Topic B1: Thermal Energy Transfers

Thermal Energy

the total kinetic energy of particles in a system due to their random motion

Heat

the transfer of energy from one system to another due to a temperature difference.

systems….

move towards thermal equilibrium, where no net energy is transferred

Conduction

Energy transfer through particle collisions in a medium

Convection

Energy transfer via fluid motion

Radiation

Energy transfer through electromagnetic waves (does not require a medium)

Solids (Molecular Behaviour)

Particles vibrate about fixed positions; strong intermolecular forces

Liquids (Molecular Behaviour)

Particles move more freely; weaker intermolecular forces than solids

Gases (Molecular Behaviour)

Particles move randomly at high speeds; negligible intermolecular forces

Key Assumptions of the Kinetic Theory

Molecules are in constant random motion, Collisions between particles are elastic (no energy loss), Average kinetic energy is proportional to absolute temperature (T)

Internal Energy (definition)

Total energy of a system, including both kinetic and potential energy of particles

Internal Energy (for an ideal gases)

For an ideal gas, U is purely kinetic and proportional to T

Temperature (defintion)

A measure of the average kinetic energy of particles in a system, does not depend on the total amount of matter

Celcius (°C)

Freezing 0, Boiling 100

Kelvin (K)

Absolute temperature scale with 0K as the point where particle motion theoretically stops

Conversion (Celcius to Kelvin)

T(K)=T(°C)+273.15

Absolute Zero

The lowest possible temperature, 0 K or −273.15 C, where particles have minimal energy.

Relationship with Energy (Kinetic energy and Kelvin temperature)

The kinetic energy of a particle is directly proportional to the Kelvin temperature (Ek​∝T(K))

Specific Heat Capacity (definition)

The energy required to raise the temperature of 1 kg of a substance by 1 K

Specific Heat Capacity Equation

Q=mcΔT

Q=

Thermal Energy (J)

m=

Mass (Kg)

c=

Specific Heat Capacity (JKg^-1K^-1)

ΔT=

Change in Temperature (K)

Latent Heat (Definition)

Energy required to change the phase of a substance without changing its temperature.

For solid ↔ liquid transitions

Latent heat of fusion (Lf)

For liquid ↔ gas transitions

Latent heat of vaporisation (LV)

Latent Heat Equation

Q = mL

L =

Latent Heat (JKg^-1)

Features During Phase Changes

potential energy changes while kinetic energy remains constant, temperature remains constant during phase transitions (e.g., melting, boiling)

Heat Conduction Equation

ΔQ/Δt = kAΔT/​Δx

Heat Conduction Equation

Rate of Heat Transfer Through a Material via Conduction

k =

Thermal conductivity ( Wm^-1K-1)

A =

Area (m²)

​Δx =

Thickness (m)

Conduction features

Energy transfer due to particle collisions and/or free electron movement (in metals)

Convection Features

Energy transfer by the bulk movement of a fluid due to density differences, occurs in liquids and gases, forming convection currents (e.g., boiling water, sea breezes)

Radiation Features

Energy transfer via electromagnetic waves, primarily infrared, does not require a medium

Stefan-Boltzmann Law

P=eσAT^4

Stefan-Boltzmann Law meaning

power radiated by a black body due to thermal radiation

e =

emissivity (0 ≤ e ≤ 1)

σ =

Stefan-Boltzmann Constant

Wein’s Displacement Law

λmax​T=2.90×10^−3mK

Wein’s Displacement Law

the relationship between the temperature of a black body and the wavelength at which it emits radiation most intensely

b =

Weins Constant, 2.90×10^−3mK

λmax =

Maximum wavelength

Black Body (Definition)

An idealised object that absorbs all incident radiation and emits energy at all wavelengths

Perfect Emitter →

Emissivity = 1

Black Body Radiation meaning

the electromagnetic radiation emitted by a perfect black body

Black Body Radiation features

Depends only on temperature, not material composition, higher temperature leads to more radiation and shorter peak wavelength

Relationship between Temperature and Peak intensity on wavelength vs intensity graphs

As temperature increases, Peak intensity increases

Relationship between Temperature and Peak wavelength on wavelength vs intensity graphs

As temperature increases, Peak wavelength decreases (λmax​)