3.5 Ways of transferring thermal energy

• When an object is heated, energy is transferred to its thermal energy store
• Heating can take place in three different ways depending on the medium involved:

  • For solids, heat is transferred by conduction
  • For fluids, heat is transferred via convection
  • Heat is transferred through empty space in the form of radiation
• Conduction:

  • Vibrating particles transfer energy to neighboring particles
  • Particles vibrate faster and collide with more energy, passing along the energy until heat is spread out evenly
  • Conduction mainly occurs in solids due to closely held particles
• Thermal conductivity:

  • Metals have high thermal conductivity
  • Plastics have low thermal conductivity and are used as insulators
  • Pretty much all fluids have low thermal conductivity
• Convection:

  • Mainly occurs in fluids (liquids and gases)
  • Particles in fluids move around faster by random diffusion
  • Higher energy particles move away from the warmer region towards the cooler region, causing expansion and less density in the warmer fluid
• Convection current:

  • Particles near the heat source gain kinetic energy and spread out, becoming less dense
  • Cooler particles sink down and take their place
  • Hot particles lose energy and cool down, while cool particles at the bottom heat up
  • This cycle repeats as long as the fluid is being heated
• Radiation:

  • Heat energy can be transferred without particles through radiation
  • Energy is carried by infrared waves
  • Objects constantly absorb and emit radiation
  • Hotter objects emit more radiation
• Infrared radiation:

  • Objects emit and absorb infrared radiation
  • Hot objects emit more infrared radiation
  • Infrared radiation is absorbed by objects
• Conduction: Conduction is the transfer of thermal energy through a material without the material itself moving. Heat travels through the substance by direct contact between particles.
• Convection: Convection is the transfer of heat through the movement of fluids (liquids or gases). As the fluid heats up, it becomes less dense and rises, while cooler, denser fluid descends, creating a circular motion that facilitates heat transfer.
• Convection Current: A convection current is the circular movement of fluids caused by the transfer of heat. Hotter, less dense fluid rises, while cooler, denser fluid sinks, creating a continuous flow that aids in distributing heat.
• Expand: To expand means to increase in size, volume, or quantity. In the context of heat, substances often expand when heated because the increase in thermal energy causes the particles to move more vigorously, leading to greater spacing between them.
• Radiation: Radiation is the transfer of heat energy through electromagnetic waves, such as light or infrared radiation. Unlike conduction and convection, radiation does not require a medium and can occur through a vacuum.
• Vigorously: Vigorously means with great force, energy, or intensity. When particles move vigorously, they move rapidly and with significant kinetic energy, often leading to expansion or increased temperature.
• Thermal Energy: Thermal energy refers to the total amount of energy possessed by an object due to its temperature. It is measured in joules (J) and represents the sum of all the individual energies of the atoms and molecules within the object.
• Solids: When solids are heated, their particles vibrate faster around fixed positions. This increased vibration, indicative of greater thermal energy, causes the particles to take up more space and the solid to expand.
• Liquids: In liquids, particles are not fixed but still vibrate. When heated, the particles vibrate faster and move around more quickly. This increased movement results in the liquid taking up more space, leading to expansion.
• Gases: Gas particles are already spaced far apart and move freely until they collide with other particles or the container walls. Heating a gas increases the speed and frequency of particle collisions, causing the gas particles to move with more force and take up more space. Consequently, gases also expand when heated.
• Conduction: Thermal energy transfer through direct contact between particles in a substance. When particles vibrate, they push against neighboring particles, transferring energy. The faster the vibration, the stronger the push, leading to increased thermal energy transfer.
• Solids: Conduction is most effective in solids where particles are closely packed and can only move by vibrating. Metals are excellent conductors due to their tightly arranged particles and the presence of free-moving electrons, which aid in transferring vibrations.
• Insulators: Materials like wool, cotton, plastics, and wood are poor conductors, making them good thermal insulators. These materials are chosen for specific applications to regulate thermal energy transfer. For instance, metal pans heat food quickly due to their conductivity, while plastic or wooden handles prevent heat transfer, ensuring comfort.
• Limitations: Conduction is inefficient in liquids and gases. In liquids, particles move more freely when heated, and in gases, particles are spaced far apart, reducing the frequency of collisions. Additionally, conduction cannot occur in a vacuum, where there are no particles available to vibrate and transfer thermal energy.
• Thermal conductivity: A measure of how easily a material transfers thermal energy through conduction. Highly conductive materials have high thermal conductivities, allowing rapid transfer of thermal energy.
• Metals: Good conductors because of their close packing of atoms and delocalized electrons that facilitate the flow of thermal energy.
• Air: Poor conductor as it has low density and contains few molecules per unit volume, resulting in fewer opportunities for molecular collisions.
• Water: Moderately good conductor but less than metals.
• Wood: Poor conductor due to its porous structure and low density, resulting in fewer collisions between particles and slower transfer of thermal energy.
• Plastic: Insulator used in building construction to reduce heat gain/loss.
• Plastic: Insulator with very low thermal conductivity.
• Convection: Thermal energy transfer in liquids and gases through the movement of particles. When heated, particles move faster, occupying more space, which decreases density. Less dense portions rise, while denser regions sink, creating a circular flow known as a convection current.
• Density and Buoyancy: Increasing volume without changing mass lowers density. Less dense substances float in denser ones, driving the upward movement of warmer fluids in convection. Conversely, cooler, denser portions sink, creating a cycle of heating and mixing within the fluid.
• Process: In convection, heated portions rise, transferring thermal energy to cooler surroundings as they ascend. Cooled regions descend, absorbing heat and repeating the cycle. This process ensures thorough heating of the entire fluid.
• Applications: Convection currents are essential in various natural and artificial processes. In a room, warm air rises, creating circulation and evenly heating the space. Similarly, in a cooking pan, convection ensures even heating of water or other liquids.
• Limitations: Convection occurs in liquids and gases where particles are free to move but not in solids, where particles are fixed in position. Additionally, convection cannot occur in a vacuum due to the absence of particles necessary for movement.
• Radiation: A method of transferring thermal energy through waves that can traverse through vacuum or transparent mediums. Unlike conduction or convection, radiation does not require particles for transmission.
• Sun-Earth Interaction: In the vast vacuum of space between the Sun and Earth, thermal energy is transferred via radiation. This form of energy transfer occurs rapidly, with solar radiation reaching Earth in approximately nine minutes.
• Emission and Absorption: All objects emit and absorb thermal energy through radiation. Hotter objects emit more radiation, while cooler objects absorb radiation from hotter sources.
• Transmission: Radiation can pass through vacuum and transparent solids, liquids, and gases unhindered. Its ability to traverse mediums makes it a vital mechanism for thermal energy transfer over large distances.
• Emissivity and Absorptivity: The efficiency of radiation emission and absorption is influenced by an object's surface properties. Dull, black objects with larger surface areas are superior emitters and absorbers, while shiny, white, or silver surfaces reflect radiation, making them poor emitters and absorbers.
• Room Heater: Thermal energy from hot water flowing through the heater is transferred to the metal by conduction. The warm metal then heats the surrounding air through conduction. As the warmed air expands, it rises, heating the air above through convection. Additionally, the metal surface emits thermal energy through radiation.