheated to 180°C. The chicken has been cooking for one hour and its temperature is also 180°C. Which of the following statements best describes this scenario?

• Thermal energy flows from the chicken into the

  hot air.

• The chicken and the air in the oven are in

  thermal equilibrium.

• Thermal energy flows from the hot air into the

  chicken.

• The chicken and the air in the in the oven are not in thermal equilibrium

Laws of Thermodynamics

• The first law of thermodynamics states that

energy simply changes from one form to another
and the total internal energy of a system is
constant.

• The internal energy can be changed by heating or

cooling, or by work being done on a system.

• Any change in the internal energy (ΔU) of a system is

equal to the energy added by heating (+Q) or
removed by cooling (-Q), minus the work done on (-W)
or by (+W) the system:
∆U = Q-W

• The internal energy (U) of a system is defined as

the total kinetic and potential energy of the
system.

Laws of Thermodynamics

• If heat is added to the system then the

internal energy rises by either increasing
temperature or by changing state.

• If work is done on a system then the internal

energy of the system rises also.

• When heat is added or work is done on a

system then is positive.

• If heat is removed, or work is done by the

system then the internal energy decreases by
either decreasing temperature or changing
state.

• When heat is removed or work is done by a

system then is negative.

Open Ended

A 1L beaker of water has 25kJ of work done on it and also loses 30kJ of thermal energy to
the surroundings. What is the change in energy of the water?

Specific Heat Capacity

• The temperature of a substance is a measure of the average kinetic energy of

the particles inside the substance.

• To increase the temperature of a substance the kinetic energy of its particles

must increase.

• This happens when heat is transferred to that substance.
• The amount the temperature increases depends on a number of factors.
• The greater the mass of a substance the greater the energy required.
• Therefore the heat required to raise the temperature is proportional to the

mass of the substance.

• As the more heat is transferred, the more the temperature rises. The amount

of energy transferred is therefore also proportional to the change in
temperature.

Specific Heat Capacity

• Heating experiments using different materials confirm these relationships.
• However the amount of energy required to head a given mass of a

material through a particular temperature also depends on the nature of
the material being heated.

• This is known as a materials specific heat capacity.
• This changes when a material changes state.

Q=mc∆T

• Where:

• Q is the heat energy transferred in J
• m is the mass in kg
• ∆T is the change in temperature (in ° Cor K)
• c is the specific heat capacity of the material in Jkg^-1K^-1

Specific Heat Capacity

• Some examples of materials

and their specific heat
capacities.

• Note the heat capacity of

water in relation to the
metals.

Open Ended

A hot water tank contains 135L of water. Initially the water is at 20°C. Calculate the amount of energy that must be
transferred to the water to raise the temperature to 70^∘C. c_water=4200Jkg^-1K^-1
• 1L of water =1kg of water
• Mass= 135kg

Specific Heat Capacity Examples

1. A hot water tank contains 135L of water. Initially the water is at 20oC. Calculate the amount of energy that must be
transferred to the water to raise the temperature to 70oC. ()
• 1L of water =1kg of water
• Mass= 135kg

2. A bath contains 75L of water. Initially the water is at 50oC. Calculate the amount of energy that must be transferred

from the water to cool the bath to 30oC.

• Mass =75kg

• 6.3MJ must be transferred from the water.

Latent Heat

• Latent heat is the energy released or

absorbed during a change of state.

• When a substance changes state the

temperature remains constant.

• The energy used for example in melting

ice into water is hidden in a sense, as the
temperature remains constant.

• Latent heat is calculated using:

• Where Q is the heat energy transferred (in J),

m is the mass (in kg) and L is the latent heat
(in Jkg-1).

Latent Heat

• When the phase change is from solid to liquid we must use the latent heat of fusion.
• The specific latent heat of fusion, L, of a substance is the heat needed to change a

mass of 1 kg the substance from a solid at its melting point into liquid at the same
temperature.

• At this point the particles begin to move further apart, reducing the strength of their

bonds.

• Instead of increasing the temperature the extra energy increases the potential

energy of the particles (i.e. no change in temperature occurs because the extra
energy is reducing the strength of the interparticle forces).

• When the phase change is from liquid to a gas, we must use the latent heat of

vaporisation.

• The specific latent heat of vaporisation, L, of a substance is the heat needed to

change the substance from a liquid at its boiling point into vapour at the same
temperature.

