Estimating the size of a nucleus using electrostatic repulsion can be traced back to the invention of an ultra-sensitive apparatus, which measured forces in terms of the twisting of a fine wire, dating from the 18th Century. This device, known as a torsion balance, is often attributed to the French engineer and physicist Charles Augustin … Continue reading Coulomb’s Law and Nuclear Radii
How do electric cells generate a current?
We take for granted the fact that an electric cell (or battery) will cause a current to flow through an external circuit. The current is often discussed in terms of a flow of electrons (like water through a pipe) or energy transferred between adjacent electrons (like a row of dominoes falling onto each other). It … Continue reading How do electric cells generate a current?
Red-Shift and the Age of the Universe
Stars can be characterised by their absorption lines, which reveal their temperature and composition. There is more detail about this in two recent posts about the HR diagram and stellar classification. A similar approach can also be used to characterise entire galaxies but in this case the aim is to determine how quickly they are … Continue reading Red-Shift and the Age of the Universe
The formation, life and death of stars
All stars are born through the same basic mechanism but their evolution depends on their size. In particular, all stars go through a hydrogen-fuel stage but the time spent fusing hydrogen is determined by the star's initial mass. The final fate of a star is also predicted by its mass; some stars will simply fade … Continue reading The formation, life and death of stars
What are lumens? (part 2)
In the previous discussion about lumens (https://physbang.com/2025/12/30/what-are-lumens/) some numbers were quoted without any explanation. This post adds a bit more detail to those numbers. Firstly, the maximum theoretical output of a light source was stated to be 683 lumens-per-watt. Why that number? The answer is to be found in the definition of the candela, which … Continue reading What are lumens? (part 2)
What are lumens?
Modern light sources are rated in lumens or, more importantly, lumens per watt. The first figure expresses the brightness of the source whereas the second indicates its efficiency. But what, exactly, is a lumen? To answer that question it is useful to recall that the first man-made light sources relied on heat and were very … Continue reading What are lumens?
Beta Decay Q-value
At GCSE level, beta decay is said to be the emission of an electron (or positron, in beta-plus decay) when an unstable nucleus transforms into a more stable nucleus. The parent and daughter nuclei are different elements with the same nucleon number (sum of neutrons and protons) but their proton numbers vary by +1 for … Continue reading Beta Decay Q-value
How high does a spring toy jump?
Determining a spring toy’s jump height is a useful exercise in the application of physics, not only in terms of this specific example but also in highlighting contrasting approaches to answering any sort of physics question. As is so often the case, there are two possible starting points; the first is theoretical whereas the second … Continue reading How high does a spring toy jump?
Beta Decay and Energy Conservation
I have just finished reading an excellent book that traces various theories about beta decay in the first third of the Twentieth Century. Controversy and Consensus: Nuclear Beta Decay 1911-1934 is an editted version of the successful PhD thesis written by Carsten Jensen, who clearly had a deep passion for unravelling physics history but died … Continue reading Beta Decay and Energy Conservation
How reliable are AI assistants?
To explore the current state of AI reliability in a physics context, two questions were put to five free AI assistants, all available via https://duck.ai/; Why don’t radioactive decay chains end with iron? Which isotope has the highest binding energy? Before revealing the AI responses, here are my own answers. Question 1 assumes that all … Continue reading How reliable are AI assistants?
