Part of the show Cambridge Science Festival Q&A
Tristan, Australia asked:
What causes the Earth to have a magnetic field?
Dave – They're not sure but they think its to do with the centre of the Earth being a liquid metal ball and you get convection currents in that. There's a strange thing where it excites itself and turns itself into a dynamo, you get electric currents which make an electro magnet and that's what they reckon is making the magnetic field.
Chris – The field does flip from time to time doesn't it? I think the average is about every 100, 000 years, but in fact its less than that.
Dave – Yes, they reckon we're due for one fairly soon as the Earth's magnetic field is getting weaker very fast.
Chris – So does the North pole become the South Pole?
Dave – Yes.