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## electric field

### 1. Series 35. Year - 5. mechanically (un)stable capacitor

Assume a charged parallel-plate capacitor in a horizontal position. One of its plates is fixed and the other levitates directly below it in an equilibrium position. The lower plate is not mechanically fixed in its place. What is the capacitance of the capacitor depending on the voltage applied? Is the capacitor mechanically stable?

Vašek wanted to grill you on a capacitor.

### 6. Series 34. Year - P. more dangerous corona

When there is a coronal mass ejection from the Sun, the mass will start to propagate with high velocity through the space. Sometimes the mass can hit the Earth and affect its magnetic field. Estimate the magnitude of the electric currents in the electric power transmission network on Earth which could be generated by such ejection. What parameters does it depend on? Comment on what effects would such event have on the civilisation.

### 5. Series 34. Year - 1. the charge of the Earth

Find the total electric charge, that the Earth would need to let all electrons close to its surface fly away. How would this charge differ if it had to deflect protons?

Karel likes planetary problems.

### 3. Series 34. Year - P. wavy electromagnetism

What if the laws of nature weren't the same throughout the whole universe? What if they somehow changed with location? Let's focus on electromagnetic interaction. What would be the minimal change of the Coulomb's law constant as a function of distance, such that we could observe a deviation? How would we observe it?

Karel was watching YouTube too much.

### 2. Series 34. Year - 5. magnetic non-stationarities detector

The electrical circuit shown in the figure can serve as a non-stationary magnetic field detector. It consists of nine edges of a cube formed by electric wire. The electrical resistance of one edge is $R$. If this construction lies in a non-stationary homogeneous magnetic field, which has, for simplicity, a constant direction, and its magnitude changes slowly, then there are currents $I_1, I_2, I_3$ flowing at the marked spots. With the knowledge of these currents, determine the direction of the magnetic field in space and also the dependence of its magnitude on time.

Vašek thought that an electromagnetic induction problem would be welcome.

### 5. Series 33. Year - S. min and max

*We are sorry. This type of task is not translated to English.*

They had to wait a lot for Karel.

### 4. Series 33. Year - S.

*We are sorry. This type of task is not translated to English.*

### 5. Series 32. Year - 4. splash

Consider a free water droplet with radius of $R$. We start to charge the drop slowly. Find the magnitude of the charge $Q$ the drop needs to splash.

### 4. Series 32. Year - 3. levitating

Matěj likes levitating things and therefore he bought an infinite non-conductive charged horizontal plane with the charge surface density $\sigma $. Then he placed a small ball with given mass $m$ and charge $q$ above the plane. For which values of $\sigma $ will the ball levitate above the plane? What is the corresponding height $h$? Assume that the gravitational acceleration $g$ is constant.

Matěj would love to have levitation superability.

### 1. Series 32. Year - 3. unstable

We have 8 point charges (each of magnitude $q$) located on the vertices of a cube. Find out the value of a point charge $q_0$ that needs to be placed in the middle of the cube, so that all charges remain in balance. Is this equilibrium stable?

Matej wanted to pose a problem that even a professor couldn't work out.