**P. 4720.** The operation of the demister is the following: a fan blows the air towards a cooled surface, and the condensed water vapour is collected in a container. How much heat is given off to the air in the room by the demister, rated at 180 W, in 15 hours, if it collected 4 litres of water? The temperature of the room is \(\displaystyle 25~{}^\circ\)C.

If the demister is considered to be a heat pump, what is its coefficient of performance (the ratio of the released heat to the electrical energy consumed)?

(4 points)

**P. 4722.** There are two cylinders of cross section \(\displaystyle A=2~\rm{dm}^2\) at rest on a frictionless, horizontal surface. The pistons of the cylinders can move easily and they are connected with a rigid rod. Each cylinder is connected to an unstretched spring. The values of the spring constant of the springs are \(\displaystyle D_{\rm b}=24\) kN/m, and \(\displaystyle D_{\rm j}=16\) kN/m. The volume of the air enclosed in the left, insulated cylinder is \(\displaystyle V_{\rm 0b}=20~{\rm dm}^3,\) and that of in the right cylinder is \(\displaystyle V_{\rm
0j}=30~{\rm dm}^3\), and the initial temperature of both gases is \(\displaystyle T_0=300\) K. The temperature of the gas in the left cylinder is increased to \(\displaystyle T_{\rm b}=1200\) K. The ambient air pressure is \(\displaystyle p_0=10^5\) Pa.

\(\displaystyle a)\) What is the displacement of the two cylinders and the piston with respect to the ground if the temperature of the gas in the right cylinder is kept at its initial value?

\(\displaystyle b)\) What are the final values of the volume of the two samples of gas?

\(\displaystyle c)\) How much heat was released by the heating element which heated the gas in the left cylinder?

(5 points)

**P. 4724.** The working temperature of the tungsten filament of an incandescent lamp rated at 230 V and 100 W is \(\displaystyle 2400~{}^\circ\)C. In an experiment the voltage applied to the lamp is gradually increased from zero, and it can be seen that the filament begins to glow at 17 V. What is the temperature and the resistance of the filament in this state, and how much power is dissipated in this state of the bulb?

We can assume that the emitted power is proportional to the fourth power of the temperature measured in Kelvins, the fifth power of the resistance is proportional to the sixth power of the absolute temperature and heat conduction is negligible.

(5 points)