Superconductors as Perfect Diamagnets
In the ideal Meissner state, bulk B = 0 and the effective susceptibility is -1 in SI for suitable geometry. The critical temperature T_c(B) generally decreases as applied field increases; beyond the critical boundary the material becomes normal.
Why this shows up in the exam
Magnetic levitation · Interpreting resistance-temperature transition graphs · Setting operating limits for superconducting magnets
Learn the idea
Below its field-dependent critical boundary, a superconductor expels magnetic flux and has zero dc resistance. Cooling through the superconducting transition removes dc resistance, while screening currents oppose magnetic penetration. A stronger applied field can lower the critical temperature and eventually destroy the state.
🧠 Memory hook: Superconducting means zero resistance and field expulsion only inside the critical boundary.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- B_inside = 0 in the ideal Meissner state — perfect bulk flux expulsion under the stated ideal conditions
- chi_m = -1 — ideal SI volume susceptibility of a perfect diamagnet
- B increases implies T_c(B) decreases — qualitative critical-field trend
How to approach it
- 1Compare temperature and field with the critical boundary
- 2Place the larger-field transition at lower temperature
- 3Use B_inside = 0 only in the superconducting Meissner state
Common slip-ups that cost marks
- •Treating zero resistance alone as the full Meissner effect
- •Claiming critical temperature rises with magnetic field
- •Assuming every field strength preserves superconductivity below T_c(0)
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A charge of 2 microC moves perpendicular to a 3 T magnetic field at 4 x 10^5 m/s. Find the magnetic force.
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