Specific heat is the energy required to increase the temperature of 1 gram of a pure substance by 1 °. The parameter depends on its chemical composition and state of aggregation: gaseous, liquid or solid. After its discovery, a new round of development of thermodynamics began, the science of transient energy processes that relate to heat and the functioning of a system.
Usually, specific heat and the basics of thermodynamics are used in the manufacture radiators and systems designed for cooling cars, as well as in chemistry, nuclear engineering and aerodynamics. If you want to know how the specific heat is calculated, then check out the proposed article.
Formula
Before proceeding with the direct calculation of the parameter, you should familiarize yourself with the formula and its components.
The formula for calculating the specific heat is as follows:
- c = Q / (m * ∆T)
Knowledge of the quantities and their symbolic designations used in the calculation is extremely important. However, it is necessary not only to know their visual appearance, but also to clearly understand the meaning of each of them. The calculation of the specific heat capacity of a substance is represented by the following components:
ΔT is a symbol that means a gradual change in the temperature of a substance. The "Δ" character is pronounced delta.
ΔT can be calculated using the formula:
ΔT = t2 – t1, where
- t1 - primary temperature;
- t2 is the final temperature after the change.
m is the mass of the substance used for heating (gr).
Q - amount of heat (J / J)
On the basis of Tsr, other equations can be derived:
- Q = m * cp * ΔT - amount of heat;
- m = Q / cr * (t2 - t1) - the mass of the substance;
- t1 = t2– (Q / cp * m) - primary temperature;
- t2 = t1 + (Q / cp * m) - final temperature.
Definition and formula of the amount of heat
The internal energy of a thermodynamic system can be changed in two ways:
- doing work on the system,
- through thermal interaction.
The transfer of heat to the body is not associated with performing macroscopic work on the body. In this case, the change in internal energy is caused by the fact that individual molecules of the body with a higher temperature are doing work on some molecules of the body, which has a lower temperature. In this case, thermal interaction is realized due to thermal conductivity. Energy transfer is also possible through radiation. The system of microscopic processes (related not to the whole body, but to individual molecules) is called heat transfer. The amount of energy that is transferred from one body to another as a result of heat transfer is determined by the amount of heat that is transferred from one body to another.
Definition
Warmth
is called the energy that is received (or given away) by the body in the process of heat exchange with the surrounding bodies (environment). Heat is indicated, usually by the letter Q.
This is one of the basic quantities in thermodynamics. Heat is included in the mathematical expressions of the first and second principles of thermodynamics. Heat is said to be energy in the form of molecular motion.
Heat can be communicated to the system (body), or it can be taken away from it. It is believed that if heat is imparted to the system, then it is positive.
Instructions for calculating the parameter
Calculate from
the substance is quite simple and to do this, you need to follow these steps:
- Take the calculation formula: Heat capacity = Q / (m * ∆T)
- Write out the initial data.
- Plug them into the formula.
- Calculate and get the result.
As an example, let's calculate an unknown substance weighing 480 grams and having a temperature of 15ºC, which, as a result of heating (35 thousand J), increased to 250º.
According to the instructions given above, we perform the following actions:
We write out the initial data:
- Q = 35 thousand J;
- m = 480 g;
- ΔT = t2 – t1 = 250–15 = 235 ºC.
We take the formula, substitute the values and solve:
c = Q / (m * ∆T) = 35 thousand J / (480 g * 235º) = 35 thousand J / (112800 g * º) = 0.31 J / g * º.
Quantity of heat
The amount of heat is the energy that the body loses or gains during heat transfer. This is also clear from the name. When cooling, the body will lose a certain amount of heat, and when heated, it will absorb. And the answers to our questions showed us what does the amount of heat depend on? First, the greater the body mass, the more heat must be spent on changing its temperature by one degree. Secondly, the amount of heat required to heat a body depends on the substance it consists of, that is, on the kind of substance. And thirdly, the difference in body temperatures before and after heat transfer is also important for our calculations. Based on the above, we can determine the amount of heat by the formula:
Q = cm (t_2-t_1),
where Q is the amount of heat, m is the mass of the body, (t_2-t_1) is the difference between the initial and final temperatures of the body, c is the specific heat capacity of the substance, is found from the corresponding tables.
Using this formula, you can calculate the amount of heat that is necessary to heat any body or that this body will release when it cools.
The amount of heat is measured in joules (1 J), like any type of energy. However, this value was introduced not so long ago, and people began to measure the amount of heat much earlier. And they used a unit that is widely used in our time - a calorie (1 cal). 1 calorie is the amount of heat required to heat 1 gram of water by 1 degree Celsius. Guided by these data, those who like to count the calories in food eaten can, for the sake of interest, calculate how many liters of water can be boiled with the energy they consume with food during the day.
Payment
Let's perform the calculation CP
water and tin under the following conditions:
- m = 500 grams;
- t1 = 24ºC and t2 = 80ºC - for water;
- t1 = 20ºC and t2 = 180ºC - for tin;
- Q = 28 thousand J.
To begin with, we determine ΔT for water and tin, respectively:
- ΔТв = t2 – t1 = 80–24 = 56ºC
- ΔТо = t2 – t1 = 180–20 = 160ºC
Then we find the specific heat:
- с = Q / (m * ΔТв) = 28 thousand J / (500 g * 56ºC) = 28 thousand J / (28 thousand g * ºC) = 1 J / g * ºC.
- s = Q / (m * ΔTo) = 28 thousand J / (500 g * 160ºC) = 28 thousand J / (80 thousand g * ºC) = 0.35 J / g * ºC.
Thus, the specific heat of water was 1 J / g * ºC, and that of tin was 0.35 J / g * ºC. Hence, we can conclude that with an equal value of the supplied heat of 28 thousand J, tin will heat up faster than water, since its heat capacity is less.
