**Nov. 24, 2024, 23:59, CET**.

# Assignment of Series 2 of Year 38

About the Competition Rules How to Write Solutions Results#### 1... ecological bitcoin

#### 3 points

A certain computer science student is mining bitcoins. His graphics cards yield $0.2\,\mathrm{BTC}$ per year and have the energy consumption of $3~000\,\mathrm{W}$. However, the student reaches an epiphany thinking about how much carbon dioxide he releases into the Earth's atmosphere. He therefore decides to use the money gained from Bitcoin mining to buy trees which act as natural carbon capture mechanism. What would the price of a bitcoin have to be to make such an activity profitable? Suppose the price of one tree is $1~000\,\mathrm{CZK}$ and each tree can capture $25\,\mathrm{kg\cdot year^{-1}}$ $CO_2$. Consider two energy sources – coal with the price $5.32\,\mathrm{CZK\cdot kWh^{-1}}$ and emissions of $0.82\,\mathrm{kg\cdot kWh^{-1}}$ and a hydroelectric power plant with the price of $4.00\,\mathrm{CZK\cdot kWh^{-1}}$ and emissions of $0.012\,\mathrm{kg\cdot kWh^{-1}}$.

#### 2... (not really) capillary phenomena

#### 3 points

Monika has attempted to create her own kind of tree. However, as a physicist, she disregarded all biological phenomena and aspects. For her tree, she took a long capillary tube with a diameter of $d=0.1\,\mathrm{mm}$. How high does the water rise in this physics-based tree? For which fluid are such trees the tallest? Additionally, by neglecting which physical principle does the height of this “tree” differ so significantly from that of real trees?

#### 3... floating pyramid

#### 5 points

Consider a homogeneous pyramid with density $\rho_{\mathrm{j}}=250\,\mathrm{kg\cdot m^{-3}}$ floating on water with density $\rho=1~000\,\mathrm{kg\cdot m^{-3}}$. While floating, its axis is vertical. Is the position more stable when the apex of the pyramid is pointing up or down? The height of the pyramid is $h=20\,\mathrm{cm}$ and the surface area of its base is $S=49\,\mathrm{cm^2}$.

#### 4... a tap and a container

#### 8 points

We have an empty container of height $H$. The container has a square base of side length $a$. A faucet is positioned directly above the container, and at time $t =0\,\mathrm{s}$, water begins to flow out of the faucet with an initial velocity $v_0$. Calculate the dependence of the water level in the tank on time $t$. The volumetric flow rate of water $Q$ is constant. Assume that $Q$ is small enough for the water level in the container to settle instantaneously at a uniform height. However, do not forget about the time it takes the liquid to fall.

#### 5... focal point in a cylinder

#### 9 points

Consider a cylindrical capacitor with internal radius $r_1$ and external radius $r_2$. The capacitor is charged so that the voltage between the two electrodes is $V$. Electrons with a small angle distribution $\Delta \alpha$ are flying out perpendicular to the radius of the cylinder at a distance $r$ ($r_1 < r < r_2$) and at such speed that their distance from the center of the cylinder is approximately constant. Determine the location of the first point in which the electrons focus again. The situation is planar and do not consider the space charge of the electrons.

#### P... cold lovers' association

#### 10 points

Let the quality of acclimatization to cold temperatures be defined based on the thermal power that a person must exert. Is it better to acclimatize under flowing water or by immersing oneself in a bathtub of cold water? Consider at least the different water temperatures, flow rate, and the temperature of the surroundings.

#### E... tension in the kitchen

#### 12 points

Measure the deformation curve for an ordinary rubber band.

Instructions for Experimental Tasks#### S... electrochemistry 2 – potentials

#### 10 points

- Determine the voltage of the following electrochemical reactions under standard
conditions. Does the reaction occur spontaneously? (2 points)
- $\ce{CaCl_2(l) -> Ca(s) + Cl_2(g) }$,
- $\ce{Pb(s) + PbO_2(s) + 2H_2 SO_4(aq) -> 2PbSO_4(s) + 2H_2 O(l)}$.

- Determine which of the following electrochemical half-cells will reduce and
what will be the voltage after they combine. (2 points)
- $\ce{Ni^{2+}(aq)|Ni(s)}$ a $\ce{Au^{3+}(aq)|Au(s)}$,
- $\ce{(NO_3)^-(aq)|NO(g)|Pt(s)}$ in an acidic solution and $\ce{Fe^{2+}, Fe^{3+}|Pt}$

- Let us consider a fuel cell which, under standard conditions, produces electric energy during the water-producing reaction $\ce{2H_2(g) + O_2(g) -> 2H_2 O(g)}$. Determine the energy density in relation to the mass of hydrogen (in $\mathrm{J\cdot kg_{\ce{H_2}}^{-1}}$) that is released in this situation. Determine the constant of equilibrium $K$ for this reaction and discuss its value. (2 points)
- Now, let us consider a electrochemical
cell $\ce{Cu|Cu^{2+}(aq)||Ag^{+}(aq)|Ag}$. The initial concentration of copper
in the solution is $\left[\ce{Cu^{2+}}\right] = 0.40\,\mathrm{M}$ while the
concentration of silver is $\left[\ce{Ag^{+}}\right] = 0.50\,\mathrm{M}$. What is the
concentration of silver when the cell's voltage is equal to $0.40\,\mathrm{V}$? (4 points)
*Special motivation:*You might encounter a problem like this on the electrochemistry exam during your master's studies at MFF. Can you solve it already in high school?