What is superconductivity and why does it happen near absolute zero?

In one line

The rule

Superconductivity is a phenomenon where a material exhibits zero electrical resistance and expulsion of magnetic fields when cooled below a certain critical temperature. (NCERT)

• Formula for resistance in a superconductor: R = 0 (below critical temperature)
• Formula for magnetic field inside: B = 0 (inside the superconductor)

Let's understand it simply

Imagine you are riding your cycle on a smooth, empty road. You hardly need to pedal — you keep moving easily. Now, think about cycling through thick mud. It’s hard, you slow down, and you get tired quickly. Usually, electricity faces 'mud' inside wires — this is called resistance (the wire slows the current and wastes energy as heat).

But in some special materials, if you make them very, very cold (close to absolute zero, which is −273°C), the 'mud' disappears. The electricity can flow forever, like a cycle on a perfect, endless road. This is superconductivity — zero resistance, no energy wasted.

How it works

1. Normal state (room temperature): Atoms in the wire vibrate a lot. Electrons (which carry current) bump into them and lose energy as heat.
2. Cooling down: As you cool the material, atoms vibrate less, so electrons move more easily, but there is still some resistance.
3. Critical temperature (Tc): If you cool it below a special temperature (called the critical temperature), something amazing happens.
4. Electron pairing: Electrons team up in pairs called Cooper pairs. These pairs move together smoothly, without bumping into atoms.
5. Zero resistance: Now, the current can flow with no resistance at all. If you start a current, it can keep going forever (unless the material warms up again).
6. Expulsion of magnetic field (Meissner effect): The superconductor pushes out all magnetic fields from inside it, another key sign of superconductivity.

The story behind it

Superconductivity was discovered in 1911 by Dutch scientist Heike Kamerlingh Onnes. He was studying how metals behave at very low temperatures. In his lab in Leiden, Netherlands, he cooled mercury to about −269°C and found its resistance suddenly dropped to zero. He was trying to understand how cold affects electricity, and this discovery shocked the world. Onnes later won the Nobel Prize for this work.

Where you see it in real life

• MRI machines: Hospitals use superconducting magnets to make strong, stable magnetic fields for scanning your body.
• Maglev trains: These trains float above the tracks using superconducting magnets, so there’s no friction — they can go very fast.
• Particle accelerators: Superconducting wires guide and speed up particles in huge labs like CERN.
• Electric power cables: In some cities, superconducting cables carry electricity with no energy loss.
• Quantum computers: Superconductors help make special circuits that store and process quantum information.
• Magnetic levitation toys: Some science kits use superconductors to make small objects float above magnets.

Why it matters

• Saves energy: Superconductors can carry current without wasting energy as heat, making power grids much more efficient.
• Strong magnets: They make very powerful magnets for medicine, research, and transport.
• New technology: Superconductors are key for future tech like quantum computers and superfast trains.
• Understanding physics: Studying superconductivity helps us learn about quantum mechanics (the rules for tiny particles).

Worked example

Let’s say you have a loop of superconducting wire. You start a current of 5 A (amperes) in it at −270°C. How long will the current last?

Given: Superconducting wire, R = 0, Current started = 5 A
Step 1: In a normal wire, current stops after some time due to resistance (I = 0 eventually)
Step 2: In a superconductor, resistance R = 0, so no energy is lost
Step 3: The current will keep flowing forever (as long as the wire stays cold)
Answer: The current lasts forever (theoretically infinite time)

Common mistakes

• Thinking all metals become superconductors: Only some materials do, and only below their critical temperature.
• Believing superconductivity happens at any low temperature: It only starts below a specific temperature (Tc), which is different for each material.
• Forgetting the Meissner effect: Superconductors not only have zero resistance, they also push out magnetic fields.
• Assuming room temperature superconductors exist: Most need very cold temperatures, though scientists are searching for ones that work warmer.

Remember it

Superconductivity = zero resistance + no magnetic field inside, but only when it’s super cold!