Thermal Properties and Energy Transfer Notes

2.2 Thermal properties and temperature

2.2.1 Thermal expansion of solids, liquids, and gases

• When something is heated, it expands because the molecules take up more space.
• Solids:
  • When a solid is heated, the molecules vibrate more but stay in place. The expansion is relatively small.
• Liquids:
  • When a liquid is heated, it expands for the same reason as a solid.
  • Intermolecular forces are less, so it expands more than solids.
• Gases:
  • When a gas is heated, the molecules move faster (increase in the average kinetic energy of the molecules) and further apart.
  • The expansion is the greatest.

Table comparing thermal expansion:

Applications and Consequences of Thermal Expansion:

• Railway tracks have small gaps to prevent buckling when they expand.
• The liquid in a thermometer expands with temperature and rises up the glass.

2.2.2 Specific Heat Capacity

Rise in Temperature and Increase in Internal Energy

• Internal Energy: The total energy stored in an object due to the motion and arrangement of its particles.
• Effect of Temperature Rise: When the temperature of an object increases, its internal energy increases because the particles move faster.
  • Solids: Particles vibrate more rapidly.
  • Liquids and Gases: Particles move faster and more freely.

Thermal Capacity (Heat Capacity)

• Specific heat capacity (c): Energy required per unit mass per unit temperature increase, measured in joules per kilogram per degree Celsius (J/(kg°C)).
• Finding specific heat capacity:
  • Insulate the material and place a thermometer and an immersion heater inside.
  • Divide the total work done by the heater (power (current times potential difference) times time) by the mass of the material and the change in temperature.
• Thermal capacity: How much energy needs to be put in to raise its temperature by a given amount.
• Thermal capacity of a system is given by: Thermal_capacity = mc

2.2.3 Melting, boiling and evaporation

Melting and Boiling

• Melting: The process where a solid turns into a liquid.
  • During melting, temperature does not change until all the solid has melted, even though energy is still being absorbed.
• Boiling: The process where a liquid turns into a gas throughout the entire volume of the liquid.
  • Boiling happens at a specific temperature, and like melting, the temperature does not change during the process until all the liquid has boiled.
• Melting point of water: 0℃ (273 K) at standard atmospheric pressure
• Boiling point of water: 100℃ (373 K) at standard atmospheric pressure

Condensation and Solidification

• Condensation (Gas to liquid): As gas cools, particles lose energy and move close together, forming a liquid. It’s the reverse of boiling.
• Solidification (Liquid to solid): When a liquid cools, its particles lose kinetic energy and settle into a fixed position, forming a solid. This is the reverse of melting.

Evaporation

• Evaporation: The process where particles at the surface of a liquid escape into the gas phase.
  • Particles with higher kinetic energy escape first, leaving behind particles with lower energy
  • Unlike boiling, evaporation happens at any temperature.

Evaporation Causes Cooling

• Evaporation cools the liquid because the most energetic particles leave, reducing the average energy of the remaining particles.
• The decrease in energy leads to a lower temperature.

Differences Between Boiling and Evaporation

Factors Affecting Evaporation

• Temperature: Higher temperature increases particle energy, speeding up evaporation.
• Surface Area: Larger surface area allows more particles to escape, increasing the rate of evaporation.
• Air Movement: Moving air (wind) carries away evaporated particles, preventing saturation and promoting further evaporation.

2.3 Transfer of thermal energy

• 2.3.1 Conduction

• 2.3.2 Convection

• 2.3.3 Radiation

• 2.3.4 Consequences of thermal energy transfer

• Energy will always try to flow from areas at high temperatures to areas at low temperatures. This is called thermal transfer.

2.3.1 Conduction

• Good Conductors: Metals like copper and aluminum transfer heat quickly.
  • Example: A metal rod heated at one end will transfer heat to the other quickly, making the whole rod hot.
• Bad Conductors (Insulators): Materials like wood, plastic, and air do not transfer heat well.
  • Example: Place a metal spoon and a wooden spoon in a cup of hot water. The metal spoon becomes hot faster, while the wooden spoon remains cool.

2.3.2 Convection

• Convection is a major method of thermal energy transfer in liquids and gases (fluids).
• It occurs when warmer, less dense fluid rises and cooler, denser fluid sinks, forming a cycle.

2.3.3 Radiation

• Thermal radiation is mainly infra-red waves (chapter 3) but very hot objects also give out light waves.
• Infra-red radiation is part of the electromagnetic spectrum.
• Effect of Surface Color and Texture on Infrared Radiation:
  • Black surfaces: Absorb and emit infrared radiation more efficiently than white surfaces.
  • Shiny surfaces: Reflect infrared radiation making them poor emitters and absorbers.
  • Dull surfaces: Are better absorbers and emitters compared to shiny ones.

2.3.4 Consequences of thermal energy transfer

Basic everyday application of Conduction, Convection, and Radiation:

• Heating Objects via Conduction, Convection and Radiation.
• Heating a room by convection
  • Heater warms air, warm air rises, cooler air sinks. Cycle repeats. Convection current.