Heat is a kind of energy which gives us the sensation of hotness. Its S.I. unit is the joule. Temperature is the amount of coldness or hotness of an object. When heat is given to an object, its temperature rises. The two words are not interchangeable, and telling them apart is the whole business of this topic.
Which energy do we need in order to dry grain and clothes and to cook food? What are the sources of that energy? The Sun is the main source of heat energy, and various jobs can be done with the heat obtained from it. Drying clothes, firewood and biskun, warming yourself in the sun in winter, and taking salt from sea water are all done with solar heat. Heat can also be obtained from sources other than the Sun. Firewood, fossil fuel and an electric heater are all sources of heat.
Your hand can rank, but it cannot measure
Look at the three glasses in the picture. One holds lukewarm water, another holds water straight from the tap, and the third holds very cold water. Touch the glasses and discuss for a moment: could you tell which one has tap water, which one is lukewarm and which one is very cold?
In that situation, just by touching the glasses, it is possible to say which one is lukewarm, which is tap water and which is very cold. But now suppose you were given one glass of lukewarm water, a second a little hotter, and a third of boiling water. It becomes hard to separate them, because lukewarm, a little hotter and boiling are all hot. Even though they are all hot water, the amount of hotness in each is different. The water with more hotness in it has the higher temperature, and the rate of vibration of the molecules in that water is higher.
That little experiment is not a warm-up. It is the argument for why thermometers exist at all. Your hand is quite good at putting things in order when they are far apart, so cold, tap and lukewarm are easy. It is hopeless at putting a number on any of them, and it fails completely once the three are close together or all on the same side of your own body temperature. Worse, your hand is not even consistent with itself. Put one hand in cold water and one in hot for a minute, then put both into the same bowl of tap water, and the same water will feel warm to one hand and cool to the other at the same moment. Two honest answers, one bowl, and no way to choose between them. That is precisely why science replaces a judgement with an instrument. A thermometer is not a more sensitive hand. It is a way of getting an answer that does not depend on whose hand it was.
Heat and temperature are not the same thing
In ordinary speech we use hot for both, and that is where the confusion begins. In science they are two different quantities with two different units, and once you have the distinction you have most of this unit.
| Heat | Temperature | |
|---|---|---|
| What it is | A kind of energy, which gives us the sensation of hotness | The amount of coldness or hotness of an object |
| Unit | Joule (J), because it is an energy | Degree Celsius, degree Fahrenheit, and the S.I. unit Kelvin |
| Does the amount of substance matter? | Yes. A large pot of warm water holds far more heat than a small cup of it | No. A cup and a pot at the same hotness have the same temperature |
| How to picture it | The total energy in the movement of all the molecules of the object | How fast those molecules are vibrating. Faster vibration means higher temperature |
| Measured with | Not measured directly in school. It is worked out from the temperature change | A thermometer |
Here is the comparison that settles the difference for good. A small cup of tea might be at eighty degrees Celsius, and a bucket of warm bathing water at forty. The tea is at the higher temperature, and nobody disputes that. But the bucket contains far more heat, because heat depends on how much substance there is as well as on how hot it is, and there are thousands of times more molecules in the bucket. If you wanted to warm a cold room, you would rather be given the bucket. If you wanted to burn your tongue, the cup would do it. So temperature is a measure of intensity, of how hot, while heat is a measure of quantity, of how much energy is in there altogether. That is why they need different units, and why a question asking for heat and a question asking for temperature are asking two different things.
The thermometer
Have you had your body temperature taken with a thermometer when a fever came on? The instrument needed for measuring the amount of hotness of any object is called a thermometer. The thermometer used for measuring the temperature of the body is called a clinical thermometer. These days, in order to measure a person's temperature in a clean and safe way, a digital thermometer is used in place of a clinical thermometer. A thermometer used in daily life or in the laboratory for measuring the temperature of various objects is called a laboratory thermometer.
How a laboratory thermometer is built
The outer part of a laboratory thermometer is cylindrical in shape and is made of glass. At one end of it there is a bulb with a thin glass wall, and joined to that is the stem. From the bulb a very fine tube begins and runs to the other end of the stem. This fine tube is called the capillary, and it is surrounded all round by the thick-walled glass of the stem. The bulb of the thermometer is filled with alcohol or with mercury. When the temperature rises or falls, the liquid placed in the thermometer goes down or up in the capillary tube. A laboratory thermometer is marked with numbers from minus ten degrees Celsius up to one hundred and ten degrees Celsius.
To measure the temperature of any object, the bulb of the thermometer is placed touching that object or inside it. When the bulb is placed touching an object, the effect of the heat in that object makes the volume of the liquid in the bulb increase or decrease. When the volume increases the liquid in the thermometer rises up the capillary tube, and when the volume decreases it goes down. The level to which the liquid in the thermometer has fallen or risen gives the value of the temperature of the object.
The whole instrument rests on one ordinary fact: a liquid takes up slightly more room when it is warmed. But slightly is the problem. Heating the alcohol in a thermometer bulb from twenty degrees to thirty changes its volume by only a tiny fraction, and if that extra liquid had to spread out across a wide tube it would rise by less than the thickness of a hair, and nobody could read it. Squeeze the same tiny extra volume into a capillary as fine as a thread and it has nowhere to go but upwards, so it climbs a visible distance up the stem. The thinness is not a manufacturing accident. It is a magnifying trick, and it is why the tube must be the same width all the way along, since a tube that widened in the middle would make the scale uneven and the readings wrong.
Units of temperature
Estimate for a moment what the temperature around you is at present. By measuring a person's body temperature you find out whether they have a fever or not. In which unit is body temperature written when it is measured like that? Generally our body temperature is measured in the unit degree Fahrenheit. Our body temperature in the normal state is 98.6 degrees Fahrenheit, but when a fever comes the body temperature rises, so the body temperature of a person with a fever is higher than 98.6 degrees Fahrenheit.
Temperature is also measured in the unit degree Celsius. Although degree Celsius and degree Fahrenheit are the units in common use, the S.I. unit of temperature is the Kelvin.
The book says a digital thermometer is now used in place of a clinical one for measuring people, in order to do it cleanly and safely, and both of those words are doing real work. Safe first: a traditional clinical thermometer is a thin glass tube containing mercury, and mercury is poisonous. If the glass breaks in a mouth or under an arm, small silvery beads scatter and are difficult to collect, and their vapour is harmful. A digital thermometer has no glass to break and no mercury in it at all. Clean second: a digital thermometer reads in seconds rather than minutes, its tip is easy to wipe between patients, and many are read without contact at all, which matters a great deal when one instrument is used for many people in a clinic.
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Heat is a kind of energy which gives us the sensation of hotness, and its S.I. unit is the joule.
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Heat Is a Kind of Energy
- It gives us the sensation of hotness. Its S.I. unit is the joule
- The Sun is the main source. Firewood, fossil fuel and heaters are also sources
- Drying clothes and biskun, warming in winter, taking salt from the sea
- Give heat to an object and its temperature rises