IB DP Physics: HL复习笔记8.2.1 Conduction, Convection & Thermal Radiation

• Thermal energy transfers from hotter areas to cooler areas by the processes of:
  • Conduction
  • Convection
  • Radiation

• Objects will always lose heat until they are in thermal equilibrium (same temperature) with their surroundings
  • For example, a mug of hot tea will cool down until it reaches room temperature

Conduction

• Conduction is the main method of thermal energy transfer in solids
• Conduction occurs when:

Two solids of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the cooler object

• Metals are the best thermal conductors
  • This is because they have a high number of free electrons
• Non-metals, such as plastic or glass, are poor at conducting heat
  • Poor conductors of heat tend to be poor conductors of electricity
  • This suggests a link between the mechanisms behind both types of conduction
• Liquids and gases are even poorer thermal conductors
  • This is because the atoms are further apart, hence, the intermolecular forces are weaker

Conduction: the atoms in a solid vibrate and bump into each other

• Conduction can occur through two mechanisms:
  • Atomic vibrations
  • Free electron collisions

• When a substance is heated, the atoms, or ions, start to move around (vibrate) more
  • The atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end
  • As they do so they bump into each other, transferring energy from atom to atom
  • These collisions transfer internal energy until thermal equilibrium is achieved throughout the substance
  • This occurs in all solids, metals and non-metals alike

• Metals are especially good at conducting heat due to their high number of delocalised electrons
  • These can collide with the atoms, helping to transfer the vibrations through the material
  • This, therefore, allows metals to achieve thermal equilibrium faster than non-metals

Step 1: Use dimensional analysis to determine the equation for the number of free electrons

• • Units for number of free electrons per cubic centimetre, [n] = cm⁻³
  • Units for density, [ρ] = g cm⁻³
  • Units for Avogadro's number, [N_A] = mol⁻¹
  • Units for relative atomic mass, [A] = g mol⁻¹

[n]^a = [ρ]^b [N_A]^c [A]^d

(cm⁻³)^a = (g cm⁻³)^b (mol⁻¹)^c (g mol⁻¹)^d

• • The only unit present on both sides is cm⁻³, therefore:

a = b = 1

• • No other units are present on both sides, so:

c + d = 0

b + d = 0

∴ d = −1, c = 1

Step 2: Write out the equation for the number of free electrons per cubic centimetre

[n]¹ = [ρ]¹ [N_A]¹ [A]⁻¹

Step 3: Calculate the number of free electrons in each metal

• • Copper:

• • Steel:

• • Aluminium:

  

Step 4: Rank the metals from best thermal conductor to worst

• • Best thermal conductor = copper (highest number of free electrons)
  • Worst thermal conductor = aluminium (lowest number of free electrons)

Convection

• Convection occurs when:

A fluid is heated causing the movement of groups of atoms or molecules due to variations in density

• Convection is the main way that heat travels through liquids and gases
  • Convection cannot occur in solids
• When a fluid (a liquid or a gas) is heated from below:
  • The heated molecules gain kinetic energy and push each other apart, making the fluid expand
  • This makes the hot part of the fluid less dense than the surrounding fluid
  • The hot fluid rises, and the cooler (surrounding) fluid moves in to take its place
  • Eventually, the hot fluid cools, contracts and sinks back down again
  • The resulting motion is called a convection current

A convection current caused by the heat transfer from the fire

Step 1: Draw a diagram to illustrate the convection currentsStep 2: Describe the situation

• • One method of observing a convection current is by heating a beaker of water containing potassium permanganate crystals
  • Heat is initially transferred through the glass wall of the beaker by conduction
  • The water in the region of the Bunsen flame is heated and expands, becomes less dense and rises
    • This causes the dissolved purple crystal to flow up with the water
  • Meanwhile, when the water at the top of the beaker cools, it becomes denser again and falls
  • The process continues which leads to a convection current where heat is transferred through the liquid
    • The dissolved purple crystal follows this current which is what is observed during this experiment

• • Other examples of convection include:
    • Atmospheric convection / winds / sea breezes
    • Thunderheads (a cloud that appears before a thunderstorm)
    • Convection currents in the Earth's mantle (which can lead to continental drift)
    • Ocean currents
    • Solar conventions / sunspots / flares

Thermal Radiation

• All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves
• These electromagnetic waves usually lie in the infrared region of the spectrum
  • Black-body radiation can also be emitted in the form of visible light or other wavelengths, depending on the temperature
• Thermal radiation is defined as:

Heat transfer by means of electromagnetic radiation normally in the infrared region

• The hotter the object, the more infrared radiation it radiates in a given time
  • This is because atoms and molecules above absolute zero are in constant motion
  • Electric charges within the atoms in a material vibrate causing electromagnetic radiation to be emitted
  • Therefore, the higher the temperature, the greater the thermal motion of the atoms and the greater the rate of emission of radiation

• Thermal radiation is the only method of thermal energy transfer that does not require matter in order to move or propagate
  • Therefore, thermal radiation is the only way heat can travel through a vacuum

All objects above absolute zero emit infrared radiation

Step 1: Identify the types of thermal energy transfer

• • An object can lose energy through conduction, convection or radiation
  • In this case, the hot meteorite will only be able to lose energy via conduction and radiation

Step 2: Explain these choices

• • The meteorite can lose heat energy through conduction because it is in contact with the surface of the Moon
  • The Moon does not have an atmosphere, so convection is not possible
  • Infrared photons emitted by the meteorite are able to travel through a vacuum, so heat loss via radiation is possible