Thermal Energy Transfers

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Solids, Liquids & Gases

The three states of matter are solid, liquid and gas; the kinetic theory model treats particles as small spheres.
Solid: particles closely packed in a fixed lattice, only vibrate about fixed positions; fixed shape and volume, very difficult to compress, highest density.
Liquid: particles closely packed but randomly arranged, can flow past each other; fixed volume but no fixed shape, difficult to compress, medium density.
Gas: particles far apart and randomly arranged, moving in all directions at various speeds; no fixed shape or volume, easily compressed, lowest density.
Intermolecular forces are strongest in solids and weakest in gases: solid > liquid > gas.
Particle energy increases from solid to liquid to gas.

Particle spacing in solids, liquids and gases.

Density

Density is the mass per unit volume of an object: ρ = m / V.
If two objects occupy the same volume, the one with lower density has lower mass (e.g. feathers vs sand).
Units: g cm⁻³ when mass in g and volume in cm³; kg m⁻³ when mass in kg and volume in m³.
Gases are less dense than liquids due to their molecular arrangements.
Volume may need to be calculated from the dimensions of the object using the appropriate formula.

Measuring the Density of a Liquid

Temperature Scales

Absolute zero is the lowest possible temperature: 0 K or −273 °C.
Absolute zero is the temperature at which molecules have zero kinetic energy; no more energy can be removed from a system at 0 K.
Temperature in kelvin is never negative.
Convert with: T / K = θ / °C + 273, or θ / °C = T / K − 273.
The divisions on both scales are equal, so a change of 1 K equals a change of 1 °C.

Conversion chart relating the temperature on the Kelvin and Celsius scales

Temperature & Kinetic Energy

Molecules in a gas have a range of speeds.
For an ideal gas, average kinetic energy is Eₖ = (3/2)k_BT, where k_B = 1.38 × 10⁻²³ J K⁻¹.
Absolute temperature is directly proportional to the average kinetic energy of the molecules.
Equating (3/2)k_BT = ½mv² allows calculation of average molecular speed.

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

Internal energy is the sum of the total kinetic energy and the total intermolecular potential energy of the particles in a substance.
Thermal energy transferred to a substance can increase the average kinetic energy of its molecules, the potential energy of its molecules, or both.
Temperature is a measure of the average kinetic energy of the molecules.
Only a change in average kinetic energy changes the temperature; a change in average potential energy does not.
During all state changes (melting, boiling), only potential energy changes, so temperature stays constant.
A change in internal energy does not necessarily correspond to a change in temperature.

Internal energy of particles in a liquid

Thermal Equilibrium

Thermal energy is always transferred from a hotter region to a cooler region.
Thermal equilibrium is reached when two substances in contact no longer exchange heat energy and both reach an equal temperature.
The regions must be in thermal contact for equilibrium to occur.
The hotter region cools down and the cooler region heats up until they reach the same temperature.
Example: ice in room-temperature water — the ice gains energy and the water cools until equilibrium.

Changes of State

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

Particle arrangement and motion in solids, liquids and gases

Specific Heat Capacity

The thermal energy needed to change an object's temperature depends on the temperature change ΔT, the mass m, and the specific heat capacity c.
Equation: Q = mcΔT, where c is in J kg⁻¹ K⁻¹.
Specific heat capacity is the energy required to change the temperature of 1 kg of a substance by 1 K (or 1 °C).
The higher the specific heat capacity, the longer the substance takes to warm up or cool down.
Water has a high specific heat capacity (4200 J kg⁻¹ K⁻¹), so it heats and cools much slower than metals such as copper (390 J kg⁻¹ K⁻¹).
ΔT can be expressed in °C or K because temperature differences are the same on both scales.

Diagram of apparatus for investigating specific heat capacity

Specific Latent Heat

During a phase change, thermal energy is transferred without a change in temperature: Q = mL.
Specific latent heat is the energy required to change the state of 1 kg of a substance without changing its temperature; units J kg⁻¹.
Specific latent heat of fusion L_f applies to solid↔liquid changes; specific latent heat of vaporisation L_v applies to liquid↔gas changes.
For any given substance, L_v > L_f, because intermolecular forces must be completely overcome for vaporisation but only partially overcome for melting.
Heating/cooling curves: flat sections show phase changes (potential energy changes only); non-flat sections show temperature change (kinetic energy changes).
On a heating curve, temperature rises to the melting point, stays constant during melting, rises to the boiling point, then stays constant during boiling.

