If the sun suddenly became a black hole, would the earth be sucked in?

According to a widespread belief, black holes suck, inexorably, any object within the range of their gravitational force, regardless of the kinematic state of the poor prey. In fact, this is misconception. A black hole’s gravitational field is no different from the one of any other body. Thus, any object passing nearby, would follow the same trajectory it would have followed if the black hole was not such, but its mass was distributed on the volume of the star from which it originated.

In other words, if our Sun became a black hole (it won’t, because it does not have…


Thank you for your interesting considerations (as usual). It took some time to me to respond because in this period I am very busy with teaching.

I mostly agree with what you say about the equivalence sign "=". On the other hand, I must admit that in certain cases (like this) I suspect that this is not a mere equivalence in terms of numbers. Of course, I admit that my position about that has no scientific grounds, so I will not elaborate on that. I strongly believe that not all of out thoughts must be solidly fixed on scientific arguments…


Using Arduino, it is straightforward to investigate the Faraday-Neumann-Lenz Law

The pandemic has severely restricted the possibilities to attend practicals at schools and university. Arduino can be exploited, not only to mitigate the effects of the lockdowns, but to introduce a new way of engaging students in laboratory activities. One you start using Arduino for teaching, you will never return to traditional instruments and methods.

The first observation of electromagnetic induction, consisting in the development of a current in a coil when the magnetic flux varies through a surface that has the coil as a boundary, is credited to Michael Faraday, who reported it in 1831, formulating the law that…


Teaching energy before forces is not only possible but desirable.

This post was motivated by a tweet by Frank Noschese, reproduced below, to which I responded that I am a big fan of the “energy first” approach.

I must say that the approach I am advocating is not an idea of myself. I started thinking about it after reading a textbook by C. J. Fischer: “The Energy of Physics”, from which I derived my own approach. Fischer, in fact, introduces the concept of energy almost axiomatically, while I always prefer an approach based on experience.

In order to observe that free fall is a uniformly accelerated motion with acceleration g=9.8


Using Arduino we can measure temperature and pressure of a gas and study its equation of state

In my last post, I show how to obtain experimental data from Arduino via BLE. As promised, I now come to the collection and the interpretation of these data, in the case of an experiment in which a gas is subject to an isovolumetric process, i.e., a process in which the pressure p and the temperature T can change, while volume V remains constant.

The experiment is, indeed, very simple. You just need a BLE capable Arduino board, a barometric and temperature sensor, a power source and a jar. I used an Arduino MKR 1010 WiFi connected to a BME180…


How to collect and publish data using Bluetooth Low Energy with Arduino

In this post, I illustrate operation of an Arduino with BLE (Bluetooth Low Energy). As usual, physics being my field, I do not discuss a generic example (the equivalent of “Hello, World!”), but a concrete physics application. Sometimes, for brevity or clarity, I will not be rigorous, and purists may turn up their noses, but, as I read on a A. Zee’s books, “sometimes, too much rigor soon leads to rigor mortis”.

WARNING: I assume you are familiar with Arduino and C/C++ programming. If you are not, you will hardly understand the content of this article.

What I write here…


General relativity is one of the most technically difficult physics theories, as it requires uncommon knowledge and mathematical skills. However, it is possible to calculate the relativistic effects induced by the curvature of space-time with good precision, and even to derive its formula, with a few simple arguments.

We know from special relativity that time flows more slowly in a moving system than in a stationary one. Given t as the time elapsed aboard the moving system, the time elapsed for the stationary one is 𝛾t, with 𝛾>1. In other words, time for those moving flows more slowly. A striking…


Almost like a flush toilet, indeed…

Transistors are the building blocks of our electronic gadgets: from smartphones to computers, toys, cooking devices, cleaning tools, etc.

Despite their ubiquity, only a few people know about how they work. In fact, understanding their principles of operation is not so hard.

The diode: a transistor building block

In order to understand how a transistor work, we need to understand what a diode is. In fact, a transistor, in practice, is nothing but two diodes.

A diode, essentially, works as a valve for electric current. It lets a flow of electrons flow in one direction only, opposing the movement of electrons in the opposite direction, just…


So, how does it melt the ice?

In a previous post of mine, I illustrated a brief history of temperature scales in which I recalled that the reason for which 0 °F corresponds to -18 °C is that the latter was the lowest temperature that Daniel Gabriel Fahrenheit could achieve in his laboratory.

Such a temperature can be easily reached mixing water, ice and ammonium chloride. This was, in fact, a very common method to produce a frigorific mixture before the invention of the freezer. The melting of the salts in water, in fact, is a chemical reaction that requires heat to proceed. Indeed, as every Italian…


The most widely adopted temperature scale is the Celsius or centigrade scale. Nowadays, we use these terms as synonyms, but, in fact, they were not such, originally. The Celsius temperature scale was invented by Anders Celsius that, in 1742, proposed its use by publishing “Observationer om twänne beständiga grader på en thermometer” (Observations of two constant gradations on a thermometer).

In his paper, Celsius reported the observations, according to which the temperature of melting ice (marked as C) and that of boiling water (marked as D) appeared to be constant, if measured with a mercury thermometer.

He then proposed, on…

Giovanni Organtini

Professor of Physics at Sapienza Università di Roma. Member of the CMS and PADME collaborations. Arduino advocate and phyphox ambassador.

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