Recent content by Wille

Ok. I found this explanation: https://www.quora.com/Why-does-PCA-choose-covariance-matrix-to-get-the-principal-components-of-features-X It says: "This is because covariance matrix accounts for variability in the dataset, and variability of the dataset is a way to summarize how much information...

I don't know. Do you mean the svd function? Matlab or not, which of the two approaches is correct to use?

I wonder how to accurately perform data compression on the m x n matrix X using PCA. Each row is a data point, and each column is a feature. So m data points with n features. If I like to go k < n dimensions, how is the correct way of doing so? How to I accurately create the matrix W_k, which...

Ok thanks for your answer. What about the microwave oven, is it the same way of thinking? One cannot say that it is either the electric field or the magnetic field of the electromagnetic radiation that oscillates the water molecule? Or is something different in that case? And also the part...

As I understand it, it is the magnetic part of electromagnetic radiation which creates a current (AC) in a radio receiver (i.e in an antenna) which can be translated into sound/graphics and so on. Is this correct? I mean in the sense that it is the magnetic field in the incoming radiation which...

@Dr_Nate Ok I thought that that is how electrons move. I.e. a current is free electrons moving from atom to atom. As the electrons collide with atoms, the kinetic energy of those atoms increases, i.e. heat is created.

@scottdave Thanks! Great answer! That make sense and explains why the metal has to be magnetic and why it is not enough with a metal being a good electrical conductor. It is foremost not a matter of moving electrons. Many sites say that it is a matter of current and resistance though, for...

I can create a fairly strong magnetic field by leading current through a copper coil. However, I cannot then with that magnetic field create a current (not a useful one at least) in an object made out of copper. I understand that this is because copper lacks of magnetic properties. For instance...

@Drakkith I have one final question in this matter. I thought I could continue this thread instead of writing a new one. The lightning can heat the air to around 30 000 degrees C. The electrons move through the air due to the large potential between cloud and ground. Is it again the same reason...

Ok, but what makes the windshield go below zero degrees C then?

Hi, I want to model the phenomena that a windshield gets covered in ice during a night with clear sky even tough the air holds a temperature of a few degrees above zero Celsius (at which water freezes for sea level pressure). Clearly the windshield gets a temperature below zero Celsius, and...

Thanks for this great answer. So it is the same for lamps then? It is not mainly electron transitions that make the metal thread emit light? But rather the accelerations of the atoms/molecules in the metal thread? And those accelerations I suppose happen due to the electrons trying to move...

I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. Or possibly the other way around; the more vibrations, the more heat we say that the object has (right?). But what is happening when an...

Again thank you for the answer. Just one more thing: would you say that this https://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission is a correct way of describing what is happening? Is it correct to think in terms eigenfrequencies of the electrons...

Ah, thank you for the answer. Yes, I think the line you wrote "Or perhaps you're asking why the emission spectrum isn't exactly the same as the absorption spectrum" is spot on. Now I think I understand. The new photon can be emitted with a frequency belonging to the (for instance) infrared...