First law of thermodynamics

Thermodynamic process

Heat (Q) is the energy that moves from one object to another because of the temperature difference. About systems and environments, heat is energy moving from system to environment or energy moving from environment to system, due to the temperature difference. If the system temperature is higher than the ambient temperature, heat will flow from the system to the environment. If the ambient temperature is higher than the system temperature, then heat flows from the environment to the system.

Heat (Q) is energy that moves due to the temperature difference, whereas Work (W) is related to energy transfer through work. For example, if the system does work on the environment, then energy moves from system to environment. Conversely, if the environment does work on the system, then energy moves from environment to system.

Internal energy and the first law of thermodynamics

The internal energy (U) of the system is the sum of all the molecular kinetic energy in the system, plus the amount of all potential energy arising from the interaction between molecules in the system. If heat flows from the environment to the system (the system receives energy), the energy in the system increases. Conversely, if the system does work on the environment (the system releases energy), energy in the system decreases.

Based on the conservation of energy, the internal energy changes in the system = heat is added to the system (the system receives energy) minus the work performed by the system (the system releases energy).

First law of thermodynamics 1

ΔU = internal energy changes, Q = Heat, W = Work

Internal energy is the microscopic quantity of the system so that its value can not be known. While changes in internal energy can be known through energy added to the system and energy released by the system in the form of heat and work. In contrast, macroscopic quantities such as temperature (T), pressure (p), volume (V) and mass (m) or some moles (n), are known.

Heat and work are only involved in the energy transfer process between the system and the environment. Heat and work is not a quantity that states the state of the system.

Sign rules for Heat (Q) and Work (W)

The sign rules for Heat and Work are adjusted to the equation of First Law of Thermodynamic. Heat (Q) in the above equation is the heat that is added to the system (Q positive), while Work (W) in the above equation is the work done by the system (W positive). If heat leaves the system, then Q is negative. If work is done on the system, then W is negative.

Example 1 :

If heat 2000 Joule is added to the system, while the system does work 1000 Joule. How many the change of system internal energy?

Solution

ΔU = Q – W = 2000 – 1000= 1000 Joule.

The system absorbs heat 2000 Joule (system receiving energy), the system also delivers 1000 Joule of work (system release energy). System energy change = 1000 Joule.

Example 2 :

Heat 2000 Joule leaves the system. The system does a 1000 Joule work. Calculate the system’s internal energy changes.

Solution

If system release heat, then Q is negative.

ΔU = Q – W = -2000 – 1000= -3000 Joule.

The system releases 2000 Joule heat. The system also performs 1000 Joule work (system release energy). Thus, the internal energy of the system is reduced by 3000 J.

Example 3 :

2000 Joule heat is added to the system, and 1000 Joules work is performed on the system. Calculate the system’s internal energy changes.

Solution

Work was done on the system, so W is negative.

ΔU = Q – W = 2000 – (-1000) = 3000 Joule.

The internal energy of the system increases by 3000 Joules.

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