Principles of Heat

People refer to temperature when they describe the weather or food. Scientists, however, would define temperature as a measure of the average kinetic energy of the particles in a substance.

The SI unit for temperature is Kelvin, but degrees Celsius is also used. You may also be familiar with the Fahrenheit temperature scale, but it is not often used in science. When the temperature of a substance goes up, the kinetic energy of its particles increases. This energy, combined with its potential energy, is called thermal energy.

Particles not only have thermal energy, they also transfer energy to their surroundings. A person who stands in the sunlight or takes a warm shower can feel the thermal energy from the Sun or water. The transfer of thermal energy is called heat, which is measured using the SI unit joule (J), the same unit that is used for all other forms of energy. Thermal energy is a form of energy that is transferred as heat.

Heat is the flow of thermal energy between substances at different temperatures. The direction of heat flow is always from hotter to colder substances. The hotter substance cools as it warms the cooler substance. Heating continues until the energy is distributed uniformly throughout both substances and both substances are at the same temperature. Heat can also cause change in the state of a substance-it can cause a solid to change to a liquid or a liquid to change to a gas. In each case, the kinetic energy of the particles increases.

Early scientists and philosophers thought heat was a substance that flowed from one substance to another. Later observations and experiments showed that heat results from the movement of particles. Scientists observed that the source of heat does not determine the effect heat has on its surroundings. Whether from chemical reactions, friction, nuclear reactions, or electricity, heat has the same effect.

Think about Science

Directions: Answer the following questions.

  1. [ blank ] measures the average kinetic energy of the particles in a substance.
  2. [ blank ] is the flow of energy that warms a person standing next to a campfire.
  3. Which unit listed below describes the amount of thermal energy transferred from one substance to another? A. degrees Fahrenheit B. Kelvin C. joules D. degrees Celsius
  4. One kilogram of which of these substances will have the lowest average thermal energy? A. tap water B. ice C. ice water D. steam

Kinetic Energy Transfer

Kinetic energy is transferred as heat by three methods: conduction, convection, and radiation. You may have experienced conduction if you have ever been burned by a hot substance, such as the handle of a heated pan. You can observe convection by adding hot water to one end of a full bathtub and noticing that the water in the entire tub becomes warmer, not just the water near the faucet. You can feel heat transferred by radiation by putting your hands near a radiator or a fire.


Conduction is the transfer of thermal energy between particles that collide with each other. When these particles make contact, the particle with more kinetic energy passes some energy to the particle with less energy. For example, electric stove burners transfer thermal energy by conduction to the bottom of a pan of water. The hot pan’s particles transfer their energy to the cooler water in the pan. Another example occurs when heat is conducted from your hand to a cold snowball. In scientific terms, the snow does not make your hand cold. Instead, your hand warms the snow, and because the thermal energy is transferred, your hand feels cooler. In all cases of conduction, thermal energy is transferred by the collisions between particles resulting from contact.


Unlike particles in solids, particles in liquids and gases can mix and flow past one another. This allows heat to be transferred within the substance as particles with more thermal energy move and mix with particles with less thermal energy. The transfer of thermal energy by the movement and mixing of particles is called convection. Convection occurs in gases and liquids.

Particles with more thermal energy move faster and tend to spread apart. When thermal energy increases in a fluid, the fluid expands. When the fluid expands, the density of the warmer fluid is less than the density of the cooler fluid that surrounds it. Think about how an electric burner conducts heat to a pan of water. The water being warmed at the bottom of the pan becomes less dense than the cooler water above it. The cool, dense water sinks, pushing the warm water upward. That warm water then cools and sinks to the bottom of the pan, and the process continues. The rising and sinking action of the water is a convection current.



Thermal energy is transferred through the air to your hand when you put your hand near a radiator or fireplace. This energy is transferred by radiation, which is thermal energy transferred as waves. Unlike conduction and convection, radiation does not need matter to travel through, so it can travel through empty space. The Sun’s energy radiates through space to Earth 150 million kilometers away.

Think about Science

Directions: Fill in the blank.

  1. An ice-cube in a cup of water is another example of [ blank ]. Cold water melting from the ice-cube [ blank ]. As the cooler, denser water sinks, it pushes the warm water to the [ blank ] where it cools and continues the cycle.
  2. You walk across the sand on the beach. The heat from the sand is transferred to your feet by [ blank ]. Your skin feels warm as the Sun heats it by [ blank ].

Energy Conversions Involving Heat

What happens when you rub a piece of wood with a cloth compared to when you rub the same piece of wood with sandpaper? The amount of friction produced when a cloth is rubbed across a piece of wood is much less than it is when sandpaper is used. This is because the friction between two surfaces depends on the types of surfaces involved and how hard the surfaces are pressed together. When a larger force is pushing two substances together, particles on the surfaces that would not normally come into contact do so, creating more friction between the substances.

If you continue to rub the cloth or sandpaper against the piece of wood, your hand would start to feel warm because friction converts the kinetic energy of rubbing to heat. Which would heat up faster, the cloth or the sandpaper? Because the sandpaper produces more friction, sanding the wood will produce more heat.


Think about Science

Directions: Answer the following questions.

  1. Why does snow melt when you walk on it?
  2. Why does a spacecraft get very hot as it re-enters the atmosphere?
  3. You erase a mistake on your paper. Why is your paper warm?