Physics · Chapter 9
Study notes aligned to the official NEB syllabus.
All matter is made up of a very large number of tiny particles (atoms and molecules) that are in a state of continuous, random motion. Because of this motion each molecule possesses kinetic energy, and because of the forces between molecules each also possesses potential energy. The study of heat begins with the idea that these microscopic motions and interactions are what we experience, on a large scale, as hotness, warmth, and the flow of heat.
Three closely related ideas run through this chapter: thermal energy, heat, and temperature. They are often confused in everyday speech, but in physics each has a precise and distinct meaning.
Thermal energy (more completely, the internal energy) of a body is the total energy possessed by all of its molecules on account of their random motion and their mutual positions. It is the sum of:
The greater the number of molecules and the more vigorously they move, the larger the thermal energy of the body. Its SI unit is the joule ($\text{J}$).
Temperature is the physical quantity that measures the degree of hotness or coldness of a body. On the molecular scale, temperature is a measure of the average kinetic energy of the random (translational) motion of the molecules of the body.
For an ideal gas, the average translational kinetic energy of a single molecule is related to the absolute temperature $T$ by:
$$\bar{E}_k = \frac{3}{2}k_B T$$
where $k_B = 1.38 \times 10^{-23}\ \text{J K}^{-1}$ is the Boltzmann constant. This shows the key point: temperature depends on the average energy per molecule, not on how many molecules there are. Its SI unit is the kelvin ($\text{K}$).
Because it depends on the average and not the total, a cup of boiling water at $100^\circ\text{C}$ is at a higher temperature than a large bucket of warm water at $40^\circ\text{C}$, even though the bucket contains far more total thermal energy.
Heat is the thermal energy that is transferred from one body to another (or between parts of a body) solely because of a difference of temperature between them. Heat is therefore energy in transit. Once the energy has been delivered, it becomes part of the internal energy of the receiving body and is no longer called "heat". Its SI unit is the joule ($\text{J}$); an older unit is the calorie ($1\ \text{cal} = 4.186\ \text{J}$).
On the molecular level, the fast-moving molecules of the hotter body collide with the slower molecules of the colder body at the contact surface. In these collisions energy is handed on from the energetic molecules to the less energetic ones, so on average energy passes from hot to cold until the average molecular kinetic energies (and hence the temperatures) become equal.
All matter is made up of a very large number of tiny particles (atoms and molecules) that are in a state of continuous, random motion. Because of this motion each molecule possesses kinetic energy, and because of the forces between molecules each also possesses potential energy. The study of heat begins with the idea that these microscopic motions and interactions are what we experience, on a large scale, as hotness, warmth, and the flow of heat.
Three closely related ideas run through this chapter: thermal energy, heat, and temperature. They are often confused in everyday speech, but in physics each has a precise and distinct meaning.
Thermal energy (more completely, the internal energy) of a body is the total energy possessed by all of its molecules on account of their random motion and their mutual positions. It is the sum of:
The greater the number of molecules and the more vigorously they move, the larger the thermal energy of the body. Its SI unit is the joule ().
Temperature is the physical quantity that measures the degree of hotness or coldness of a body. On the molecular scale, temperature is a measure of the average kinetic energy of the random (translational) motion of the molecules of the body.
For an ideal gas, the average translational kinetic energy of a single molecule is related to the absolute temperature by:
where is the Boltzmann constant. This shows the key point: temperature depends on the average energy per molecule, not on how many molecules there are. Its SI unit is the kelvin ().
Because it depends on the average and not the total, a cup of boiling water at is at a higher temperature than a large bucket of warm water at , even though the bucket contains far more total thermal energy.
Heat is the thermal energy that is transferred from one body to another (or between parts of a body) solely because of a difference of temperature between them. Heat is therefore energy in transit. Once the energy has been delivered, it becomes part of the internal energy of the receiving body and is no longer called "heat". Its SI unit is the joule (); an older unit is the calorie ().
On the molecular level, the fast-moving molecules of the hotter body collide with the slower molecules of the colder body at the contact surface. In these collisions energy is handed on from the energetic molecules to the less energetic ones, so on average energy passes from hot to cold until the average molecular kinetic energies (and hence the temperatures) become equal.