Most of us had a mild shock when we fiddled with wires, switches, and light bulbs in physics class and tried to understand the basic principles of electricity.
The most important of these laws is Ohm’s Law, which describes the relationship between voltage, resistance, and current.
This law was formulated by the German physicist Georg Ohm after a series of experiments he conducted in the mid-1820s.
It is easier to understand Ohm’s law if we compare electric current to the flow of water in a river. Here, the voltage will correspond to the force needed to push the water across the river bed. The electrical resistance will correspond to the amount the water flow is slowed by obstacles in the path.
Logically speaking, the current – or the strength of the current – will depend on both the force pushing the water through it and the resistance it encounters. The greater the power, the greater the current – and the greater the resistance, the lower the current.
However, it’s not always that simple.
Temperature undermines the law
Ohm’s law states that there is a linear relationship between voltage and current. If we increase the voltage by ten percent, the current will also increase by ten percent.
But this only applies if the resistance is constant, which rarely happens. In most materials it changes with temperature.
This means, for example, that the hotter a regular light bulb gets, the greater the resistance. So, although Ohm’s law is a useful tool, it is not a true law of nature.
In so-called superconducting materials, the Ohm formula completely decomposes. Superconductors have no resistance at all. According to the formula, keeping the voltage constant means that the current will be infinitely high, which makes no sense.
However, this is not a problem for physicists. They would gladly sacrifice Ohm’s law for all the advantages offered by superconducting materials.
Specifically, because they have no resistance, they are not heated by the current passing through them. Therefore, they are, for example, well suited to electromagnets that can create very strong magnetic fields.
Powerful electromagnets could, for example, be used in trains hovering over railway tracks. They will be indispensable in a future fusion power plant, where they will keep the 100-million-degree hot fuel floating in the reactor.
So physicists are working hard to develop superconductors that operate at normal temperatures. So far, superconductivity has only been achieved near absolute zero, i.e. -273.15°C.
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