Thermal Energy Transfer Notes

Transfer of Thermal Energy

  • Thermal energy can be transferred through three main methods:
    • Conduction
    • Convection
    • Radiation

Conduction

  • Energy is transferred by direct contact between objects or particles.

Convection

  • Energy is transferred by the mass motion of molecules within fluids (liquids and gases).

Radiation

  • Energy is transferred by electromagnetic radiation.

Objectives

  • Demonstrating Conduction
  • Conduction, Convection, Radiation
  • The Greenhouse Effect
  • Investigating IR Radiation
  • Consequences of Thermal Energy Transfer

Thermal Conductors

  • Good thermal conductors:
    • Materials that easily transfer heat.
    • Examples: metal pan, ceramic teacup.
  • Bad thermal conductors (insulators):
    • Materials that do not transfer heat well.

Explanation of Thermal Conductivity

  • Tiles and Stone:
    • Good conductors of heat.
    • Transfer heat away from the foot, making it feel cold.
  • Carpet/Rugs:
    • Good insulators (poor conductors of heat).
    • Heat is not readily transferred away from the foot, so the foot stays at its starting temperature.

Comparing Conduction

  • Experiment comparing conduction in wood and metal:
    • Materials: Wood, metal, paper, Bunsen flame.
    • Procedure: Wrap wood and metal with paper, hold above a cool Bunsen flame, and rotate.
    • Observation:
      • Metal side: Heat is transferred from the paper into the metal, preventing the paper from burning.
      • Wood side: Heat is not transferred from the paper, causing the paper to burn.

Conduction Mechanism

  • Conduction
    • Transfer of heat from one region to another through particle vibrations.
    • Movement of free/delocalized electrons (primarily in solids).

Thermal Conduction in Solids

  • Conduction occurs through two mechanisms:
    • Atomic vibrations: Faster vibrating ions collide with slower vibrating ions, transferring energy.
    • Free electron collisions: Free electrons transfer energy through the metal.

Requirements for Thermal Conduction

  • For thermal conduction to occur, particles need to be close together.
    • Vibrations are passed along
    • Liquids and gases are poor conductors of heat because their particles are more spread out.

Relative Thermal Conductivity

  • Better thermal conductors have delocalized electrons, which can easily transfer energy.
  • Examples of materials by conductivity (better conductors to better insulators):
    • Copper
    • Iron
    • Carbon
    • Water
    • Air
    • Glass
    • Rubber

Convection

  • Thermal energy transfer in liquids and gases ONLY.
  • Hot gases or liquids rise due to the change in density when heated (they become less dense).

Demonstrating Convection Currents

  • Demonstration: Using KMnO4 (potassium permanganate) in water.
    • KMnO4 dissolves and rises with warm water, showing convection current.
    • As water cools, it becomes more dense and falls, carrying KMnO4 with it.

Radiation

  • All hot objects give off thermal radiation.
  • Thermal radiation is infrared radiation (part of the electromagnetic spectrum).
  • Radiation is the only way heat can travel through a vacuum.
  • Does not need a medium to travel, unlike convection and conduction.

Factors Affecting Thermal Energy Transfer Rate

  • Surface color of the object:
    • Black = more radiation emitted and absorbed.
  • Texture of the surface:
    • Shiny surfaces = less radiation emitted and absorbed.

Examples of Radiation

  • Black objects absorb thermal radiation well (black clothes feel hotter in sunny weather).
  • Black objects emit thermal radiation well (laptop chargers and car radiators are colored black to help them cool down).

Infrared Radiation Detection

  • Infrared radiation emitted from a hot object can be detected using a special camera.
    • The dull black side of a cube glows brighter with infrared radiation than the shiny light side.
  • Shiny objects reflect thermal radiation and so absorb and emit very little.

Thermal Equilibrium

  • When heated:
    • Object absorbs thermal radiation and becomes hotter.
    • As it gets hotter, it emits more thermal radiation.
  • Thermal equilibrium:
    • Reached when the object is absorbing radiation at the same rate as it is emitting radiation.
    • At thermal equilibrium, an object has a constant temperature.

Greenhouse Effect

  • If the Earth had no atmosphere, the temperature on the surface would drop to about -180 °C at night because the surface would be emitting all the radiation from the Sun into space.

