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## magnetic field

### 6. Series 21. Year - E. magnetic lock

[on request you can get a piece of magnet] Measure the dependence of magnetic force between 2 magnets.

Hint: place a piece of paper between magnets and measure the force needed to separate the magnets.

Vyplodil Honza Prachař.

### 2. Series 21. Year - 4. charged aerial

Two identical charges are at the end of stiff non-conductive rod. What power will be needed to rotate rod at constant angular speed with axis going through the middle of the rod? The friction is negligible

Úlohu vymyslel Martin Výška.

### 6. Series 19. Year - 3. spinning the electrical motor

On the axle of motor the sting is winded up. At the end of the string a small mass $m$ is fixed. If the motor is connected to the ideal voltage source of voltage $U$, small mass travels up at speed $v_{1}$. What speed will the small mass reach, if the voltage source is disconnected and the motor contacts are connected (shorted)? Assume friction-less devices.

Našel Matouš v sovětské sbírce.

### 1. Series 19. Year - 4. get a loop

Assume circular loop made from wire. The current is fed by radial wires (see figure). What is the magnetic field in the middle of the loop? The radius of the loop is $R$, the angle between radial wires $φ$ and the current in the wire is $I$. 