Thermal Energy Transfers

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Solids, Liquids & Gases

The three states of matter are solid, liquid and gas; the kinetic theory of matter models particles as small spheres to explain their properties.
In a solid, particles are closely packed in a fixed pattern (lattice), can only vibrate about fixed positions, and have low energy; solids have fixed shape, fixed volume, high density, and are very difficult to compress.
In a liquid, particles are closely packed but randomly arranged, can flow past each other, and have medium energy; liquids have fixed volume but take the shape of their container, and have medium density.
In a gas, particles are far apart (about 10 times the spacing in solids/liquids), move randomly at various speeds, and have high energy; gases have no fixed shape or volume, low density, and are compressible.
The intermolecular forces are strongest in solids, weaker in liquids, and weakest in gases; particles in a gas have enough energy to completely overcome these forces.
Density is mass per unit volume; gases are much less dense than liquids and solids due to their molecular arrangement.

Particles in a solid are packed close together in a fixed pattern; in a liquid they are still close but free to move past each other.

Temperature Scales & Kinetic Energy

Absolute zero is the lowest possible temperature, equal to 0 K or −273 °C, at which molecules have zero kinetic energy.
Convert between Celsius and Kelvin using: T / K = θ / °C + 273 or θ / °C = T / K − 273.
A change of 1 K is equal to a change of 1 °C; temperature differences are the same on both scales.
For an ideal gas, the average kinetic energy of molecules is Ek = (3/2) kBT, where kB = 1.38 × 10⁻²³ J K⁻¹.
The absolute temperature of an ideal gas is directly proportional to the average kinetic energy of its molecules.

A large iceberg has more total kinetic (thermal) energy than a few small ice cubes of the same substance and temperature, because it is made of far more particles.

Internal Energy & Thermal Equilibrium

Internal energy is the sum of the total kinetic energy and total intermolecular potential energy of all particles in a substance.
When thermal energy is transferred to a substance, its internal energy increases; this can increase the average kinetic energy (temperature rises) or the potential energy (state changes).
Temperature is a measure of the average kinetic energy of molecules; a change in average potential energy does not affect temperature.
Thermal equilibrium is reached when two substances in contact no longer exchange heat energy and have the same temperature.
Thermal energy always transfers from a hotter region to a cooler region until thermal equilibrium is reached.

Changes of State

A change of state (phase change) occurs when matter changes between solid, liquid and gas; thermal energy is transferred but the temperature remains constant.
During a phase change, only the potential energy of the molecules changes (spacing between particles); the average kinetic energy does not change.
The four main phase changes are: melting (solid→liquid), freezing (liquid→solid), vaporisation/boiling (liquid→gas), and condensation (gas→liquid).
Each substance has its own melting/freezing point and boiling point; for water, freezing is at 0 °C and boiling at 100 °C.
Melting and boiling absorb thermal energy; freezing and condensation release thermal energy.

Particle arrangement and motion in solids, liquids and gases

Specific Heat Capacity

The thermal energy needed to change the temperature of a substance depends on its mass, the temperature change, and its specific heat capacity.
Specific heat capacity is the energy required to change the temperature of 1 kg of a substance by 1 K (or 1 °C).
The equation for thermal energy transferred is Q = mcΔT, where m is mass (kg), c is specific heat capacity (J kg⁻¹ K⁻¹), and ΔT is temperature change (K or °C).
A higher specific heat capacity means the substance heats up or cools down more slowly; water has a high specific heat capacity (4200 J kg⁻¹ K⁻¹) compared to metals like copper (390 J kg⁻¹ K⁻¹).
In thermal equilibrium problems, energy lost by a hot object equals energy gained by a cold object: −Qc = Qw.

Diagram of apparatus for investigating specific heat capacity

Specific Latent Heat

During a phase change, thermal energy is transferred without changing temperature; the energy is calculated using Q = mL.
Specific latent heat is the energy required to change the state of 1 kg of a substance without changing its temperature; units are J kg⁻¹.
Specific latent heat of fusion (Lf) applies to solid↔liquid changes; specific latent heat of vaporisation (Lv) applies to liquid↔gas changes.
For any substance, Lv > Lf because completely overcoming intermolecular forces (vaporisation) requires more energy than partially overcoming them (fusion).
Heating/cooling curves show flat sections during phase changes (temperature constant) and sloped sections when temperature changes.