Thermal Conduction

Conduction is the main method of thermal energy transfer in solids; it occurs when two solids at different temperatures are in contact.
Metals are the best thermal conductors because they have a high number of free (delocalised) electrons.
Conduction occurs via atomic vibrations and free electron collisions; collisions transfer internal energy until thermal equilibrium is reached.
Non-metals (plastic, glass) are poor conductors; liquids and gases are even poorer because their atoms are further apart.
Thermal conductivity k quantifies the ability of a substance to transfer heat via conduction; units W m⁻¹ K⁻¹.
Rate of heat transfer: ΔQ/Δt = kAΔT/Δx, where A is cross-sectional area and Δx is thickness.
The rate of thermal energy flow is uniform across a temperature gradient if A is constant, analogous to current in a series circuit.

Conduction in metals

Thermal Convection

Convection is the main way heat travels through liquids and gases (fluids); it cannot occur in solids because particles cannot travel relative to one another.
When a fluid is heated from below, heated molecules gain kinetic energy, push apart, expand, and become less dense.
The less dense hot fluid rises, and cooler surrounding fluid moves in to take its place.
The hot fluid eventually cools, contracts and sinks, forming a convection current.
Examples: heating water with potassium permanganate crystals, atmospheric winds, sea breezes, convection currents in the Earth's mantle, ocean currents.

Thermal energy is transferred from hot coffee by convection

Thermal Radiation

All bodies emit a spectrum of thermal radiation as electromagnetic waves, usually in the infrared region.
Thermal radiation is heat transfer by electromagnetic radiation, normally in the infrared region.
The hotter the object, the more infrared radiation it radiates per unit time.
Thermal radiation is the only method of thermal energy transfer that does not require matter, so it is the only way heat can travel through a vacuum.
Dark, dull surfaces are better emitters and absorbers of radiation; light, shiny surfaces are worse.
The amount of radiation emitted depends on surface colour, texture, and surface area.
A perfect black body absorbs all radiation incident on it and does not reflect or transmit any radiation.

Hotter objects emit more thermal radiation

Black-Body Radiation & Wien's Law

Black-body radiation is the thermal radiation emitted by all bodies; it can be infrared, visible or other wavelengths depending on temperature.
A black-body radiation curve shows the intensity and wavelength distribution of emitted radiation.
The lower the wavelength at which peak intensity occurs, the higher the peak intensity.
Wien's Displacement Law: λ_max ∝ 1/T, so the peak wavelength is inversely proportional to absolute temperature.

Stefan-Boltzmann Law & Luminosity

The power output of a black body depends on its surface temperature and its radius.
Apparent brightness and luminosity relate to the radiation received from a star or black body.
The Stefan-Boltzmann law relates the power radiated to the surface temperature and surface area of a black body.

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1.What are the three states of matter?
Easy
- ASolid, liquid and gas
- BSolid, fluid and vapour
- CLiquid, gas and plasma
- DSolid, liquid and vacuum
2.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
3.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 has no change; average random kinetic energy decreases
- CAverage potential energy decreases; average random kinetic energy has no change
- DAverage potential energy has no change; average random kinetic energy has no change
4.Which of the following correctly identifies the properties of the molecules of a substance that determine the substance's internal energy?
Easy
- AThe total gravitational potential energy and random electrostatic potential energy
- BThe total potential energy and random kinetic energy
- CThe random kinetic energy only
- DThe total potential energy only
5.The specific latent heat of vaporisation is the energy required to change the phase of:
Easy
- Aone kilogram of a liquid to gas at constant temperature
- Ba solid at constant temperature
- Cone kilogram of a gas to liquid at constant temperature
- Da gas at constant temperature
6.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
7.In which type of material is conduction the main type of thermal transfer?
Easy
- AGas
- BLiquid
- CFluid
- DSolid
8.Which of the following statements about convection is correct?
Easy
- 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

