HEAT: FORMULAS AND UNITS, CHARACTERISTICS, HOW TO MEASURE IT, EXAMPLES - PHYSICAL - 2024

The heat in physics is defined as the thermal energy transferred whenever they contact objects or substances that are at different temperatures. This energy transfer and all the processes related to it, is the object of study of thermodynamics, an important branch of physics.

Heat is one of the many forms that energy takes, and one of the most familiar. So where does it come from? The answer lies in the atoms and molecules that make up matter. These particles inside things are not static. We can imagine them as small beads linked by soft springs, capable of shrinking and stretching with ease.

Atoms and molecules vibrate within substances, which is translated into internal energy. Source: P. Tippens. Physics: Concepts and Applications.

In this way, the particles are able to vibrate and their energy can be easily transferred to other particles, and also from one body to another.

The amount of heat that a body absorbs or releases depends on the nature of the substance, its mass and the difference in temperature. It is calculated like this:

Where Q is the amount of heat transferred, m is the mass of the object, C _e is the substance's own specific heat and ΔT = _final T - _initial T, that is, the temperature difference.

Like all forms of energy, heat is measured in joules, in the International System (SI). Other appropriate units are: ergs in the cgs system, Btu in the British system, and the calorie, a term commonly used for the energy content of foods.

Heat characteristics

The heat from the campfire is energy in transfer. Source: Pixabay

There are several key concepts to keep in mind:

-Heat is about energy in transit. Objects do not have heat, they only release or absorb it depending on the circumstance. What objects do have is internal energy, by virtue of their internal configuration.

This internal energy, in turn, is composed of kinetic energy associated with vibratory movement and potential energy, typical of the molecular configuration. According to this configuration, a substance will transfer heat more or less easily and this is reflected in its specific heat C _e, the value that was mentioned in the equation to calculate Q.

-The second important concept is that heat is always transferred from the hottest body to the coldest one. Experience indicates that the heat from hot coffee always passes towards the porcelain of the cup and plate, or the metal of the spoon with which it is stirred, never the other way around.

-The amount of heat transferred or absorbed depends on the mass of the body in question. Adding the same amount of calories or joules to a sample with X mass does not heat in the same way another whose mass is 2X.

The reason? There are more particles in the larger sample, and each one would be receiving on average only half the energy of the smaller sample.

Thermal equilibrium and conservation of energy

Experience tells us that when we put two objects at different temperatures in contact, after a while the temperature of both will be the same. Then it can be stated that the objects or systems, as they can also be called, are in thermal equilibrium.

On the other hand, reflecting on how to increase the internal energy of an isolated system, it is concluded that there are two possible mechanisms:

i) Heating it, that is, transferring energy from another system.

ii) Carry out some kind of mechanical work on it.

Taking into account that energy is conserved:

In the framework of thermodynamics, this conservation principle is known as the First Law of Thermodynamics. We say that the system must be isolated, because otherwise it would be necessary to consider other energy inputs or outputs in the balance.

How is heat measured?

Heat is measured according to the effect it produces. Therefore it is the sense of touch that quickly informs how hot or cold a drink, a food or any object is. Since transferring or absorbing heat results in changes in temperature, measuring this gives an idea of ​​how much heat has been transferred.

The instrument used to measure temperature is the thermometer, a device equipped with a graduated scale to carry out the reading. The best known is the mercury thermometer, which consists of a fine capillary of mercury which expands when heated.

A thermometer with graduation in Celsius and Fahrenheit scales. Source: Pixabay.

Next, the mercury-filled capillary is inserted into a glass tube with a scale and is placed in contact with the body, the temperature of which must be measured until they reach thermal equilibrium and the temperature of both is the same.

What is required to make a thermometer?

To begin with, you need to have some thermometric property, that is, one that varies with temperature.

For example, a gas or a liquid such as mercury, expand when heated, although an electrical resistance also serves, which emits heat when a current passes through it. In short, any thermometric property that is easily measurable can be used.

