Heat, Internal Energy And Work - Thermodynamics - Practice Questions & MCQ

Quantities Involved in First Law of Thermodynamics -

(a) Heat (Q) : It is the energy that is transferred between a system and its environment because of the temperature gradient.

(b) Work (W) : Work can be defined as the energy that is transferred from one body to the other owing to a force that acts between them.

(C)  Internal energy (U) : Internal energy of a system is the energy possessed by the system due to molecular motion and molecular configuration.

         Types of internal energy - 

• Due to molecular motion internal energy is kinetic internal energy (UK).

• Due to molecular configuration, it is called internal potential energy (UP).

Important points :

1. Heat and work are path dependent quantities and Internal energy is point function.

   $\mathrm{\Delta }W=P\mathrm{\Delta }V=P(V_f-V_i)$

   $\mathrm{\Delta }W=$ positive if $V_f>V_i$ i.e. system expands against some external force.
   2. $\mathrm{\Delta }W=$ negative if $V_f<V_i$ i.e. system contracts because of some external force exerted by the surrounding.

2. The area of P-v diagram on volume axis give the work done in a reversible process. Also for quasistatic process work is given by

                                                         

$W={\int }_{V_1}^{V_2}P.dV$

4. And for a cyclic process the clockwise area will show positive work and anticlockwise area will show the negative work done.
5. Internal energy of an ideal gas is totally kinetic and is given by

$U=\frac{3}{2}\cdot n\cdot R\cdot T$

So, Internal energy of an ideal gas is the function of temperature only.
6. For heat transfer -

$\begin{array}{rl}& \mathrm{\Delta }Q=m{L}_{\text{(for change of state) }}\\ & \mathrm{\Delta }Q=ms\mathrm{\Delta }{T}_{\text{(for change in temperature) }}\end{array}$

or,

$\mathrm{\Delta }Q=nc\mathrm{\Delta }T$

Where, $\mathrm{c}=$ molar specific heat capacity
Sign of dQ(Heat)
$dQ>0$ if heat is given to the system
$dQ<0$ if heat is extracted from the system