# Resistance Is Futile

I will bravely assume that any electrical apparatus in your household does not have a 100% energy efficiency, i.e. the useful energy you obtain is less than the energy you put in. It is obviously preferred to have 100% efficiency because not only do you get more out of your money, but it also saves more fossil fuels, a finite resource. Energy inefficiency is an issue that has plagued the civilisation for centuries, and scientists strive to find ways of obtaining maximum efficiency wherever possible.

One of the adversaries in the war against inefficiency is electrical resistivity. Resistivity is an intrinsic property of a material (each material has a specific value of resistivity at a particular temperature) that measures how good the material is at conducting electricity. By changing the dimensions of a conductor, you change its electrical resistance. The resistivity of a conductor depends only on its material; its resistance depends on both its material and dimensions.

We can use a simple model to describe how resistance causes a loss in energy and therefore a decrease in energy efficiency. An electric current is provided by the flow of charged particles, usually electrons (which are negatively charged). The more quickly they flow, the higher the current, and the more slowly they flow, the lower the current.

A conductor (for example a wire) consists of oscillating metal ions arranged in a lattice. As electrons flow, they collide with the metal ions. As a result, the electrons lose some energy and the current decreases. The remaining energy is dissipated as heat, which is why electrical components often heat up after you use them for a period of time.

In 1908 Dutch physicist Heike Kamerlingh Onnes became the very first to produce liquid helium, a feat that required the lowering of temperatures to below 4.2 K (-269 °C). With this new tool under his belt, he could begin investigating the behaviour of materials as they approached absolute zero (0 K, or -273 °C). One of the major theories at that time was that the resistivity of a material would decrease steadily to zero with temperature. However, while investigating mercury Kamerlingh Onnes found that its resistivity suddenly dropped to zero at around 4.2 K. He had discovered superconductivity.

In the following decades new materials were found that, when cooled to below their critical temperatures, they would exhibit zero resistivity. These superconductors generally consisted of alloys of a variety of metals and metal oxides. It was then a puzzle of finding the optimal proportions of the optimal elements to produce superconductors that had high critical temperatures.

In a typical circuit, the wire will be a source of resistance. However, a current flowing through a loop of superconducting wire will persist indefinitely with no power source. No energy is wasted; no heat is dissipated. There are high costs to maintaining a low-temperature environment of course, so to this day scientists continue to search for high-temperature superconductors.

This remarkable property entitles superconductors to a significant and diverse range of applications. The ability of superconductors to carry high electrical currents without heat loss allows them to produce strong magnetic fields. Such electromagnets have applications in medicine, for example in magnetic resonance imaging (MRI) scanners. MRI is a method of seeing inside the human body without invasive surgery. A superconducting magnet produces a magnetic field, which in combination with radio waves can examine the distribution of water in the body. MRI has allowed medical diagnoses to be far more accurate and therefore life-saving.

Besides zero resistivity, superconductors have another very unique property. As soon as a material has obtained superconductivity, it will not allow magnetic fields to pass through it. Consequently, when a superconductor is placed above a magnet, it forces magnetic fields to pass around it, causing itself to be levitated. This property is exploited in magnetically levitated (maglev) trains. Maglev trains levitate above their rails, essentially removing all friction between the train and the rails, leaving only air resistance to be the principle source of energy inefficiency. Maglev trains can reach extremely high speeds, and the Shanghai Maglev Train holds the record for the fastest commercial train in operation with a top speed of 430 kmh-1.

Superconductors possess an inexhaustible list of potential future applications – they are one step closer to the desired world where energy inefficiency is negligible.

Yanhao