If the temperature t is directly proportional to the thermometric property X, then it can be written:

Where k is the constant of proportionality to be determined when two appropriate temperatures are set and the corresponding values ​​of X are measured. Appropriate temperatures mean easy to obtain in the laboratory.

Once the pairs (t ₁, X ₁) and (t ₂, X ₂) have been established, the interval between them is divided into equal parts, these will be the degrees.

Temperature scales

The selection of the temperatures necessary to construct a temperature scale is made with the criterion that they are easy to obtain in the laboratory. One of the most widely used scales around the world is the Celsius scale, created by the Swedish scientist Anders Celsius (1701-1744).

The 0 on the Celsius scale is the temperature at which ice and liquid water are in equilibrium at 1 atmosphere of pressure, while the upper limit is chosen when liquid water and water vapor are equally in equilibrium and at 1 atmosphere of pressure. This interval is divided into 100 degrees, each of which is called a degree centigrade.

This is not the only way to build a scale, far from it. There are other different scales, such as the Fahrenheit scale, in which the intervals have been chosen with other values. And there is the Kelvin scale, which only has a lower limit: absolute zero.

Absolute zero corresponds to the temperature at which all movement of particles in a substance ceases completely, however, although it has come quite close, it has not yet been able to cool any substance to absolute zero.

Examples

Everyone experiences heat on a daily basis, either directly or indirectly. For example when you have a hot drink, in the midday sun, examining the temperature of a car engine, in a room full of people and in countless other situations.

On Earth, heat is necessary to maintain life processes, both that which comes from the Sun and that which comes from the interior of the planet.

Likewise, the climate is driven by changes in thermal energy that occur in the atmosphere. The Sun's heat does not reach everywhere equally, at equatorial latitudes it reaches more than at the poles, so the hottest air in the tropics rises and moves north and south, to achieve thermal balance that was talked about before.

In this way, air currents are established at different speeds, which transport clouds and rain. On the other hand, the sudden collision between hot and cold air fronts causes phenomena such as storms, tornadoes and hurricanes.

In contrast, at a closer level, the heat may not be as welcome as a sunset on the beach. Heat causes operating problems in car engines and computer processors.

It also causes electrical energy to be lost in conduction cables and materials to expand, which is why heat treatment is so important in all areas of engineering.

Exercises

- Exercise 1

The label of a candy reads that it provides 275 calories. How much energy in joules is this candy equivalent to?

Solution

At the beginning, the calorie had been mentioned as a unit for heat. Food contains energy that is usually measured in these units, but dietary calories are actually kilocalories.

The equivalence is the following: 1 kcal = 4186 J, and it is concluded that the candy has:

275 kilocalories x 4186 joule / kilocalorie = 1.15 10 ⁶ J.

- Exercise 2

100 g of a metal are heated to 100 ° C and placed in a calorimeter with 300 g of water at 20 ° C. The temperature that the system acquires when it reaches equilibrium is 21.44 ° C. You are asked to determine the specific heat of the metal, assuming the calorimeter does not absorb heat.

Solution

In this situation, the metal gives up heat, which we will call Q _given and a sign (-) is placed before it to indicate loss:

For its part, the water in the calorimeter absorbs heat, which will be denoted as Q absorbed:

Energy is conserved, from which it follows that:

From the statement you can calculate ΔT:

Important: 1 ºC is the same size as 1 kelvin. The difference between the two scales is that the Kelvin scale is absolute (Kelvin degrees are always positive).

The specific heat of water at 20ºC is 4186 J / kg. K and with this the absorbed heat can be calculated:

To conclude, the specific heat of the metal is cleared:

References

1. Bauer, W. 2011. Physics for Engineering and Sciences. Volume 1. McGraw Hill.
2. Cuellar, JA Physics II: Approach by Competences. McGraw Hill.
3. Kirkpatrick, L. 2007. Physics: A Look at the World. 6 ^ta Editing abbreviated. Cengage Learning.
4. Knight, R. 2017. Physics for Scientists and Engineering: a Strategy Approach. Pearson.
5. Tippens, P. 2011. Physics: Concepts and Applications. 7th Edition. Mcgraw hill