Soal lainnya

9.Which region of the electromagnetic spectrum is responsible for thermal radiation from an object at room temperature?
Easy
- AUltraviolet
- BMicrowaves
- CInfrared
- DVisible light
10.What is the correct definition for a perfect black body?
Easy
- 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
11.Absolute zero is the temperature at which the molecules in a substance have zero kinetic energy.
Easy
True or false?
12.A change in temperature of 1 K is equal to a change in temperature of 1 °C.
Easy
True or false?
13.During a phase change, the thermal energy transferred changes only the potential energy of the molecules, not their average kinetic energy.
Easy
True or false?
14.Which of the following statements about the three states of matter are correct? (select all that apply)
Medium
- AParticles in a solid are arranged in a fixed pattern
- BParticles in a gas are closely packed in a lattice
- CLiquids have a fixed volume but not a fixed shape
- DGases have the lowest densities of the three states
- EParticles in a liquid cannot flow past each other
15.Which of the following are properties of a black body? (select all that apply)
Medium
- AIt absorbs all radiation incident on it
- BIt reflects most incident radiation
- CIt is the best possible emitter of thermal radiation
- DIt only emits radiation in the visible region
- EIts emission spectrum depends on its temperature
16.Which of the following factors affect the amount of thermal radiation emitted by an object? (select all that apply)
Medium
- AThe surface colour of the object
- BThe texture of the surface
- CThe surface area of the object
- DThe volume of the object
- EThe mass of the object
17.Match each phase change to its description.
Medium
- Melting
- Freezing
- Vaporisation
- Condensation
- Solid to liquid, absorbing thermal energy
- Liquid to solid, releasing thermal energy
- Liquid to gas, absorbing thermal energy
- Gas to liquid, releasing thermal energy
18.Match each key term to its definition.
Medium
- Density
- Thermal equilibrium
- Specific heat capacity
- Specific latent heat
- Mass per unit volume of an object
- When two substances in contact no longer exchange heat energy and reach equal temperature
- 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
19.Order these stages as a solid is heated at a constant rate from below its melting point until it becomes a gas.
Medium
- Solid heats up to its melting point
- Solid melts at constant temperature
- Liquid heats up to its boiling point
- Liquid boils at constant temperature
20.A change in temperature of 1 K is equal to a change in temperature of:
Easy
- A1 °C
- B273 °C
- C0 °C
- D1/273 °C
21.A solid is heated at a constant rate. Its temperature is recorded every minute. During the melting stage, the temperature:
Medium
- Aremains constant
- Bincreases linearly
- Cdecreases
- Dincreases then decreases
22.A substance has a specific latent heat of fusion of 2.0 × 10⁵ J kg⁻¹ and a specific latent heat of vaporisation of 4.0 × 10⁶ J kg⁻¹. For 0.5 kg of the substance, how much more energy is needed to vaporise it than to melt it?
Medium
- A1.9 × 10⁶ J
- B2.0 × 10⁶ J
- C2.1 × 10⁶ J
- D1.0 × 10⁵ J
23.A metal rod of length 0.5 m and cross-sectional area 2 × 10⁻⁴ m² has a temperature difference of 100 °C between its ends. If its thermal conductivity is 400 W m⁻¹ K⁻¹, what is the rate of heat transfer?
Medium
- A16 W
- B8 W
- C160 W
- D0.16 W
24.Which of the following statements about thermal radiation are correct? (select all that apply)
Medium
- AAll objects emit thermal radiation regardless of temperature.
- BThermal radiation is mainly in the infrared region for objects at room temperature.
- CThermal radiation requires a medium to propagate.
- DDark, dull surfaces are better emitters of thermal radiation than shiny surfaces.
- EThe rate of thermal radiation emission decreases as the object's temperature increases.
25.Which of the following factors affect the rate at which an object emits thermal radiation? (select all that apply)
Medium
- ASurface colour
- BSurface texture
- CSurface area
- DMass of the object
- EVolume of the object
26.Convection can occur in solids.
Easy
True or false?
27.The specific latent heat of vaporisation of a substance is always greater than its specific latent heat of fusion.
Medium
True or false?
28.Match each thermal energy transfer process to its primary mechanism.
Medium
- Conduction
- Convection
- Radiation
- Transfer by electromagnetic waves, mainly infrared
- Transfer through particle collisions and free electron movement
- Transfer by the bulk movement of a fluid due to density differences
29.Match each term to its correct definition.
Medium
- Specific heat capacity
- Specific latent heat of fusion
- Specific latent heat of vaporisation
- Energy required to change 1 kg of a substance from solid to liquid at constant temperature
- Energy required to raise the temperature of 1 kg of a substance by 1 K
- Energy required to change 1 kg of a substance from liquid to gas at constant temperature
30.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
31.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
32.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.
33.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
