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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?

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Muscle power

Photo:
Two people jogging.


Information and ideas:
An example of the process whereby chemical energy is converted into mechanical energy.

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Magnetic energy

Overview graphic:
Two manifestations of magnetic energy are compared: the magnetic energy of a current-bearing coil and that of an elementary magnet.

Magnetic energy is the energy that is stored in a current-bearing coil in the form of its magnetic field. It is the result of the work that the current has to perform in opposition to the induced voltage (Faraday?s law of induction). Conversely, this magnetic energy is released again in the form of electric current when the magnetic field collapses. Magnetic energy is also stored in a magnetized material. It is equivalent to the work that must be expended in order to align the magnetic dipoles of this material in an external magnetic field. In ferromagnetic materials, the magnetic dipoles align themselves in small zones ("Weiss domains"), even without an external magnetic field. If the Weiss domains are now aligned by an external magnetic field, a permanent magnet is produced.
Incidentally: If a permanent magnet is heated above a critical temperature, it loses its magnetization. The magnetic energy is released as additional heat at this so-called Curie temperature.

Information and ideas:
A simple experiment on magnetization: If you pass a permanent magnet over an iron nail, it magnetizes the nail. What work has to be expended for this, apart from the friction work? Is the permanent magnet or its magnetic energy "used up" in the process?

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Lightning - electrical energy from the sky

Photo:
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.

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Phase diagram of water

Diagram:
A P-T diagram for pure water. The lines indicate the temperature and the pressure at which the solid, liquid, and vapor phases exist in equilibrium. All three phases exist in equilibrium only at the triple point; otherwise, there are a maximum of two phases.

In addition to the equilibrium curves (melting pressure curve, sublimation curve, vapor pressure curve), the diagram also includes the pressure and temperature data for the melting, boiling, triple, and critical points.
Attention: The axes of the diagram are not shown true to scale.

Information and ideas:
This diagram also reflects the density anomaly of water (lower density in the solid state than in the liquid state): The melting pressure curve shows a negative slope. The reason for the density anomaly is the hydrogen bonds.

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Xenon auto light bulb

Photo:
A Xenon auto light bulb.


Information and ideas:
An example of the conversion of electrical energy into radiant energy.

Medientypen

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Lernalter

6-18

Schlüsselwörter

Energy

Sprachen

Englisch

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Thermal energy

Diagram:
Formulas for the thermal energy of gases and temperature as a function of their molar heat capacity at constant volume.

The thermal or internal energy of a substance is the sum of the kinetic energies of its atoms or molecules. This energy is measurable as temperature. If you supply heat to the substance, the particle speed increases and the temperature rises. In the case of molecular gases, the supply of heat in addition to the translatory motion can excite other forms of motion (rotation and oscillation). This finds expression in the stepped curve of the molar heat capacity (diagram at the right). The molar heat capacity of a substance is the amount of energy required to raise 1 mole of a substance by 1°C. For gaseous substances, the following applies: If the gas particles move only linearly (translation), the amount of heat that is required to raise the gas by 1°C remains constant at 3R/2. In the case of molecular gases, the molecules start to rotate when a specific temperature is reached. In this area (linear increase in the diagram), more energy must be supplied to raise the temperature by 1°C, since the energy goes not only into the translatory motion, but also into exciting the rotation. If all particles are made to rotate, the energy required to raise the temperature by 1°C is again constant at 5R/2. The rise at the point of transition from rotation to oscillation can be explained in a similar way.

Information and ideas:
The overview graphic summarizes the topic of thermal energy using the example of gases. You will find detailed explanations as well as an explanation of the heat in solid bodies in the guideline "What is energy?"

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Burning candle

Photo:
A burning candle.


Information and ideas:
An example of the conversion of chemical energy into thermal energy.

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Bicycle dynamo

Photo:
A bicycle dynamo.


Information and ideas:
An example of an electricity generator that converts mechanical energy into electrical energy.

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Steam pressure curve and phase diagram of water

Charts:
The steam pressure curves (p-V diagram) and the phase diagram (p-T diagram) of water are compared.

If you heat water to 100 °C at normal atmospheric pressure, it turns into steam. But what effect does raising or lowering the pressure have on the vaporization temperature?
The answer to this is given by the steam pressure curve (T-curves in the p-V diagram on the left) and the phase diagram (p-T diagram of the right) of the water. Steam pressure is the term for the pressure at which gas and liquid are in equilibrium, i.e. the same number of molecules evaporate as condense back into water. Above the critical temperature (numerical values are given) the water is always gaseous, regardless at what temperature, and it can be treated as a real gas (Van der Waals equation, formula is given). At every temperature below the critical temperature there is a steam pressure for which there is a two-phase zone (liquid and gaseous). In the liquid phase range it is possible to recognize from the steep rise in the curves that liquid substances are barely compressible.
The critical temperature must not be confused with the triple point temperature (see p-T diagram). This characterizes the values of temperature and pressure at which all phases (solid, liquid and gaseous) are present simultaneously.

Information and ideas:
At what temperature does water boil on Mount Everest? So-called "Steam pressure tables" provide information about this. It would also be interesting to refer to the phase transition points as temperature critical points. At the phase transition from liquid to gaseous the energy applied does not initially lead to an increase in temperature. The same applies to the melting of ice. Not until all the water has evaporated or melted does the temperature rise further.