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In Our Time newsletter: Conductors and Semiconductors

Friday 24 February 2012, 18:03

Melvyn Bragg Melvyn Bragg

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Editor's note: In Thursday's programme Melvyn Bragg and his guests discussed the Conductors and Semiconductors and the physics of electrical conduction. As always the programme is available to listen to online or to download and keep - PM.

Conductor and superconductor image

Hello

That was a tough one if your knowledge of physics ended in your mid-teens. But once again, particularly by vivid use of easy-to-understand analogies, the three contributors got over to the rest of us what the achievement was.

It's the speed of the whole thing that is astonishing, isn't it?

Just over a hundred years or so ago, electricity was a bit of fun in a circus, or just a marginal part of what scientifically-inclined persons did. And now, with the discoveries swiftly and brilliantly made by one group of scientists after another, we have the forces which go into lightning being controlled in such a way that they make much of the world go round.

I don't want to bang on about this, but if ever there was a staring-you-in-the-face example of pure research leading to a general improvement in the condition of mankind, a multitude of highly skilled jobs and increased global wealth, then this is it.

So afterwards skated down to the office on what seems like a spring day and from there to lunch with Tom Morris, where I discovered that, among other things, he plays a game called rackets, which seems to me to be squash hell multiplied by ten.

The day ties up in a neat circle, because tonight I'll be at the Guildhall as Chancellor of the University of Leeds for a banquet to celebrate The Queen's Anniversary Prizes for Higher and Further Education.

This is a particularly special event because it's the jubilee year and the University of Leeds, with about a dozen other universities, has won one of these prizes. So I'll be surrounded by men and women who have devoted their lives just to pure research, and they'll be in a most ancient hall, and I'll be in the presence of people who are the generators of the future prosperity of these islands.

It's great to go to these ceremonial events now and then. The dressing up in university garb. The processions. The speeches - yes, even the speeches - and meeting a group of likeminded people whose object is to find out more about the way the world works and to bring these discoveries to the rest of us.

The knot will be tied tomorrow morning when we go to Buckingham Palace, where those who did the work will receive the medals from HM The Queen; just in passing, in these pessimistic times, they are the world leaders in their field. Before that I'll be on breakfast television talking about Class and Culture; the first part of my three-part series goes out on BBC Two in the evening. It might seem a long jump. It might seem part of a positive spectrum.

Best wishes

Melvyn Bragg

PS Friday AM. It's one of these "is it real?" situations, wandering around St James's Park early for an appointment with HM The Queen in Buckingham Palace. 'Tis passing strange.

PPS Another freak spring day. In St James's Park I'm sure I can hear the bulbs and the buds having a serious conference. "You go first," they're saying, "I'll just give it another day or two."

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    Comment number 1.

    Pure research, that is open minded and free of pre determitive constraints as to the rationalistic spirit of the methodology and homogeneiety of particular exponents of it would show that nice days in the spring were more frequent than interpolated popular memory may recollect-or now expect.

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    Comment number 2.

    I thought that this was a missed opportunity to explain to the non scientist how super conductivity works.
    Whilst talking about the resistance of a normal conductor the speakers mentioned the electron collisions both with themselves and with the ions that make up the conductor. What they failed to say was that the electron- electron collisions do not contribute to the resistance, they just redistribute the electron energy amongst the electrons. It is the electron-ion collisions that cause resistance and in this sense the analogy to a fluid in a pipe works quite well. The energy that is given up to the ions is randomised because the ions are relatively hot and any the energy gets spread out.
    In a super conductor, however, the energy given to the ions causes the ion lattice to vibrate and the temperature is low enough that the coherence of the vibration is not lost before a second electron colliding with the ions can recover the energy. The second electron does not have to hit the same ion as the first one as the ion vibrations cover a macroscopic region of the lattice. In this way electrons are paired (Cooper pairs) together although not close, hence the comparison with the jive. A comparison that I thought worked well.

    Then there was the discussion about the Meissner effect, where the magnetic field gets excluded from a super conductor. This is some what harder to explain physically but the important point is that the two electrons in a Cooper pair must have opposite spin for super conductivity to work. This is the opposite of a magnetised conductor where electrons must have similar spins. The interaction of the Cooper pairs with the lattice must force the spin on the second electron to be the opposite to that on the first one. In doing so the process pushes the magnetic field out of the super conductor. The Meissner effect is the true test of super conductivity.

 
 

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