IB Physics HL B.1 Thermal energy transfers Notes

Use the particle model to connect macroscopic properties to molecular behaviour. Fixed particle positions explain why solids keep shape. Mobile but close particles explain why liquids keep volume but flow. Large spacing and random motion explain why gases expand to fill a container and are compressible. Melting, freezing, boiling, evaporating, and condensing involve changes in molecular separation and potential energy.

Compare solids, liquids, and gases in terms of particle arrangement, spacing, and motion. Then explain why temperature remains constant during a phase change.

Density connects a macroscopic measurement to particle spacing. For the same mass, a smaller volume means a larger density. Gases have low density and are compressible because there is much empty space between particles. In calculations, most mistakes come from unit conversion rather than the formula itself.

Define density and state the SI unit. Explain why gases are usually much less dense than solids or liquids using the particle model.

Celsius is convenient for everyday reference points, but Kelvin is the thermodynamic temperature scale used in physics equations that depend on absolute temperature. Because the scales have the same interval size, ΔT in kelvin has the same numerical value as Δt in degrees Celsius. The common trap is adding 273 to a temperature change or using Celsius in equations that need absolute temperature.

Explain the relationship between the Celsius and Kelvin scales, including absolute zero and the numerical size of a temperature change.

There are two different tasks: converting a temperature reading and converting a temperature change. A temperature reading needs the 273 offset because Kelvin and Celsius have different zero points. A temperature change does not need the offset because both scales step by the same amount. Always look for Δ before deciding what to do.

A sample warms from 20 °C to 55 °C. State the initial temperature in kelvin and the temperature change in kelvin.

The kinetic theory link is about microscopic random motion. A hotter ideal gas has faster molecules on average, but individual molecules still have a range of speeds. Do not use Celsius in this relationship, and do not confuse average energy per molecule with total energy, which also depends on the number of molecules.

State how the average random translational kinetic energy of ideal gas molecules depends on temperature, and explain why the temperature must be in kelvin.

Internal energy is a microscopic energy store of the whole sample. Heating within a phase usually increases the random kinetic component, so temperature rises. During a phase change, energy changes the potential component as particles separate or bond, so temperature can remain constant while internal energy changes.

Define internal energy and explain why internal energy can change during a phase change even when temperature is constant.

Temperature tells the direction of net energy transfer, not the amount of energy stored. A small hot object can have less internal energy than a large cooler object, but net thermal transfer still begins from hot to cold. Mechanisms such as conduction, convection, and radiation explain how the transfer occurs; this card focuses on the direction.

A small metal ball at 90 °C is placed in a large water bath at 30 °C. State the direction of net thermal energy transfer and describe when the transfer stops.

Phase change is not just “particles get hotter”. While a pure substance is melting or boiling, added energy goes into changing the molecular arrangement and separation rather than increasing average kinetic energy. After the phase change is complete, further energy transfer within one phase changes temperature again.

Explain why the temperature of a pure substance remains constant while it is boiling, even though energy is being supplied.

The decision is made from the physical process, not from the numbers given. A sloped section of a heating curve uses Q = mcΔT because temperature changes. A plateau uses Q = mL because the state changes while temperature stays constant. Use latent heat of fusion for solid-liquid changes and latent heat of vaporisation for liquid-gas changes.

A sample is heated from below its melting point to above it. Explain how to decide which parts of the calculation use Q=mcΔT and which use Q=mL.

The mechanism depends on what is carrying the energy. In conduction, neighbouring particles or free electrons pass energy through a material. In convection, warmer and cooler regions of a fluid move because of density differences. In radiation, energy is carried by electromagnetic waves, so no particles are needed between source and absorber.

Distinguish conduction, convection, and radiation in terms of how energy is transferred and whether a material medium is required.

At the hot end of a solid, particles vibrate more strongly. These vibrations pass energy to neighbouring particles, so energy moves through the solid from hot to cold. Metals conduct especially well because mobile free electrons can move through the lattice and transfer kinetic energy rapidly. The particles of the solid do not travel from one end to the other as a bulk flow.

Explain how thermal energy is transferred by conduction along a metal rod heated at one end.

The equation applies when the temperature difference is maintained and the slab has uniform material and thickness. The temperature difference can be in kelvin or degrees Celsius because it is a change. Use the magnitude of ΔT for the rate; direction is from the hotter side to the colder side.

A wall has area A, thickness L, thermal conductivity k, and temperature difference ΔT across it. State the conduction-rate equation and explain one way to reduce the rate of energy transfer.

Convection begins with a temperature difference inside a fluid. Heating reduces density in one region, so buoyancy makes that warmer fluid rise while cooler fluid sinks. This circulation carries internal energy through the fluid. The key distinction from conduction is that the material itself moves in convection.

Explain how a convection current forms in water heated from below.

Black-body radiation is thermal electromagnetic radiation from an ideal absorber and emitter. The model removes material details: the spectrum depends on temperature, not on paint colour or surface chemistry. For a fixed area, the total power emitted rises with the fourth power of absolute temperature, so T must be in kelvin.

Define a black body and state how the total power it emits depends on surface area and absolute temperature.

Apparent brightness describes what reaches the detector, not how much power the source produces in total. If a source radiates equally in all directions, its luminosity is spread over a sphere whose area grows as d^2. That geometric spreading gives the inverse-square relation.

A star has luminosity L and is distance d from Earth. Define apparent brightness and state the equation linking b, L, and d.

The model has two different areas. The star surface area A controls how much power the star emits, through the Stefan-Boltzmann law. The sphere of radius d around the star controls how that emitted power is spread out before it reaches the observer. Keeping R and d separate prevents most calculation errors.

A star may be modelled as a black body of radius R and surface temperature T. State an expression for its luminosity and then for its apparent brightness at distance d.

On a black-body spectrum, λ_max is the wavelength at which the emitted intensity is greatest. Wien’s law says this peak wavelength is inversely proportional to absolute temperature. The law is useful for estimating the surface temperature of stars from the wavelength at the peak of their spectrum.

A star approximates a black body and its spectrum peaks at wavelength λ_max. State Wien’s displacement law and explain how the peak changes if the star is hotter.

B.1 is strongest when each calculation is tied to its physical model. Start with the particle description of matter, decide whether energy changes temperature or phase, identify the transfer mechanism, then use the radiation and luminosity equations only under their black-body or inverse-square assumptions.