Change Internal Energy: The change in the internal energy of a system is the sum of the heat transferred and the work done. The heat flow is equal to the change in the internal energy of the system plus the PV work done. Read What is Physics?
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are two ways to change the internal energy of a body or body system. The first way is to do the work. The second method is heat transfer.
If the work is done by the body itself, then its internal energy decreases. If work is done on the body, then the internal energy of the body increases. In this case, there is a conversion of mechanical energy into internal, or, conversely, internal into mechanical.
If, as a result of heat transfer, the body cools down, then its internal energy decreases. If heat is transferred to the body, then its internal energy increases.
An example of increasing the internal energy of a body through work: compressing a gas (while it heats up).
An example of reducing the internal energy of a body through work: the expansion of a gas (with the gas cooled).
Heat transfer is a way of changing the internal energy of the body without doing work. In this case, the transmitted energy is called the amount of heat (Q, J). The body receives exactly the amount of heat that another body gives it, that is, heat transfer obeys the law of conservation of energy.
In any case, the change in the internal energy of the body is reflected in the change in its temperature.
The change in the internal energy of the body due to work and heat transfer can occur at the same time, that is, these two methods work together.