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Heat and Temperature
[Haba dan Suhu]


Boyle's Law / Hukum Boyle

Thermometers

Heat From The Sun [Haba dari Matahari]

Linear expansion [Pengembangan Linear ]

Assignment Guide / Panduan tugasan

Assignment Guide 1(a) / Panduan tugasan 1(a)

Molecule motion / gerakkan molekul

Triple Point / Takatigaan

Motion - Velocity - time

Motion - Excercise

The First Law of Thermodynamics

The first law of thermodynamics is basically the principle of conservation of energy. It says that energy will not vanish into nothing or arise out of nothing. This principle is as important to thermodynamic analysis as does Newton's second law of motion in mechanics. What we want to learn in this tutorial is how to apply this simple law to problems in heat transfer.

Applying the first law requires that we keep track of all forms of energy and make sure that every bit of energy is accounted for. In many cases, though, we ignore some of the forms that we deem small in comparison to other. Let's look at a simple example:


Here a certain amount of air with a known temperature and flow rate enters a duct (or a pipe), gets heated up as it travels through the duct and exits at the other end. Conservation of energy tells us that the energy of the air at the exit is equal to its energy at the entrance plus the amount of energy added to it during its travel through the pipe, or


In heat transfer we are mainly dealing with a form of energy called HEAT. Heat is the form of energy that is transferred from one place to another as a result of temperature difference. There is an important distinction between what a thermodynamic analysis gives us versus what we get from a heat transfer analysis. In thermodynamics, we are looking at a system at two different states of equilibrium and can use the first law to determine the energy exchange in going from one state to another. Heat transfer, on the other hand, deals mainly with the rate of heat transfer or the time it takes for the heat transfer to occur.

Let's write the first law in terms of an equation starting from

This simple equation says that the only way to change the total energy of a system is to have an imbalance between what we give to the system and what we take out of it (kind of like your bank account). Writing this equation in a rate form and using E to represent energy, we have:


In heat transfer, we are mainly interested in the transfer of heat caused by a temperature difference. We separate the energy terms into the heat energy and other energies and lump all of this "other" forms of energy into a heat generation term. The above rate equations then become:


So, we have reduced the first law of thermodynamic to a simple energy balance. Applying the first law to heat transfer problems is nothing but balancing the thermal energy items.

Closed Systems and Open Systems

It is a common practice to conside two different cases of closed systems (where mass does not cross the boundary of the system) and open or flowing system (where mass does cross the system boundary). For most applications the total energy is equal to the internal energy. The internal energy is the sum of all microspcopic forms of energy. For a constant volume system we define the specific heat at constant volume as the energy required to raise the temperature of a unit mass of the substance by one degree as the volume is held constant. Mathematically:



Which is basically the definition of the specific heat. We just took the energy required per unit mass and per degree and multiplied it by the mass and by the temperature difference to get the internal energy. If m and Dt are unity, the internal energy is equal to the specific heat. With this, the energy balance for a closed system becomes:

where Q is the NET amount of heat transfer assuming that no work is crossing the boundary of the system.

We can write the net rate of heat transfer for a steady flow (open) system as:



The mass flow rate through a given cross section A is calculated as:

Let's now consider a simple example of air flow through a duct with a constant cross section. We know the inlet and outlet temperatures and we know the flow rate. We want to calculate the heat loss (or gain) through the duct

We will use the magic equation to calculate the heat transfer through the duct walls:

We get the value of Cp and density from air property tables using the average temperature between inlet and out let. The inlet velocity is given and the cross-sectional area is equal to a x b. Substituting in the heat balance equation gives us the net heat transfer.

