You have probably noticed that your computer’s power supply or laptop’s battery gets warmer as you use it. When you touch the power supply, the heat you feel is thermal energy transformed from electrical energy. This becomes unwanted energy because instead of being used for electrical energy, it is being lost to heat. Since the world we live in mostly depends on efficiency, wouldn’t it be nice if we could have no ‘lost’ energy? This would require zero resistance ... so is this possible? Let’s look at the concept of superconductivity!
What are superconductors?
All conductors offer some amount of resistance to the flow of current, even the best ones possible. As current passes through any conductor, it heats up, resulting in a rising temperature and eventually an increase in the resistance offered by the particular conductor. If the temperature of a regular conductor decreases, the resistance it offers also decreases.
However, there is a slight difference for superconductors because they have a characteristic called the critical temperature (Tc). When a superconductor’s temperature drops below the critical temperature, its resistance suddenly drops down to zero.
The resistance of Mercury
One of the most common examples of a superconductor is Mercury at the critical temperature of 4.2 Kelvin (-269.2°C). This was discovered by the Dutch physicist Heike Kamerlingh Onnes in 1911.
The resistance of Mercury relative to the temperature in Kelvin, Oğulcan Tezcan - Vaia Originals
Onnes was trying to examine the resistance of different metals relative to the temperature using liquid helium to cool the materials. When he was cooling the sample of Mercury, he realized that as the temperature dropped below 4.2 Kelvin, the resistance suddenly dropped to zero. This temperature is now known as the critical temperature (Tc), and the phenomenon is known as superconductivity.
When a superconductor’s temperature drops below the critical temperature, its resistance suddenly drops to zero. This is known as the critical temperature (Tc), and the phenomenon is called superconductivity.
If we connect a three-digit ohmmeter across a conductor below the critical temperature, the device will read 0.00Ω. This is not because the actual value of the resistance is zero – it is instead less than 0.01Ω, which a three-digit ohmmeter cannot read.
There are several methods to measure such low resistances, but using an ohmmeter is not one of them when working with superconductors. This also shows us that even a superconductor’s resistance can't be absolute zero (even though it is theoretically accepted as zero).
Conditions for superconductivity
So if superconductors offer near-zero resistance and are very high in efficiency, why are they not used in everything for conducting electricity? This is because the conditions for superconductivity are not that easy to achieve, and common conductors such as copper, gold, or silver don’t exhibit superconductivity.
Three conditions should be fulfilled for materials to achieve superconductivity. These are:
- Critical temperature (Tc). The temperature of the conductor must be below a certain temperature called the critical temperature (Tc).
- Critical current density (Jc). The current flowing through a specific cross-section of the conductor should be below a certain value called the critical current density (Jc).
- Critical magnetic field strength (Hc). The magnetic field strength to which the conductor is exposed must be below a certain value called critical magnetic field strength (Hc).
These values are individual characteristics for different superconductors.
Critical temperature
Let’s focus on the more important and commonly known condition, critical temperature.
Three conditions affecting superconductivity, Oğulcan Tezcan - Vaia Originals
Scientists have been observing different materials in order to find a superconductor with a high critical temperature value for quite some time. In 1911, the critical temperature of Mercury was observed to be 4.2K (-269.2°C). Since then, the superconductor with the highest critical temperature is Mercury Barium Thallium Copper Oxide, which has a critical temperature of 139 Kelvin (-134.15°C).
This is much higher than Mercury’s but still really cold compared to room temperatures. This is the main reason why superconductors are not used in every device or project because they are not so cost-effective considering the cooling requirements.
Here is a table of the critical temperatures and critical magnetic fields for different materials.
Material | Symbol | Critical temperature Tc (K) | Critical magnetic field Hc (T) |
Mercury | Hg | 4.15-3.95 | 0.04 |
Lead | Pb | 7.19 | 0.08 |
Cadmium | Cd | 11.4 | 4.00 |
Titanium | Ti | 0.39 | 0.01 |
Aluminium | Al | 1.20 | 0.01 |
Although zero resistance to current flow is interesting enough in superconductivity, it is not the only interesting phenomenon. The Meissner effect, which is the exclusion of magnetic fields, is also super cool!
The Meissner effect, Wikimedia Commons
The Meissner effect can be observed when a permanent magnet is placed on top of a superconducting material that is below the critical temperature. This levitation will be static since there is an exclusion of magnetic fields.
High-speed trains use this effect to levitate over very strong superconducting magnets. This eliminates the force applied by friction, and when the friction is eliminated, the trains can go as fast as 603 kilometres per hour!
What are the applications of superconductors?
Superconductors are highly important for devices that need low resistance and a high magnetic field. The applications of superconductors include MRI scanners, generators, high-speed trains, and particle accelerators.
Another application of superconductors is called the superconducting quantum interference device, known as SQUID. SQUID is a device that is classified as a highly sensitive magnetometer and is used to measure minuscule magnetic fields. The working process of the SQUID depends on superconducting loops containing two Josephson junctions, as shown in the image below.
When a minuscule magnetic field is present around the SQUID, there will be a present interference effect, which depends on the strength of that magnetic field.
Josephson junction is a device that has a supercurrent continuously flowing across it. Supercurrent is the current that flows through superconducting materials without any dissipation.
A diagram of SQUID, Oğulcan Tezcan - Vaia Originals
Superconductivity - key takeaways
- Superconductors offer zero resistance to the flow of current. There is no energy loss as a result.
- When the temperature of a conductor that exhibits superconductor properties falls below the critical temperature (Tc), it shifts into a superconductive state.
- There are three conditions that affect superconductivity: critical temperature (Tc), critical current density (Jc), and critical magnetic field strength (Hc).
- Superconductors are used in various applications, such as MRI scanners, high-speed trains, and particle accelerators.
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