Scientists have documented the psychological and physical stresses of heat, which disproportionately impact lower-income people and countries. This is especially true when heat events are prolonged.

To learn more about the nature of heat, students can use this science lesson to investigate which material is the best insulator by conducting an experiment with bubble wrap and paper towel. They can also explore the distinction between heat and temperature by completing this worksheet.

What is Heat?

For centuries the concept of heat was thought to be a fluid that was contained within substances and always conserved. It was not until the late 19th century that scientists began to realize that this view of heat was flawed.

Rather than being a fluid, what we now know is that all objects possess internal energy which can be transferred to and from the environment by various processes. When such transfers occur, the internal energy of the object changes and this is what we now call heat.

All matter is composed of tiny particles known as atoms and molecules. These atoms and molecules are constantly bumping into each other and vibrating, which creates a form of energy called thermal energy. When this energy is transferred between two systems, the atoms in one system move into the other and they have to disperse their energy, which is what we call heat.

In thermodynamic terms, the process of transfer is defined by the law of conservation of energy and is characterized by an adiabatic component that leaves the body and a non-adiabatic component that enters the body. The body is able to convert its non-adiabatic energy into adiabatic thermal energy, which in turn can be converted to work, such as moving the body’s constraining walls, so that it can expand from its initial to its final volume. This energy is what we now call heat and it is measured in joules (J).

Heating

Heat is the flow of energy that changes the internal energy, or kinetic energy, of particles in matter. When particles gain more kinetic energy, they become hotter, and when they lose kinetic energy, they cool down. The process is called convection. All matter contains heat energy.

Physicist James Prescott Joule, who invented the standard joule unit, carried out several experiments to test the caloric theory of heat. In one experiment, he used falling weights to turn a paddle wheel submerged in water. He found that the amount of mechanical work done to turn the wheel was directly proportional to the temperature rise of the water.

When heated, matter expands. Solids expand the most, liquids expand less, and gases expand the least. The expansion of matter is a result of the transfer of energy from the hotter particles to the cooler ones.

Heat has a wide range of physiological impacts on humans, including increasing the risk of illness and death, amplifying existing conditions, and reducing productivity. High temperatures also cause disproportionate effects among the poor, who often live in areas with higher rates of extreme weather. In a recent study, researchers examined data from 21 million PSAT, or standardized tests taken by high school students, to correlate test scores with the daily maximum temperatures in each city where the students lived.

Cooling

Heat affects many aspects of human health, including physical functioning, mental health and well-being, and social systems. Its impacts range from the acute, such as injuries and deaths related to specific temperatures, to the long-term, such as decreased productivity and diminished quality of life. These effects are often considered in the context of climate adaptation planning efforts, including assessments of infrastructure and population vulnerabilities to climate extremes and development of plans to reduce their adverse impacts.

Until the nineteenth century, scientists generally thought of heat as a fluid that was either latent or stored in combustible materials and that could be released by those materials through thermal energy exchange. Noted chemist Antoine Lavoisier described caloric energy as being in two forms — latent and hidden within substances, and sensible, observable through a temperature change.

One example of a sensible form of heat is the kinetic energy of the particles in a gas. When the particles move, they have kinetic energy, and that kinetic energy can cause them to bump into each other. This can have a net negative effect on the particles, as they may be deflected from their original paths and sent in new directions.

The more energy a particle has, the more likely it is to bump into another molecule and send it in a different direction. This is why the particles in a gas are said to have a lot of energy.

Insulation

A material that prevents the transfer of heat from one place to another is called a thermal insulator. In general, insulators prevent the flow of heat by reducing conduction, convection and/or radiation.

Less dense materials insulate better than more dense ones. This is because the closer together the atoms are in a material, the more energy they have. Metals, for example, conduct electricity very well because the electrons can easily move from one atom to the next, but wood insulates well because its atoms are farther apart.

The insulating ability of different types of materials can be compared by comparing their thermal conductivity, also known as the k-value. A high k-value indicates a poor insulator, while a low k-value indicates a good insulator.

In a building, insulation reduces unwanted heating and cooling demands by slowing the flow of heat through walls, windows, floors and roofs. It also reduces the loss of energy by lowering heating and cooling costs.

In general, a higher R-value means a better insulator. However, there are a number of other factors to consider, including the temperature gradient across an assembly, whether it’s a wall or a ceiling, and what kind of air infiltration it has. In many cases the performance of a building is best evaluated using computer simulations. However, these can be expensive and time consuming to develop.