I mentioned a couple of weeks ago that I am teaching thermodynamics at the moment [see ‘Conversations about engineering over dinner and a haircut‘ on February 16th, 2022]. I am using a blended approach [see ‘ Blended learning environments‘ on November 14th, 2018] to deliver the module to more than 300 first year undergraduate students with one hour in the lecture theatre each week while the students follow the components of the MOOC I developed some years ago [see ‘Free: Energy! Thermodynamics in Everyday Life‘ on November 11th, 2015, and ‘Engaging learners online‘ on May 25th, 2016]. I have found that first year undergraduates are reluctant to participate in the online discussions that are part of the MOOC and so last year I asked them to discuss each topic in small groups with their academic tutor. I got some very positive feedback from tutors who had interesting and stimulating discussions with their students. We are repeating the process again this year. The first discussion is about energy transformations: noting that energy is always conserved but constantly transformed into different forms, each student is asked to start from an energy state of their choice and to trace the transformations backwards until they can go no further. In the lecture preceding the discussion with their tutor I provide some examples for starting states, including breakfast cereal, a pole vaulter in mid-jump and a bullet train. I also describe the series of transformations from the Big Bang to tectonic plate movement: after the initial expansion caused by the Big Bang, the universe cooled sufficiently to allow the formation of sub-atomic particles followed by atoms of hydrogen and some helium and lithium that gravity caused to coalesce into clouds which became the early stars, or solar nebula. A crust formed on the solar nebula which broke away to form planets. Our planet has a molten core with temperatures varying from 4,400 to 6000 degrees Celsius, compared to around 5,500 degrees on the surface of the sun. The temperature variation in the Earth’s core cause thermal currents which drive the movement of tectonic plates and so on [see ‘The hills are shadows, and they flow from form to form, and nothing stands‘, on February 9th, 2022]. Most chains of energy transformation lead backwards to the sun and forwards to dissipation of energy into some unusable form which we might call ‘entropy’ [see ‘Life-time battle‘ on January 30th, 2013].
When I was writing about cosmic heat death a couple of weeks ago [see ‘Will it all be over soon?’ posted on November 2nd, 2016], I implied that our sun would expire on a shorter timescale of about 4 to 5 billion years but without mentioning what we expect to happen. The gravitational field associated with every piece of matter is proportional to the mass of the piece of matter and inversely proportional to distance from its centre. The size of the sun implies it should collapse under its own gravitational forces, except that the fusion of hydrogen in its core causes an outwards heat transfer, which prevents this from happening. The sun remains a sphere of hot gases with diameter of about 864,000 miles by ‘burning’ hydrogen. When the hydrogen runs out, the gravitational field will take over and the sun is expected to collapse to a 30,000 mile diameter ball of atoms and free electrons, or a white dwarf.
These are all spontaneous processes and so the total entropy must increase although there are some local reductions. The heat dissipated following the fusion of two hydrogen nuclei generates more entropy in the surroundings than the local reduction caused by the fusion. The collapse to white dwarf would appear to represent a substantial reduction of entropy of the sun because the atomic particles are crushed together. However, this is countered by the release of photons to the surroundings which ensures that the entropy of the surroundings increases sufficiently to satisfy the second law of thermodynamics.
Isaac Asimov, The roving mind: a panoramic view of fringe science, technology, and the society of the future, London: Oxford University Press, 1987.
An extract is available in John Carey (editor), The Faber Book of Science, London: Faber & Faber, 2005.