Thermal Conduction

Conduction is the main method of thermal energy transfer in solids; it occurs when two solids of different temperatures are in contact.
Metals are the best thermal conductors because they have a high number of free (delocalised) electrons that collide with atoms and transfer energy quickly.
Non-metals like plastic and glass are poor conductors; liquids and gases are even poorer because their atoms are further apart.
Conduction occurs through atomic vibrations and free electron collisions; atoms at the hotter end vibrate more and bump into neighbouring atoms, transferring energy.
Thermal conductivity (k) quantifies a material's ability to transfer heat via conduction; units are W m⁻¹ K⁻¹. High k means fast energy transfer.
The rate of heat transfer via conduction is given by ΔQ/Δt = kAΔT/Δx, where A is cross-sectional area, ΔT is temperature difference, and Δx is thickness.

Conduction in metals

Thermal Convection

Convection occurs when a fluid (liquid or gas) is heated, causing movement of groups of atoms or molecules due to density variations.
Convection is the main way heat travels through liquids and gases; it cannot occur in solids because particles cannot travel relative to one another.
When heated from below, the hot fluid expands, becomes less dense, and rises; cooler, denser fluid moves in to take its place, forming a convection current.
Eventually, the hot fluid cools, contracts, and sinks back down, creating a continuous circulation.
Examples of convection include atmospheric winds, sea breezes, convection currents in the Earth's mantle, and ocean currents.

Thermal energy is transferred from hot coffee by convection

Thermal Radiation

Thermal radiation is heat transfer by electromagnetic waves, usually in the infrared region of the spectrum.
All bodies emit thermal radiation; the hotter the object, the more infrared radiation it emits per second.
Thermal radiation is the only method of thermal energy transfer that does not require matter; it can travel through a vacuum.
The amount of thermal radiation emitted depends on surface colour (black emits more), texture (shiny emits less), and surface area (greater area emits more).
Dark, dull objects are better emitters and absorbers of radiation; light, shiny objects are worse.
A perfect black body absorbs all radiation incident on it and reflects or transmits none; black-body radiation is the thermal radiation emitted by all bodies.

Hotter objects emit more thermal radiation

Stefan-Boltzmann & Wien's Laws

The Stefan-Boltzmann law relates the power output of a black body to its surface temperature and radius.
Wien's Displacement Law states that the peak wavelength of emitted radiation is inversely proportional to absolute temperature: λmax ∝ 1/T.
For a black body, the lower the wavelength at which peak intensity occurs, the higher the peak intensity.
The power output of a black body depends on its surface temperature and radius.

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1.Thermal energy is transferred from a solid. Three properties of the solid are: I. Specific heat capacity; II. Mass; III. Area. Which of the above properties determine the decrease in temperature of the solid?
Medium
- AII only
- BI and III only
- CI and II only
- DI only
2.What are the units of the ratio (specific latent heat of fusion of iron)/(specific heat capacity of iron)?
Medium
- AJ K
- BNo units
- CK⁻¹
- DK
3.The strength of intermolecular forces varies between the different states of matter. What is the order from highest to lowest strength of intermolecular forces?
Easy
- Asolid > liquid > gas
- Bsolid > gas > liquid
- Cliquid > gas > solid
- Dgas > liquid > solid
4.Molecules fuse from water vapour to form water. The vapour and the water have the same temperature. What is the change of the average potential energy and the change of the average random kinetic energy of these molecules when they move from the vapour to the water?
Medium
- AAverage potential energy decreases; average random kinetic energy decreases
- BAverage potential energy no change; average random kinetic energy decreases
- CAverage potential energy decreases; average random kinetic energy no change
- DAverage potential energy no change; average random kinetic energy no change
5.Which of the following correctly identifies the properties of the molecules of a substance that determine the substance's internal energy?
Medium
- AThe total gravitational potential energy and random electrostatic potential energy
- BThe total potential energy and random kinetic energy
- CThe random kinetic energy
- DThe total potential energy
6.The specific latent heat of vaporisation is the energy required to change the phase of:
Medium
- Aone kilogram of a liquid to gas
- Ba solid at constant temperature
- Cone kilogram of a gas to liquid at constant temperature
- Da gas at constant temperature
7.A sample of solid aluminium is heated beyond its melting point. The graph of temperature against time shows flat sections. During which stage(s) is the aluminium melting?
Medium
- AQ only
- BP, Q and R
- CP only
- DP and R only
8.What is the correct comparison of the specific latent heat of fusion Lf to the specific latent heat of vaporisation Lv for any substance?
Medium
- ALf = Lv
- BLf > Lv
- CLf < Lv
- DDepends on the substance.

