Thermal Conduction
- Thermal energy can be transferred from a hotter area to a cooler area through one of the following mechanisms:
- Conduction
- Convection
- Radiation
Thermal conduction, convection and radiation in a mug of tea
- Objects will always lose heat until they are in thermal equilibrium 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 also 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
Conduction of Heat in a Metal
During conduction, the atoms in a solid vibrate and collide with 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, or 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, increasing the rate of transfer of vibrations through the material
- This allows metals to achieve thermal equilibrium faster than non-metals
Worked example
Determine which of the following metals is likely to be the best thermal conductor, and which is likely to be the worst.
Metal | Density / g cm−3 | Relative atomic mass |
Copper | 8.96 | 63.55 |
Steel | 7.85 | 55.85 |
Aluminium | 2.71 | 26.98 |
Assume that each metal contributes one free electron per atom.
Answer:
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, [NA] = mol−1
- Units for relative atomic mass, [A] = g mol−1
[n]a = [ρ]b [NA]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 [NA]1 [A]−1
Step 3: Calculate the number of free electrons in each metal
- Avogadro constant, NA = 6.02 × 1023 mol−1 (this is given in the data booklet)
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)
Exam Tip
If a question mentions thermal energy transfers and metals, the answer will likely be about conduction!