Latent Heat Examples

1. How much energy must be removed from 2.5L of water at 0oC to produce a block of ice at 0oC? Express your
answer in kJ. ()
• m=2.5kg

2. 50mL of water is heated from a room temperature of 20oC to its boiling point of 100oC. It is boiled at this
temperature until it is completely evaporated. How much energy in total was required to raise the temperature
and boil the water? ()
• 50ml=50g=0.05kg

• J

Evaporation and Cooling

• A liquid changes to a gas at room temperature in a process

known as evaporation.

• If the particles of the liquid have sufficient energy they are

able to escape from the surface of the liquid into the air.

• This is more noticeable in volatile liquids such as methylated

spirits, perfume and turpentine.

• The surface bonds are weaker in these liquids and they

evaporate readily.

• Whenever evaporation occurs higher energy particles escape

leaving lower energy particles behind.

• As a result the average kinetic energy of the particles

remaining in the liquid drops and the temperature decreases.

• This cooling principle is the reason behind sweating to stay

cool.

Conduction

• There are three ways that heat can be transferred: conduction, convection and

radiation.

• Conduction is the process where heat is transferred from one place to another

without a net movement of particles.

• This can occur within a material, or between two materials that are in contact.
• Whilst this can occur (to some extent) in all materials, the effect of conduction is

only really significant in solids.

• Materials that conduct heat readily are called conductors.
• Materials that are poor conductors are called insulators.
• Conduction can happen in two ways.

• Energy transfer through particle collisions
• Energy transfer through free electrons

• Refer to boardworks Conduction PPT

Thermal Conductivity

• Thermal conductivity describes the

ability of a material to conduct heat.

• It is temperature dependent and

measured in Watts per metre per Kelvin
().

• Factors that affect thermal conduction

include:

• Nature of the material
• Temperature difference between the two

objects

• Thickness of the material
• Surface area

Thermal Conductivity

• The rate of energy transfer by conduction through

a material can be calculated using:

• Where:

• is the rate of energy transferred in
• k is the thermal conductivity of the material in
• A is the surface area perpendicular to the heat flow in
•is the temperature difference across the material in

Kelvins or degrees Celcius

• d is the thickness of the material through which the

heat is being transferred in m

• Clothing designers use this relationship when

calculating the insulating ability of parkas and
cold-weather clothes.

• Architects and builders use it to calculate the

efficiency of building insulation.

e.g. Calculate the rate of energy
transfer by conduction through a house
wall which is a section of wood that is
10cm thick. Assume that the inside
temperature is and the outside
temperature is (Assume the
conductivity of wood is 0.15 )
•
• k=0.15, A=1, , d=0.1

Convection

• Convection is the transfer of thermal energy within a fluid

by the movement of hot areas from one place to another.

• Unlike conduction and radiation, convection involves the

mass movement of particles within a system over
considerable distance.

• As a fluid is heated the particles within it gain kinetic

energy and push apart due to the increased vibration of
particles.

• This causes the heated fluid to become less dense and

then to rise.

• Colder fluid (with slower moving particles) is more dense

(heavier) and falls taking the place of the warmer fluid.

• A convection current forms when there is warm fluid

rising and cool fluid falling.

Convection

• It is difficult to quantify the thermal energy

transferred via convection but some
estimates can be made.

• The rate at which convection occurs is

affected by:

• The temperature difference between the heat

source and the convective fluid

• The surface area exposed to the convective fluid

• There are two main causes of convection:

• Forced convection (e.g. ducted heating)
• Natural convection (e.g. heating water in a kettle)

• See boardworks Convection PPT

Radiation

• Radiation is as means of heat transfer without the

movement of matter.

• In this context radiation means electromagnetic radiation.
• Electromagnetic radiation travels at the speed of light.
• When it hits an object it is partially reflected, partially

transmitted and partially absorbed.

• The absorbed part transfers thermal energy to the

absorbing object and causes an increase in temperature.

• Electromagnetic radiation is emitted by all objects that are

at a temperature above absolute zero.

• The wavelength and frequency emitted depends on the

internal energy of the object.

• The higher the temperature of the object, the higher the

frequency and the shorter the wavelength of the radiation
emitted.

Radiation

• All objects both emit and absorb thermal energy by

radiation.

• If an object absorbs more energy than it emits then

its temperature increases.

• If it emits more energy than it absorbs then its

temperature decreases.

• A number of factors affect both the rate of emission

and absorption:

• Surface area
• Temperature
• Wavelength of the incident radiation (reflective surfaces

reflect all wavelengths, white surfaces reflect visible light,
but still absorb infrared radiation)

• Surface colour and texture (matt black surfaces absorb

and emit radiant energy faster than shiny white surfaces)