The Greenhouse Effect

  • The greenhouse effect:
    • Some gases in the atmosphere, such as water vapor, methane, and carbon dioxide (greenhouse gases), absorb radiation and then emit it back to the surface, preventing it from escaping into space.
    • These gases keep the Earth's temperature at a reasonable level.
    • Process makes the Earth warmer than it would be if these gases were not in its atmosphere.

Radiation and the Greenhouse Effect

  • Diagram illustrating the greenhouse effect:
    • Thermal radiation from the Sun is absorbed by the Earth.
    • Thermal radiation is emitted by the Earth.
    • Thermal radiation is reflected by greenhouse gases.

Impact of Excess Carbon Dioxide

  • Adding an excess of carbon dioxide to the Earth's atmosphere would lead to an increase in the surface temperature of Earth:
    1. Radiation from the Sun is absorbed by the Earth.
    2. Some is re-emitted by the Earth.
    3. CO_2 reflects a fraction of this emitted radiation and heat the Earth's surface.
    4. Excess CO_2 will cause more of the Earth's emitted radiation to be reflected back to the Earth.
    5. The rate of radiation absorbed by the Earth's surface increases, so its temperature increases.

Consequences of Energy Transfer

  • Good conductors help transfer thermal energy quickly:
    • Metal pans to heat food quickly
    • Metal radiators to transfer heat from water inside to the surrounding air quickly.

Convection Example

  • Which of the following is not an example of convection?
    • A a spoon handle becoming hot when it is placed in a hot drink (Correct Answer)
    • B a thermal up-draught in the atmosphere that glider pilots use
    • C flames and smoke rising from a fire
    • D warming up water in a kettle on a gas ring

Radiation Emission Example

  • The back of the hand held near side Y feels warmer than the hand held near side X.
  • Which row identifies the black side and correctly compares the rate of emission of thermal radiation from each side?
    • C Y; greater for Y (Correct Answer)

Consequences of Energy Transfer: Insulators

  • Bad conductors (insulators) help retain thermal energy as they transfer heat slowly:
    • Plastic handles of saucepans to slow thermal energy transferred to hands.
    • Air spaces in the walls or windows of some houses help to retain heat, as air is a poor conductor.

Double Glazed Window

  • Double Glazed Window vs SINGLE GLAZED WINDOW
    • Air insulates
    • Temperature comparision: Shows how double glazed windows work at retaining heat due to the air gaps

Examples of Convection

  • Steam rising which cools a hot liquid.:
    • STEAM RISES AWAY FROM THE COFFEE THROUGH THE PROCESS OF CONVECTION

Convection in Radiators

  • Radiators use convection to raise the temperature of a room in a building:
    1. The metal radiator is hot and transfers thermal energy to air nearby.
    2. The particles of this hot air spread out, making it less dense.
    3. The spread-out air is less dense than the air above it, so this hot air rises.
    4. The radiator heats the cold air which replaces the hot air .
    5. The newly heated air also rises, cools and sinks (as it contracts and increases in density)

Cooking Pan Materials

  • Which row is correct for the materials used to make the base of the pan and the handle of the pan?
    • base of pan: good thermal conductor
    • handle of pan: poor thermal conductor (Correct Answer)

Solar Heating Panels

  • Solar heating panels consist of pipes carrying water that absorb radiation from the Sun.
  • Which texture and colour is the surface of the pipes so that the temperature of the water rises at the quickest rate?
    • A dull black (Correct Answer)

Multiple Paths of Energy Transfer

  • Conduction: by direct contact
    • Coffee and solid sides of cup; cup to the surface it is sitting on
  • Convection: surface of coffee to air directly above it.
  • Radiation: from the sides of hot cup, in all directions to the surroundings

Wood/Coal Fire Example

  • Radiation: fuel in the fire transfers thermal energy through the room by radiation.
  • Air surrounding fire rises, forming a convection current.
  • Cooler, denser air replaces it

Car Radiator

  • Transfers heat away from engine.
  • Liquid travels between radiator and engine.
  • Over the engine, by conduction the liquids absorbs energy.
  • Transfers heat back to the radiator by conduction
  • Radiator transfers thermal energy to the surrounding by radiation