Hukum Thermodynamics Pertama


Hukum Termodinamik pertama pada prinsipnya hanyalah prinsip keabadian tenaga. Hukum ini menyatakan tenaga tidak akan lenyap begitu sahaja atau muncul tanpa sebarang punca. Prinsip ini sama pentingnya kepada termodinamik seperti mana Hukum Newton Kedua dalam makanik. Apa yang hendak di bincangkan dalam tajuk ini ialah bagaimana menggunakan hukum mudah ini dalam masalah pemindahan haba.
Penggunaan hukum pertama ini memerlukan kita menjejaki setiap bentuk tenaga dan memastikan setiap cebisan tenaga itu diambil kira. Dalam banyak kes, kita tetap juga mengamabaikan sesetengah bentuk yang dianggap kecil berbanding dengan yang lain. Kita pertimbangkan contoh mudah berikut.
Di sini sejumlah tertentu udara yang di ketahui suhu serta kadar aliran nya, memasuki terowong (atau paip) menjadi panas dalam perjalanannya melalui terowong tersebut dan keluar melalui hujung yang satu lagi. Prinsip keabadian tenaga memberitahu kita bahawa tenaga udara di hujung keluar itu sama dengan tenaga di pintu masuknya tambah dengan tenaga yang di tambahkan semasa perjalanannya melalui paip itu.
Dalam pemindahan haba, kita berurusan dengan bentuk tenaga yang dipanggil HABA. Haba ialah satu bentuk tenaga yang di pindahkan dari satu tempat ke tempat lain disebabkan oleh perbezaan suhu. Terdapat perbezaan penting antara apa yang diberikan oleh analisa termodinamik dengan apa yang didapati dari analisa pemindahan haba. Dalam termodinamik, kita melihat sesuatu sistem pada dua keadaan keseimbangan dan boleh menggunakan hukum pertama untuk menentukan pertukaran tenaga dalam perubahan dari satu keadaan ke satu keadaan lain. Pemindahan haba pula mengendalikan kadar pemindahan haba atau masa yang diambilnya untuk pemindahan itu berlaku.
Hukum pertama ini boleh ditulis dalam bentuk persamaan bermula dari


Persamaan mudah ini menyatakan satu satu nya cara untuk menukarkan semua tenaga sesuatu sistem ialah dengan memiliki imbangan antara apa yang di beri pada sistem dan apa yang di keluarkan dari sistem itu. Dala mbentuk perkadaran dan menggunakan E bagi mewakili tenaga, kita dapati ;

Dalam pemindahan haba kita hanya berminat terhadap pemindahan haba yang di sebabkan oleh perbezaan suhu semata mata. Kita mengasingkan sebutan tenaga kepada tenaga haba dan tenaga lain serta kumpulkan semua ini "other" bentuk tenaga kepada sebutan penjana haba. Persamaan perkadaran di atas kemudian menjadi;

Jadi kita telah memudahkan Hukum termodinamik pertama kepada satu imbangan tenaga ringkas Penggunaan hukum pertama dalam masalah pemindahan haba hanyalah usaha mengimbangkan sebutan sebutan tenaga termal.

Closed Systems and Open Systems


Telah menjdi kebiasaan untuk menimbangkan dua kes berbezan bagi sistem tertutup ( jisim tidak melangkaui sempadan sistem) dan sistem terbuka atau sistem aliran ( dimana jisim merentasi sempadan sistem) . Bagi kebanyakkan kes keseluruhan tenaga adalah sama dengan tenaga dalam. Tenaga dalam ialah jumlah semua bentuk tenaga miktoskopik. Bagi sistem dengan isipadu tetap kita mentakrifkan muatan haba tentu pada isipadu tetap sebagai tenaga di perlukan untuk meingkatkan suhu seunit jisim bahan sebanyak satu darjah sementara isipadunya di kekalkan tetap.

Which is basically the definition of the specific heat. We just took the energy required per unit mass and per degree and multiplied it by the mass and by the temperature difference to get the internal energy. If m and Dt are unity, the internal energy is equal to the specific heat. With this, the energy balance for a closed system becomes:

where Q is the NET amount of heat transfer assuming that no work is crossing the boundary of the system.
We can write the net rate of heat transfer for a steady flow (open) system as:

The mass flow rate through a given cross section A is calculated as:

Let's now consider a simple example of air flow through a duct with a constant cross section. We know the inlet and outlet temperatures and we know the flow rate. We want to calculate the heat loss (or gain) through the duct

We will use the magic equation to calculate the heat transfer through the duct walls:

We get the value of Cp and density from air property tables using the average temperature between inlet and out let. The inlet velocity is given and the cross-sectional area is equal to a x b. Substituting in the heat balance equation gives us the net heat transfer.