Heat capacity is possessed not only by gases, liquids and solids, but also by food.
The formula for calculating the heat when the temperature changes
The elementary amount of heat will be denoted as. Note that the heat element that the system receives (gives up) with a small change in its state is not a complete differential. The reason for this is that heat is a function of the process of changing the state of the system.
The elementary amount of heat that is imparted to the system, and the temperature changes from T to T + dT, is equal to:
where C is the heat capacity of the body. If the body under consideration is homogeneous, then formula (1) for the amount of heat can be represented as:
where is the specific heat of the body, m is the mass of the body, is the molar heat, is the molar mass of a substance, and is the number of moles of the substance.
If the body is homogeneous, and the heat capacity is considered independent of temperature, then the amount of heat () that the body receives with an increase in its temperature by an amount can be calculated as:
where t2, t1 is the body temperature before and after heating.Please note that the temperatures when finding the difference () in the calculations can be substituted in both Celsius and Kelvin.
How to calculate the heat capacity of food
When calculating the power capacity the equation takes the following form:
c = (4.180 * w) + (1.711 * p) + (1.928 * f) + (1.547 * c) + (0.908 * a), where:
- w is the amount of water in the product;
- p is the amount of proteins in the product;
- f is the percentage of fat;
- c is the percentage of carbohydrates;
- a is the percentage of inorganic components.
Determine the heat capacity of Viola processed cream cheese... To do this, write out the required values from the composition of the product (weight 140 grams):
- water - 35 g;
- proteins - 12.9 g;
- fats - 25.8 g;
- carbohydrates - 6.96 g;
- inorganic components - 21 g.
Then we find with:
- c = (4.180 * w) + (1.711 * p) + (1.928 * f) + (1.547 * c) + (0.908 * a) = (4.180 * 35) + (1.711 * 12.9) + (1.928 * 25 , 8) + (1.547 * 6.96) + (0.908 * 21) = 146.3 + 22.1 + 49.7 + 10.8 + 19.1 = 248 kJ / kg * ºC.
What determines the amount of heat
The internal energy of the body changes when doing work or heat transfer. With the phenomenon of heat transfer, internal energy is transferred by heat conduction, convection or radiation.
Each body, when heated or cooled (during heat transfer), receives or loses some amount of energy. Based on this, it is customary to call this amount of energy the amount of heat.
So, the amount of heat is the energy that the body gives or receives in the process of heat transfer.
How much heat is needed to heat water? Using a simple example, you can understand that different amounts of heat are required to heat different amounts of water. Let's say we take two test tubes with 1 liter of water and 2 liters of water. In which case is more heat required? In the second, where there are 2 liters of water in the test tube. The second tube will take longer to heat up if we heat them with the same fire source.
Thus, the amount of heat depends on the body weight. The larger the mass, the more heat is required for heating and, accordingly, the body needs more time to cool.
What else does the amount of heat depend on? Naturally, from the temperature difference between bodies. But that's not all. After all, if we try to heat water or milk, then we will need a different amount of time. That is, it turns out that the amount of heat depends on the substance of which the body is composed.
As a result, it turns out that the amount of heat that is needed for heating or the amount of heat that is released when the body cools down depends on its mass, on temperature changes and on the type of substance that makes up the body.
Useful Tips
Always remember that:
- the process of heating the metal is faster than that of water, since it has CP
2.5 times less; - if possible, convert the results to a higher order, if conditions permit;
- in order to check the results, you can use the Internet and look at for the calculated substance;
- under the same experimental conditions, more significant temperature changes will be observed for materials with a low specific heat.
Formula for the amount of heat during phase transitions
The transition from one phase of a substance to another is accompanied by the absorption or release of a certain amount of heat, which is called the heat of phase transition.
So, to transfer an element of matter from the state of a solid to a liquid, it should be told an amount of heat () equal to:
where is the specific heat of fusion, dm is the body mass element. It should be taken into account that the body must have a temperature equal to the melting temperature of the substance under consideration. During crystallization, heat is released equal to (4).
The amount of heat (heat of vaporization) that is needed to convert a liquid into vapor can be found as:
where r is the specific heat of vaporization. When steam condenses, heat is released. The heat of vaporization is equal to the heat of condensation of equal masses of matter.
How to calculate the amount of heat to heat the body
For example, it is necessary to calculate the amount of heat that needs to be spent in order to heat 3 kg of water from a temperature of 15 ° C to a temperature of 85 ° C. We know the specific heat of water, that is, the amount of energy that is needed to heat 1 kg of water by 1 degree. That is, in order to find out the amount of heat in our case, you need to multiply the specific heat capacity of water by 3 and by the number of degrees by which you need to increase the water temperature. So this is 4200 * 3 * (85-15) = 882,000.
In parentheses, we calculate the exact number of degrees, subtracting the initial
So, in order to heat 3 kg of water from 15 to 85 ° C, we need 882,000 Joules of heat.
The amount of heat is indicated by the letter Q, the formula for calculating it is as follows:
Q = c * m * (t2-t1).
What is specific heat
Each substance in nature has its own properties, and heating each individual substance requires a different amount of energy, i.e. the amount of heat.
Specific heat of a substance Is a value equal to the amount of heat that needs to be transferred to a body with a mass of 1 kilogram in order to heat it to a temperature of 1 0C
Specific heat is denoted by the letter c and has a measurement value of J / kg *
For example, the specific heat capacity of water is 4200 J / kg * 0C. That is, this is the amount of heat that needs to be transferred to 1 kg of water in order to heat it by 1 0C
It should be remembered that the specific heat capacity of substances in different states of aggregation is different. That is, a different amount of heat is required to heat ice by 1 ° C.
