The internal energyAccording to the First Principle of Thermodynamics, it is understood as that linked to the random movement of particles within a system. For instance: batteries, stir a liquid, water vapor. It differs from the ordered energy of macroscopic systems, associated with moving objects, in that it refers to the energy contained by objects on a microscopic and molecular scale.
A) Yes, an object it can be completely at rest and lack apparent energy (neither potential nor kinetic), and yet be a hotbed of molecules in motion, moving at high speeds per second. In fact, these molecules will be attracting and repelling each other depending on their chemical conditions and microscopic factors, even though there is no observable movement to the naked eye.
Internal energy is considered a extensive magnitude, that is, related to the amount of matter in a given particle system. For it comprises all other forms of electrical, kinetic, chemical and potential energy contained in the atoms of a given substance.
This type of energy is usually represented by the sign U.
Internal energy variation
The internal energy of the particle systems it can vary, regardless of its spatial position or acquired shape (in the case of liquids and gases). For example, when introducing heat to a closed system of particles, thermal energy is added that will affect the internal energy of the whole.
However, internal energy is a status function, that is to say, it does not attend to the variation that connects two states of matter, but to the initial and final state of it. That is why the calculation of the variation of the internal energy in a given cycle will always be zero, since the initial and final states are one and the same.
The formulations to calculate this variation are:
- ΔU = UB – ORTO, where the system has gone from state A to state B.
- ΔU = -W, in cases where a quantity of mechanical work W is done, which results in the expansion of the system and the decrease of its internal energy.
- ΔU = Q, in the cases in which we add heat energy that increases the internal energy.
- ΔU = 0, in cases of cyclical changes in internal energy.
All these cases and others can be summarized in an equation that describes the Principle of Conservation of Energy in the system:
ΔU = Q + W
Examples of internal energy
- Batteries. The body of charged batteries contains usable internal energy, thanks to the chemical reactions between acids and heavy metals inside them. Said internal energy will be greater when its electrical load is complete and less when it has been consumed, although in the case of rechargeable batteries this energy can be increased again by introducing electricity from the outlet.
- Compressed gases. Considering that gases tend to occupy the total volume of the container in which they are contained, since their internal energy will vary as this amount of space is greater and will increase when it is less. Thus, a gas dispersed in a room has less internal energy than if we compress it in a cylinder, since its particles will be forced to interact more closely.
- Increase the temperature of matter. If we increase the temperature of, for example, a gram of water and a gram of copper, both at a base temperature of 0 ° C, we will notice that despite being the same amount of matter, the ice will require a greater amount of total energy to reach the desired temperature. This is because its specific heat is higher, that is, its particles are less receptive to the energy introduced than those of copper, adding heat much more slowly to its internal energy.
- Shake a liquid. When we dissolve sugar or salt in water, or we promote similar mixtures, we usually shake the liquid with an instrument to promote a greater dissolution. This is due to the increase in the internal energy of the system produced by the introduction of that amount of work (W) provided by our action, which allows a greater chemical reactivity between the particles involved.
- Steam of water. Once the water is boiled, we will notice that the steam has a higher internal energy than the liquid water in the container. This is because, despite being the same molecules (the compound has not changed), to induce the physical transformation we have added a certain amount of caloric energy (Q) to the water, inducing a greater agitation of its particles.