34.A liquid is initially at its boiling point. Energy is continuously supplied to the liquid at a uniform rate until it has completely evaporated. Which graph shows how the temperature of the liquid, T, varies with the energy, Q, supplied to the liquid?
Medium
- ATemperature remains constant and then increases.
- BTemperature increases linearly and then remains constant.
- CTemperature remains constant throughout.
- DTemperature increases and then decreases.
35.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?
Medium
- A233 W
- B240 W
- C932 W
- D260 W
36.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 = Lf; Specific heat capacity of solid aluminium = cS; Specific heat capacity of liquid aluminium = cL. Which expression gives the energy needed for this change to occur?
Medium
- Am(15cS + Lf + 60cL)
- Bm(15cS + Lf)
- Cm(Lf + 60cL)
- Dm(15cS + 60cL)
37.A student measures the temperature of a gas in a sealed container and finds it to be 27 °C. What is the average kinetic energy of the gas molecules? (Boltzmann constant k_B = 1.38 × 10⁻²³ J K⁻¹)
Medium
- A6.2 × 10⁻²¹ J
- B3.1 × 10⁻²¹ J
- C1.2 × 10⁻²⁰ J
- D4.1 × 10⁻²¹ J
38.Which of the following statements about a perfect black body are correct? (select all that apply)
Medium
- AIt absorbs all radiation incident on it.
- BIt reflects all radiation incident on it.
- CIt is a good emitter of thermal radiation.
- DIt transmits all radiation incident on it.
- EIt appears black at all temperatures.
39.Thermal radiation is the only method of thermal energy transfer that can occur through a vacuum.
Easy
True or false?
40.Complete the sentence about temperature scales.
Easy
The lowest possible temperature is called ____, which is equal to 0 K or ____ °C.
41.Name the three states of matter.
Easy
The three states of matter are ____, ____ and ____.
42.A paving slab has a mass of 73 kg and dimensions 40 mm × 500 mm × 850 mm. Calculate the density of the material, in kg m⁻³.
Medium
43.Determine the energy needed to melt 200 g of ice at 0 °C. The specific latent heat of fusion of water is 3.3 × 10⁵ J kg⁻¹. Give your answer in kJ.
Medium
44.A 2.1 kW kettle supplies energy to 1 kg of water initially at 20 °C. The specific heat capacity of water is 4200 J kg⁻¹ K⁻¹. How long does it take for the water to start boiling? Give your answer in seconds.
Medium
45.Name the region of the electromagnetic spectrum in which thermal radiation from a room-temperature object mainly lies.
Medium
46.State the name of the process by which thermal energy is transferred through a vacuum.
Medium
47.Calculate the energy required to melt 0.5 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
48.Complete the sentence about thermal energy transfer.
Medium
Thermal energy is always transferred from a ____ region to a ____ region until thermal equilibrium is reached.
49.A block of copper of mass 2.0 kg is heated from 20 °C to 120 °C. Calculate the energy transferred to the copper. The specific heat capacity of copper is 390 J kg⁻¹ K⁻¹. Give your answer in joules.
Medium
50.A 1500 W heater is used to melt ice at 0 °C. The specific latent heat of fusion of ice is 3.3 × 10⁵ J kg⁻¹. Calculate the mass of ice melted in 5 minutes. Give your answer in kg.
Medium
51.A metal rod of length 0.5 m and cross-sectional area 2 × 10⁻⁴ m² has a temperature difference of 100 °C between its ends. If its thermal conductivity is 400 W m⁻¹ K⁻¹, calculate the rate of thermal energy transfer through the rod in watts.
Medium
52.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
53.Explain why the temperature of a substance does not change during a phase change, even though thermal energy is being transferred to it.
Hard
54.Explain why metals are better thermal conductors than non-metals.
Medium
55.Explain why the temperature of a substance remains constant during a phase change, even though thermal energy is being transferred to it.
Hard
56.A paving slab has a mass of 73 kg and dimensions 40 mm × 500 mm × 850 mm. Calculate the density of the material, in kg m⁻³.
Medium
- A3865 kg/m³
- B4723 kg/m³
- C4294 kg/m³
- D5152 kg/m³
57.Determine the energy needed to melt 200 g of ice at 0 °C. The specific latent heat of fusion of water is 3.3 × 10⁵ J kg⁻¹. Give your answer in kJ.
Medium
- A73 kJ
- B66 kJ
- C59 kJ
- D80 kJ
58.A 2.1 kW kettle supplies energy to 1 kg of water initially at 20 °C. The specific heat capacity of water is 4200 J kg⁻¹ K⁻¹. How long does it take for the water to start boiling? Give your answer in seconds.
Medium
- A144 s
- B160 s
- C192 s
- D176 s
59.Calculate the energy required to melt 0.5 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
- A165000 J
- B198000 J
- C181500 J
- D148500 J
60.A block of copper of mass 2.0 kg is heated from 20 °C to 120 °C. Calculate the energy transferred to the copper. The specific heat capacity of copper is 390 J kg⁻¹ K⁻¹. Give your answer in joules.
Medium
- A93600 J
- B78000 J
- C70200 J
- D85800 J
61.A 1500 W heater is used to melt ice at 0 °C. The specific latent heat of fusion of ice is 3.3 × 10⁵ J kg⁻¹. Calculate the mass of ice melted in 5 minutes. Give your answer in kg.
Medium
- A0.36 kg
- B3.36 kg
- C2.36 kg
- D1.36 kg
62.A metal rod of length 0.5 m and cross-sectional area 2 × 10⁻⁴ m² has a temperature difference of 100 °C between its ends. If its thermal conductivity is 400 W m⁻¹ K⁻¹, calculate the rate of thermal energy transfer through the rod in watts.
Medium
- A17 W
- B18 W
- C15 W
- D16 W

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