Njira ya Thermodynamic
Kutentha (Q) ndi mphamvu yomwe imayenda kuchokera ku chinthu chimodzi kupita ku china chifukwa cha kusiyana kwa kutentha. Ponena za machitidwe ndi malo, kutentha ndi mphamvu yomwe imayenda kuchokera ku dongosolo kupita ku malo kapena mphamvu yomwe imayenda kuchokera ku malo kupita ku malo, chifukwa cha kusiyana kwa kutentha. Ngati kutentha kwa dongosolo kuli kokwera kuposa kutentha kwa malo, kutentha kudzayenda kuchokera ku dongosolo kupita ku malo. Ngati kutentha kwa malo kuli kokwera kuposa kutentha kwa dongosolo, ndiye kuti kutentha kumayenda kuchokera ku malo kupita ku malo.
Kutentha (Q) ndi mphamvu yomwe imayenda chifukwa cha kusiyana kwa kutentha, pomwe ntchito (W) imakhudzana ndi kusamutsa mphamvu kudzera mu ntchito. Mwachitsanzo, ngati dongosolo likugwira ntchito pa chilengedwe, ndiye kuti mphamvu imasuntha kuchokera ku dongosolo kupita ku chilengedwe. Mosiyana ndi zimenezi, ngati chilengedwe chikugwira ntchito pa dongosolo, ndiye kuti mphamvu imasuntha kuchokera ku chilengedwe kupita ku dongosolo.
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).
ΔU = internal energy changes, Q = Heat, W = Work
Internal energy is the microscopic quantity of the system so that its value cannot 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?
Anakonza
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.
Anakonza
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.
Anakonza
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.
