When objects are at the same temperature, they are in thermal equilibrium. There is no net flow of thermal energy between them.
Thermal energy always flows from a region of higher temperature to a region of lower temperature. Net flow occurs only when there is a temperature difference.
Thermal energy can be transferred through three processes:
Conduction
Convection
Radiation
Conduction is the transfer of thermal energy through a solid from a hotter region to a colder region.
Some materials are better thermal conductors than others. Good thermal conductors transfer thermal energy quickly, while bad thermal conductors (thermal insulators) transfer thermal energy slowly.
Metals are generally good thermal conductors. Non-metals such as glass, plastic, wood, wool, air, and water are bad thermal conductors (thermal insulators).
Examples of materials with varying thermal conductivity:
Best conductor: Diamond
Copper
Steel
Ice
Polythene
Fibreglass
Polystyrene
Air: Worst conductor
Fibreglass and polystyrene are good thermal insulators because they contain air.
Metals contain many free electrons, while non-metals do not.
In solids, particles vibrate about a fixed position, known as lattice vibration.
Particles at the hot end vibrate vigorously.
They collide with neighboring particles, making them vibrate more vigorously.
The kinetic energy of the vibrating particles at the hot end is transferred to the neighboring particles.
The neighboring region becomes hot.
Eventually, the particles at the cooler end vibrate vigorously, and the cooler end becomes hot.
Thermal energy is transferred without the transfer of particles.
Free electrons at the heated end absorb thermal energy and gain kinetic energy.
These free electrons move at greater speeds to the cooler regions of the metal, making them vibrate more vigorously.
As these electrons move, they collide with the atoms, transferring some of their kinetic energy to the atoms.
Thermal energy is transferred via the motion of the electrons. The cooler end becomes hot.
Metals are better thermal conductors due to both lattice vibrations and free electron diffusion.
Convection is the transfer of thermal energy in a fluid (liquid or gas) by means of convection currents due to a difference in density.
When the bottom of a flask of water is heated:
The water at the bottom expands.
The expanded water is less dense than the surrounding water.
The warmer, less dense water rises.
It cools down at the top of the flask, becomes denser, and sinks down again.
This process repeats until the whole flask of water is heated up.
When the air above a candle is heated:
The air expands.
The warm air is less dense than the surrounding air.
It rises out of the chimney.
Cooler, denser air sinks down to take the place of the warm air.
This movement of air forms convection currents.
Convection currents occur only in fluids (liquids and gases) because convection involves the bulk movement of the fluid.
In solids, particles are in fixed positions and cannot flow. They transfer thermal energy through lattice vibrations (and free electron diffusion in metals) without any bulk movement.
Thermal energy transfer through conduction occurs in liquids but is much slower compared to convection.
To prevent convection, a liquid must be heated from the top. For example, water at the top of a boiling tube boils while an ice cube at the bottom remains frozen because hot liquids rise instead of sink.
Thermal radiation is also known as infrared radiation. All objects absorb and emit infrared radiation, which is an invisible radiation that carries thermal energy.
Thermal radiation is the transfer of thermal energy in the form of invisible waves called infrared radiation, which can travel through a vacuum.
Unlike conduction and convection, infrared radiation does not require a medium to travel through. The Earth receives infrared radiation from the Sun.
When objects emit infrared radiation, their temperature decreases.
Good emitters give out infrared radiation at a faster rate and cool down more quickly than bad emitters.
When objects absorb infrared radiation, their temperature increases.
Good absorbers absorb infrared radiation at a faster rate than bad absorbers and heat up more quickly.
Dull black surfaces emit and absorb infrared radiation at a faster rate than shiny silver surfaces.
Shiny silver surfaces absorb less and reflect more infrared radiation.
The amount of infrared radiation absorbed or emitted depends on:
Surface color and texture
Surface temperature
Surface area
Dull and black surfaces emit and absorb infrared radiation at a faster rate than shiny and silver surfaces. Shiny and silver surfaces reflect more infrared radiation.
The higher the temperature of an object's surface relative to the surrounding temperature, the higher the rate of emission of infrared radiation.
For objects of the same mass and material, the object with the larger surface area will emit or absorb infrared radiation at a higher rate.
If an object absorbs energy at a greater rate than it emits energy, it is warming up.
If an object emits energy at a greater rate than it absorbs energy, it is cooling down.
If the rates of emission and absorption are the same, the temperature of the object will not change.
The temperature of the Earth is maintained at around 15^\circ C due to the greenhouse effect.
The greenhouse effect is a natural process that warms the Earth's surface through a balance of absorption and emission of infrared radiation.
Solar radiation from the Sun reaches the Earth.
Most of this radiation is absorbed by the Earth.
The Earth emits infrared radiation.
The atmosphere contains greenhouse gases (e.g., carbon dioxide, methane, water vapor). Some of the infrared radiation from the Earth is absorbed and re-emitted by these gases.
Human activity has increased the amount of greenhouse gases in the atmosphere, leading to increased thermal energy being radiated back to the Earth and causing global warming.