Before I can begin answering it, we must firststart by asking what thermal energy is. Allsubstances are made up of tiny particles which canhold energy. The particles will move faster whenthey gain thermal energy in the form of heat. Thechange in thermal energy of a substance isrepresented as a change in its temperature. Hotsubstances are made up of particles with highthermal energy.
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As heat is removed from thesubstance (like putting it into a refrigerator),the particles lose their thermal energy and beginto move slower. As a result, the temperature ofthe substance decreases.
The best way to measure the thermal energy ofan object is to use a thermometer. A thermometeris a glass tubefilled with a liquid (mercury) which expands orcontracts depending on the temperature of theobject it is in contact with. It measures theaverage kinetic energy (one type of thermalenergy) of the molecules of a substance indegrees Fahrenheit (F) or Celsius (C). Not allthe molecules have the same amount of kineticenergy because the molecules are traveling inrandom directions at a variety of speeds, fast andslow. This is why an average temperature istaken. What we measure as the temperature isalways related to the average speed of themolecules in an object.
Today, scientistsuse electronic sensors connected to high-techcomputers or portable machines to measuretemperature. The most common type of temperaturesensor is a thermistor. Thermistors (orthermo-resistors) are thermally sensitiveresistors that can measure temperatures veryaccurately and easily. A computer or othercircuit measures the resistance of the object andconverts it to a temperature digitally. They arevery affordable, durable, and used in a widevariety of applications that require temperaturemeasurement and control. Large scalethermistorsare used in electronic heating/air-conditioningsystems to keep homes warm in the winter and coolin the summer. Smaller-sized thermistorsare usedto measure our body temperature when we get afever. Other temperature sensors include RTDs (resistance temperature detectors) andthermocouples. Scientists like to use thesesensors because they can find temperatures fasterand with greater precision than with ordinarythermometers.
Thermal energy can be described as the amount ofenergy a body stored due to its being at a highertemperature than its surroundings . It is measuredusing something called a calorimeter.Usually, theheat released by some body at some hightemperature to its surroundings is compared withthe known heat that a fixed amount of water at thesame high temperature would release to thesurroundings. The unit of heat energy is Joule,though the older unit of calorie is alsoused.
Thermal energy is just the movement of moleculesin an object. And as you can imagine, the morejiggling there is, the more the molecules bumpinto each other, and the more the molecules spreadapart. Because of this, as you make thingshotter, they tend into actually get bigger -though some materials get bigger than others.
Sothe simplest way to measure thermal energy, is totake a glass tube and fill it up with some kind ofliquid. As the tube heats up, the liquid heats upand expands and rises in the tube. This lets youcompare the amount of thermal energy in twodifferent things. This is exactly how they usedto make thermometers, which measure temperature,though the liquid they used inside the tubes wascalled mercury and is very dangerous to touch. That"s what temperature is, though. When you sayis is 70 degrees outside, you are telling someonehow much thermal energy there is in theair.
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There are other ways to measuretemperature because the amount that molecules andatoms jiggle also affects other things - forexample, how well metals conduct electricity. Soby measuring how much electricity passes throughwires made of certain materials, you can tell howhot the wires are. And if you bring the wire intocontact with hot water, for example, you can tellhow hot the water is.