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## thermodynamics

### (3 points)4. Series 32. Year - 1. cube with the air

Consider a hollow cube with edge of $a = 20 \mathrm{cm}$ filled with air. Air as well as the enviroment has a temperature of $t_0 = 20 \mathrm{\C }$. We will cool down the air inside the cube to the temperature of $t_1 = 5 \mathrm{\C }$. Find the force acting on each of the cube's side. The cube has got a fixed volume. The pressure outside of the cube equals $p_0 = 101{,}3 \mathrm{kPa}$.

Danka was irritated by the shower curtain.

### (3 points)3. Series 32. Year - 2. efficient coffee

It is 2 am and Jáchym is going to make a coffee. He places a kettle with the heat capacity of $C_k$ on a hot plate, which is made of a cast-iron cylinder of a radius $r$ and of height $h$. The kettle contains water with a volume of $V$ with an initial temperature of $T\_v$. The rest of the system has got an initial temperature of $T\_s$. What is the overall efficiency (ratio of energy absorbed by water vs energy input) of water heating from its initial temperature $T = 100 \mathrm{\C }$ $(T\_s, T\_v < T).$ Assume, that the heat transfer is very fast and therefore there is no heat loss. You can estimate the unknown values or find them in physics tables.

Jáchym run out of energy drink

### (6 points)3. Series 32. Year - 3. heat in the Dyson sphere

What would be the diameter of a Dyson sphere that would surround a star with the luminosity of the Sun, so the temperature on the outer surface of the sphere is $t= 25 \mathrm{\C }$?. Don't consider the presence of the atmosphere in the Dyson sphere. A Dyson sphere should be a relatively thin concave structure of spherical shape surrounding the star.

Karl likes Dyson spheres.

### (3 points)2. Series 32. Year - 2. Finnish sauna

Imagine that Dan has a sauna with dimensions $2,5 \mathrm{m}$ x $3 \mathrm{m}$ x $4 \mathrm{m}$ with a relative humidity of $20 \mathrm{\%}$ and temperature of $90 \mathrm{\C }$. How much water would have to evaporate, so the relative humidity inside the sauna is $35 \mathrm{\%}$? The water evaporates inside the sauna without changing the overall temperature.

### (10 points)2. Series 32. Year - P.

Create an accurate weather forecast for address V Holešovičkách 2, Prague 8, for Wednesday 14th of November from 12:00 to 15:00. How will the weather change throughout the whole day? You are allowed to use previous data about the weather in this area (remember you are only permitted to use data until 10th of November). It is necessary to justify your weather prediction, write down references and ideally to use as many data and resources as possible.

Karl listened to radio on a motorway

### (3 points)6. Series 31. Year - 2. hot wire

Calculate the current, that needs to pass through a metal wire of a diameter $d = 0{,}10 \mathrm{mm}$ located in a vacuum bulb, so that its temperature stays at $T = 2 600 K$. Assume the surface of the wire radiates like an ideal black body and neglect any losses by heat conduction. The resistivity of the material of the wire at the given temperature is $\rho = 2{,}5 \cdot 10^{-4} \mathrm{\Ohm \cdot cm}$. \taskhint {Hint}{Use the Stefan-Boltzmann's law.}

Danka was contemplating the light bulb efficiency

### (12 points)6. Series 31. Year - E. don't play with matches

Measure the speed with which a wooden skewer burns as a function of its tilt with respect to the vertical.

Because the gasoline that Karel suggested was a bit too much.

### (7 points)5. Series 31. Year - 4. thermal losses

At what temperature does the indoor environment of the flat in a block of flats stabilise? Consider that our flat is adjacent to other apartments (except its shorter walls), in which the temperature $22 \mathrm{\C}$ is maintained. The shorter walls adjoin the surroundings where the temperature is $- 5 \mathrm{\C}$. The inside dimensions of the flat are height $h = 2{,}5 \mathrm{m}$, width $a = 6 \mathrm{m}$ and length $b = 10 \mathrm{m}$. The coefficient of the specific thermal conductivity of the walls is $\lambda = 0{,}75 \mathrm{W\cdot K^{-1}\cdot m^{-1}}$. The thickness of the outer walls and the ceilings are $D\_{out} = 20 \mathrm{cm}$, and the thickness of the inner walls are $D\_{in} = 10 \mathrm{cm}$.

How will the result be changed if we add polystyrene insulation to the building? The thickness of the polystyrene is $d = 5 \mathrm{cm}$, and its specific heat conductivity is $\lambda '= 0{,}04 \mathrm{W\cdot K^{-1}\cdot m^{-1}}$.

### (3 points)4. Series 31. Year - 1. ice-cream

Estimate how many grams of ice-cream is possible to be made out of $5 \mathrm{l}$ of liquid oxygen with temperature $-196 \mathrm{\C }$ and unlimited amount of milk and cream with room temperature $22 \mathrm{\C }$? Let's suppose that ice-cream consists of milk and cream only (same mass of both ingredients) and the temperature of the ice-cream should be $-5 \mathrm{\C }$. Use average heat capacity $c\_m = 3{,}45 \mathrm{kJ\cdot kg^{-1}\cdot K^{-1}}$ for milk and $c\_s = 4{,}45 \mathrm{kJ\cdot kg^{-1}\cdot K^{-1}}$ for cream (despite the fact that changes considerably in this temperature range). Find other needed quantities on the internet by yourself.

Michal got a taste for ice-cream.

### (3 points)1. Series 31. Year - 1. I'm gonna milk my coffee

When is it most efficient to pour cold milk into a hot coffee for the coffee to be drinkable as soon as possible? We expect a detailed description of the cooling process rather than a precise calculation.

Terka S. was baffled by the exclamation: „I've already put milk in your coffee to make it cool quicker.“