Thêm câu hỏi

9.Which of the following is numerically equal to the specific heat capacity of a substance?
Easy
- AThe thermal energy required to increase the temperature of unit mass of the substance by 1 °C
- BThe thermal energy required to increase the temperature of the substance by 1 °C
- CThe sum of the random kinetic and potential energy of all the molecules in the substance
- DThe thermal energy required to evaporate the substance
10.In which type of material is conduction the main type of thermal transfer?
Easy
- AGas
- BLiquid
- CFluid
- DSolid
11.Which of the following statements about convection is correct?
Medium
- AConvection is the main way heat is transferred through liquids and gases
- BConvection mainly occurs in metals
- CCool fluid rises and warm fluid moves in to take its place
- DHeated molecules gain energy, become denser and sink
12.Which region of the electromagnetic spectrum is responsible for thermal radiation from an object at room temperature?
Easy
- AUltraviolet
- BMicrowaves
- CInfrared
- DVisible Light
13.What is the correct definition for a perfect black body?
Medium
- AAn object that transmits all of the radiation incident on it and does not reflect any radiation
- BAn object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation
- CAn object that does not absorb any of the radiation incident on it
- DAn object that transmits all of the radiation incident on it
14.A black body radiation curve shows the intensity and wavelength distribution of the waves emitted from a black body. Which statement correctly describes the position of the peak intensity?
Medium
- AThe lower the wavelength at which peak intensity occurs, the higher the peak intensity
- BThe higher the wavelength at which peak intensity occurs, the higher the peak intensity
- CThe wider the wavelength range, the lower the peak intensity
- DThe narrower the wavelength range, the higher the peak intensity
15.Which relationship represents Wien's Displacement Law?
Medium
- Aλmax = 1/T
- Bλmin ∝ 1/T
- Cλmax ≤ 1/T
- Dλmax ∝ 1/T
16.Which two factors does the power output of a black body depend on?
Medium
- AMass and volume
- BSurface temperature and radius
- CDensity and luminosity
- DWavelength of radiation and spectral analysis
17.Liquid iron solidifies without a change in temperature. Which of the following is correct for the molecules in the solid phase compared with molecules in the liquid phase?
Medium
- APotential energy same; average kinetic energy same
- BPotential energy same; average kinetic energy less
- CPotential energy less; average kinetic energy less
- DPotential energy less; average kinetic energy same
18.Aluminium melts at 660 °C. A mass m of solid aluminium is initially at 645 °C. It is heated until it melts completely and continues to be heated to a final temperature of 720 °C. Specific latent heat of fusion = Lf, specific heat capacity of solid aluminium = cS, specific heat capacity of liquid aluminium = cL. Which expression gives the energy needed for this change?
Hard
- Am(15cS + Lf)
- Bm(15cS + Lf + 60cL)
- Cm(15cS + 60cL)
- Dm(15cS + Lf + 60cS)
19.Which of the following statements about the three states of matter are correct? (select all that apply)
Medium
- ASolids have a fixed shape and a fixed volume
- BLiquids have a fixed shape but not a fixed volume
- CGases can be compressed and expand to fill their container
- DParticles in a solid are arranged in a fixed lattice and can only vibrate about fixed positions
- EParticles in a gas are closely packed and randomly arranged
20.Which of the following are correct about internal energy and temperature? (select all that apply)
Medium
- AInternal energy is the sum of the total kinetic energy and the total intermolecular potential energy of the particles
- BA change in internal energy always results in a change in temperature
- CDuring a change of state, the temperature remains constant because only the potential energy of the molecules changes
- DTemperature is a measure of the average kinetic energy of the molecules
- EThe internal energy of a substance depends only on its temperature
21.Which of the following statements about thermal radiation are correct? (select all that apply)
Medium
- AAll bodies emit thermal radiation regardless of their temperature
