Siemens Stiftung

Redox flow cell

Schematic diagram:
The redox flow cell is an accumulator and works, so to speak, with liquid electrode materials, e.g., with zinc (Zn) and bromine (Br).

The graphic shows the flow of the electrode material during discharging of the cell. Two graphite electrodes (black surfaces) collect the current. Zinc is oxidized at its electrode, while the bromine is reduced at its electrode.
During charging, voltage is applied and the two solutions are pumped past the electrodes again.

Information and ideas:
What advantages does this process have over conventional galvanic cells?


Siemens Stiftung

Eddy currents in metal objects

The principle of eddy current brakes.

If a metallic disc moves through a static magnetic field whose field lines are perpendicular to the direction of movement, voltages are induced in the disc and eddy currents form. According to Lenz's law, the induced current generates a magnetic field to oppose the change in magnetic field that produced the current.
The braking effect is examined at the edges of the static magnetic field. At the position where the disc enters the external magnetic field, the current tries to maintain the state "without the magnetic field?. Therefore, the current is directed such that its induced magnetic field offsets the external magnetic field. The fields oppose each other (in the graphic, the induced field is directed "outward?), resulting in magnetic repulsion. The resulting force counteracts the direction of movement, slowing down the disc.
The situation is reversed at the position where the disc exits the magnetic field. The induced current tries to maintain the state "with the magnetic field?. Therefore, it is directed such that its induced magnetic field strengthens the external magnetic field (in the graphic, the induced field is directed "inward?). This results in magnetic attraction. The resulting force once more counteracts the direction of movement, slowing down the disc again.

Information and ideas:
This principle is also applied to eddy current sorting of residual waste containing aluminum.






Electricity Magnetism




Siemens Stiftung

Lightning - electrical energy from the sky

Bolt of lightning between the Earth and clouds - an excellent example of electrical energy in nature.

Rising streams of air generate electricity from mechanical energy by means of friction in the form of electrically charged clouds, up to a charge of 20 ampere-seconds (As). If the voltage difference between the storm cloud and the Earth is greater than 100 million V, a powerful discharge will occur as an electric arc. Because the discharge takes place within fractions of a second, high currents of up to 100,000 A can occur. For example, at a charge of 20 As and a discharge time of 0.4 ms, the current is 50,000 A. At this current, the power of a lightning bolt is 5 terawatts (TW). One TW equals one billion watts. Energy totaling 560 kWh is released in the process.

Information and ideas:
For further study, the physics of the gas discharge could be discussed. Another interesting exercise is to calculate the energy content of a bolt of lightning and to compare it with the calorific value of gasoline. What amount of gasoline corresponds to the energy of a bolt of lightning? Another example of the occurrence of electrical energy in nature is the electric eel, which produces electrical energy from a biochemical reaction.