Superconductivity

Zero resistance in the deep cold

Some materials lose all electrical resistance when cooled below a critical temperature, Tc. This is superconductivity. A current can then flow with no energy lost as heat.

Mercury becomes a superconductor below 4.2 K and lead below 7.2 K. Certain ceramics do so near 135 K, which is still far below room temperature.

In a normal metal, resistivity falls gradually as it cools but never reaches zero. In a superconductor, it drops suddenly to zero at Tc (Fig. 10.14).

Below Tc the resistance is not small, it is zero. No resistance means no heating and no wasted energy.

Key terms

Superconductivity

The state of a material with no electrical resistance.

Critical temperature (Tc)

The temperature below which a material becomes a superconductor.

Resistivity

How strongly a material opposes current; plotted in Fig. 10.14.

Maglev train

A train that floats above the track using superconductors, reducing friction.

Short questions with model answers

1. Q1. A sample of lead is at 10 K. Is it a superconductor? What if it is cooled to 5 K?

   • Compare with Tc.
   • Decide.

   Not at 10 K; yes at 5 K, where its resistance is zero.

2. Q2. Why would superconducting power cables save energy?

   • Normal cables.
   • Superconductors.

   No heating losses, as long as the cable is kept below Tc.

Common mistakes

• ✗ A superconductor has very low resistance.

  ✓ Below Tc its resistance is exactly zero.

• ✗ Any metal becomes a superconductor if cooled enough.

  ✓ A normal metal's resistivity falls gradually but never reaches zero (Fig. 10.14).

• ✗ Ceramic superconductors work at room temperature.

  ✓ They need about 135 K, which is still very cold.

MCQs

1. 1. Superconductivity means a material has:

   1. (a) Very high resistance
   2. (b) Resistance that rises with cold
   3. (c) Zero resistance below Tc
   4. (d) No electrons
   Show answer

   (c) p. 15.

2. 2. Mercury becomes a superconductor below:

   1. (a) 4.2 K
   2. (b) 7.2 K
   3. (c) 135 K
   4. (d) 273 K
   Show answer

   (a) p. 15. 7.2 K is for lead; 135 K for ceramics.

3. 3. Which material class has the highest critical temperature given in your book?

   1. (a) Mercury
   2. (b) Lead
   3. (c) Copper
   4. (d) Ceramics
   Show answer

   (d) MCQ 10.5 (p. 17): ceramics, about 135 K.

4. 4. As a normal metal is cooled, its resistivity:

   1. (a) Drops suddenly to zero
   2. (b) Falls gradually but stays above zero
   3. (c) Rises
   4. (d) Does not change
   Show answer

   (b) Fig. 10.14.

5. 5. Which is NOT a use of superconductors listed in your book?

   1. (a) Car radiators
   2. (b) MRI machines
   3. (c) Maglev trains
   4. (d) Particle accelerators
   Show answer

   (a) p. 16 lists MRI, maglev and particle accelerators. Radiators use water (10.2).

Quick revision

• Below its critical temperature Tc, a superconductor has zero resistance and wastes no energy.
• Mercury 4.2 K, lead 7.2 K, ceramics about 135 K; a normal metal never reaches zero.
• Uses include MRI machines, maglev trains and particle accelerators.