- BThermal radiation is the only method of thermal energy transfer that can travel through a vacuum
- CDark, dull surfaces are better at emitting and absorbing radiation than light, shiny surfaces
- DThermal radiation from objects at room temperature lies mainly in the infrared region
- EThermal radiation requires a medium to propagate
22.Absolute zero is the temperature at which the molecules in a substance have zero kinetic energy.
Easy
True or false?
23.Convection can occur in solids as well as in liquids and gases.
Easy
True or false?
24.For a given substance, the specific latent heat of vaporisation is always higher than the specific latent heat of fusion.
Easy
True or false?
25.Thermal energy is always transferred from a hotter region to a cooler region.
Easy
True or false?
26.Metals are good thermal conductors because they contain a high number of free electrons.
Easy
True or false?
27.A change in temperature of 1 K is equal to a change in temperature of 1 °C.
Easy
True or false?
28.Match each phase change to its description.
Medium
- Melting
- Freezing
- Vaporisation
- Condensation
- Solid to liquid as thermal energy is absorbed
- Liquid to solid as thermal energy is released
- Liquid to gas as thermal energy is absorbed
- Gas to liquid as thermal energy is released
29.Match each term to its correct definition.
Medium
- Internal energy
- Specific heat capacity
- Specific latent heat
- Thermal equilibrium
- The sum of the total kinetic energy and total intermolecular potential energy of the particles in a substance
- The energy required to change the temperature of 1 kg of a substance by 1 K
- The energy required to change the state of 1 kg of a substance without changing its temperature
- When two substances in contact no longer exchange heat energy and both reach an equal temperature
30.Match each thermal transfer mechanism or property to its correct description.
Medium
- Conduction
- Convection
- Thermal radiation
- Perfect black body
- Main method of thermal transfer in solids, involving atomic vibrations and free electron collisions
- Transfer of thermal energy by the movement of groups of atoms or molecules due to density variations in a fluid
- Heat transfer by electromagnetic radiation, normally in the infrared region
- An object that absorbs all radiation incident on it and reflects or transmits none
31.Order the following stages of heating a solid substance from lowest to highest temperature, starting from the solid being heated to the point where it has completely vaporised.
Medium
- Solid heating up to its melting point
- Solid melting at constant temperature
- Liquid heating up to its boiling point
- Liquid boiling at constant temperature
32.Order the following states of matter from lowest to highest average particle energy.
Medium
- Solid
- Liquid
- Gas
33.In a refrigerator, 2 kg of water cools from 30 °C to 0 °C and then freezes to form ice also at 0 °C. The whole process takes 4000 seconds. The specific heat capacity of water is 4200 J kg⁻¹ K⁻¹. The specific latent heat of fusion of ice is 3.4 × 10⁵ J kg⁻¹. What is the power required by the refrigerator during this process?
Easy
- A233 W
- B240 W
- C932 W
- D260 W
34.A 2.1 kW kettle supplies energy to water of mass 1 kg. The initial temperature of the water is 20 °C. The specific heat capacity of water can be taken to be 4200 J kg⁻¹ K⁻¹. How long does it take for the water to start boiling?
Easy
- A80 s
- B120 s
- C160 s
- D210 s
35.Aluminium melts at 660 °C. A mass m of solid aluminium is initially at a temperature of 645 °C. The aluminium is heated and melts into liquid aluminium and continues to be heated up to a final temperature of 720 °C. Specific latent heat of fusion of aluminium = L_f. Specific heat capacity of solid aluminium = c_S. Specific heat capacity of liquid aluminium = c_L. Which expression gives the energy needed for this change to occur?
Medium
- Am(15c_S + L_f + 60c_L)
- Bm(15c_S + L_f + 720c_L)
- Cm(645c_S + L_f + 720c_L)
- Dm(15c_S + L_f + 60c_S)
36.Order the following stages of heating a solid substance from lowest to highest temperature, starting from the solid being heated to the point where it has completely melted.
Medium
- Solid at room temperature
- Solid at melting point
- Solid melting (temperature constant)
- Liquid just after melting
