Maharashtra BoardEnglish MediumSTD 12 SciencePhysicsThermodynamics4 Marks
Question
State the first law of thermodynamics. Express it in mathematical form.
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Answer
First law of thermodynamics : The change in the internal energy of a system $(\Delta U)$ is the difference between the heat supplied to the system $(\mathrm{Q})$ and the work done by the system on its surroundings $(W)$. Mathematically, $\Delta U=Q-W$, which is the same as $Q=\Delta U+W$. Notes: 1. if $Q$ is positive, it means heat is added to the system. If $Q$ is negative, it means heat is given out by the system or removed from the system, 2. If $\Delta U$ is positive, it means there is increase in the internal energy of the system. If $\Delta U$ is negative, it means there is decrease in the internal energy of the system, 3. If $\mathrm{W}$ is positive, it means it is the work done by the system on its surroundings. Negative $W$ means work is done on the system by the surroundings, 4. The first law of thermodynamics is largely due to Joule. It is essentially the law of conservation of energy applied to the systems that are not isolated, i.e., the systems that can exchange energy with the surroundings. Thermodynamics was developed in 1850 by Rudolf Clausius (1822-88) German theoretical physicist, His ideas were developed in 1851 by William Thomson [Lord Kelvin] (1824-1907), British physicist and electrical engineer, 5. $Q=\Delta U+W$. Here, all quantities are expressed in the same units, e.g., cal or joule. If $Q$ and $A U$ are expressed in heat unit (cal, $\mathrm{kcal}$ ) and $W$ is expressed in mechanical unit (erg, joule) then the above equation takes the form $Q=\Delta U+\frac{W}{J}$, where $J$ is the mechanical equivalent of heat.]
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