Heat,
heating and thermal energy
"It's a hot day for Spring, you can really feel the heat! I
hope the air-conditioner is working this summer, as the heating system
did not work this winter and I was really cold. I had to wear my thermals
to school just to keep warm!"
This is the way that we generally use words like hot, cold and heating
and thermal in everyday life. In science though we need to be more precise
with our language to get hold of the concepts of heat, heating and thermal
energy as understood by scientists. Do not worry, the normal language
is pretty close, you just have to be careful how each word is used.
Thermal energy
Thermal energy is the energy in a body due to its temperature.
If you are hot because you have been out in the Sun, then scientifically
you have increased thermal energy. This energy within the body or object
includes kinetic energy from vibrations and movement of atoms and molecules
and potential energy associated with the state (solid, liquid or gas)
of the body. In short any energy that is temperature dependent.
Thermal energy and temperature
When something is cold it has less thermal energy than when it is hot,
but thermal energy is not temperature! Temperature is a measure of the
average kinetic energy of the particles in a body. Thermal energy is
the heat energy in the whole body. This takes into account the temperature,
type of material and mass of the body. For example, a lump of steel
with a mass of 100kg will have twice the thermal energy of a 50kg piece
of steel, if their average temperatures are the same. Thermal energy
is a function of temperature, type of material and mass.
Thermal energy also is dependent on the type of material the object
is made of. Some substances, like water require a lot more energy to
heat up than others like fat. The measure of this is known as "Heat
Capacity". The Heat Capacity of a substance is the energy required
to heat 1 gram of the substance by 1°C.
Heating
Heating is the process of transferring energy from one body to
another due to a difference in temperature between the bodies. Heating
can occur by conduction, convection and radiation. For example, when
someone sits out in the sunshine, they are undergoing a heating process,
increasing their thermal energy. The three processes of thermal energy
transfer are discussed in detail in the topic - Heating.
Heat
Heat is the quantity of energy that has been transferred during
the heating process. If you stay out in the Sun for an hour instead
of 5 minutes, you will be heated for longer and your body will have
increased its thermal energy more. The quantity of energy transferred
to you, heat, will be greater.
Using thermal energy
Here are a few examples of how thermal energy can be described:
- Your body transforms stored potential energy in the food you eat
into thermal energy which keeps our bodies at 37.5°C.
- The daily temperature usually rises as radiant energy from the
Sun is absorbed by the atmosphere increasing the thermal energy of
the air.
- The Sun is a major source of energy as discussed above. In
fact, the Sun supplies the vast majority of thermal energy and
chemical potential energy available to us on Earth. The Sun
directly lights our day, warms our air, drives the wind and
weather. Indirectly all the available energy in food, fossil
fuels, wood and any living or once living thing has gained its
energy from the Sun. In plants this is done by photosynthesis
and in animals by eating plants or other animals which have
eaten plants.
- The Earth also transforms other forms of energy into thermal
energy from activity deep within the Earth - this energy is
very obvious when volcanoes erupt or in the activity of hot
springs.
- Cooking is another use of thermal energy where conduction
transfers energy to the food to be cooked. Take cooking a cake;
the ingredients are mixed, poured into a metal container, then
popped into the oven - a source of thermal energy converted
from electrical energy, gas or wood. The energy is transferred
to the metal by conduction, then into the mixture where a chemical
change takes place to produce that delicious cake you enjoy
so much!
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Remember - Heat is
not a form of energy
The correct uses of the word "Heat" are:
- As a verb, describing the process of
- Transfer of energy from one body to another due to a difference
in temperature. eg.
"Heat up the water for the tea, please."
"You're cold, sit by the fire and heat yourself up a bit."
- As a noun, describing the quantity of energy that has been
transferred during a heating process.
"By sitting next to the fire I was able to feel the heat."
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