37.A block of metal has a mass of 2.0 kg and a volume of 2.5 × 10⁻⁴ m³. What is the density of the metal?
Medium
- A8.0 × 10³ kg m⁻³
- B5.0 × 10⁻⁴ kg m⁻³
- C1.25 × 10⁻⁴ kg m⁻³
- D8.0 × 10⁻³ kg m⁻³
38.A student investigates thermal conduction through a metal rod. Which of the following changes would increase the rate of thermal energy transfer through the rod? (select all that apply)
Medium
- AIncreasing the temperature difference across the rod
- BUsing a rod with a larger cross-sectional area
- CUsing a longer rod of the same material
- DUsing a material with a higher thermal conductivity
- EIncreasing the thickness of the rod
39.Which row correctly describes the arrangement and movement of particles in a gas?
Medium
- ARandomly arranged, moving at a variety of speeds in all directions
- BFixed pattern, vibrating about fixed positions
- CRandomly arranged, flowing past each other
- DClosely packed in a lattice, moving slowly
40.During a change of state, the temperature of a pure substance remains constant while thermal energy is supplied.
Easy
True or false?
41.What is the name of the process in which a gas changes directly into a liquid?
Easy
42.What is the name of the temperature scale on which absolute zero is equal to 0?
Medium
43.What is the name of the process by which thermal energy is transferred through a fluid due to variations in density?
Medium
44.Convert 27 °C to kelvin. Give your answer to the nearest whole number.
Medium
45.A substance has a specific heat capacity of 4200 J kg⁻¹ K⁻¹. Calculate the energy required to raise the temperature of 0.5 kg of the substance by 10 K. Give your answer in joules.
Medium
46.Calculate the energy required to melt 0.2 kg of ice at 0 °C. The specific latent heat of fusion of water is 3.3 × 10⁵ J kg⁻¹. Give your answer in joules.
Medium
47.What is the temperature in degrees Celsius of a gas at 300 K?
Medium
48.Complete the sentence about the definition of internal energy.
Easy
The internal energy of a substance is the sum of the total ____ energy and the total ____ energy of the particles within the substance.
49.Complete the sentence about conduction in metals.
Easy
Metals are good thermal conductors because they have a high number of ____ electrons.
50.Name the three states of matter.
Easy
The three states of matter are ____, ____ and ____.
51.A 0.50 kg block of aluminium is heated from 20 °C to 120 °C. The specific heat capacity of aluminium is 900 J kg⁻¹ K⁻¹. Calculate the thermal energy absorbed by the block. Give your answer in joules.
Medium
52.Complete the sentence about the specific heat capacity of a substance.
Easy
The specific heat capacity of a substance is the energy required to raise the temperature of 1 kg of the substance by ____.
53.Explain why the temperature of a substance remains constant during a change of state, even though thermal energy is being supplied.
Hard
54.Explain why thermal radiation is the only method of thermal energy transfer that can occur through a vacuum.
Hard
55.A hot meteorite hits the surface of the Moon. Identify and discuss the principal means by which the meteorite can dissipate thermal energy once it has landed.
Hard
56.Describe how a convection current forms when a fluid is heated from below.
Medium
57.State the definition of a perfect black body and explain how the radiation emitted by a black body changes as its temperature increases.
Medium
58.Convert 27 °C to kelvin. Give your answer to the nearest whole number.
Medium
- A270 K
- B300 K
- C330 K
- D360 K
59.A substance has a specific heat capacity of 4200 J kg⁻¹ K⁻¹. Calculate the energy required to raise the temperature of 0.5 kg of the substance by 10 K. Give your answer in joules.
Medium
- A21000 J
- B18900 J
- C25200 J
- D23100 J
60.Calculate the energy required to melt 0.2 kg of ice at 0 °C. The specific latent heat of fusion of water is 3.3 × 10⁵ J kg⁻¹. Give your answer in joules.
Medium
- A59400 J
- B66000 J
- C72600 J
- D79200 J
61.What is the temperature in degrees Celsius of a gas at 300 K?
Medium
- A30 °C
- B27 °C
- C33 °C
- D24 °C
62.A 0.50 kg block of aluminium is heated from 20 °C to 120 °C. The specific heat capacity of aluminium is 900 J kg⁻¹ K⁻¹. Calculate the thermal energy absorbed by the block. Give your answer in joules.
Medium
- A40500 J
- B54000 J
- C49500 J
- D45000 J

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