Physical meanings of some physical quantities

[challarao_confused]

Hi!

I am so confusing about the topics covered in waves and oscillations section of physics

I am not getting the physical meaning phase phi,what is phi why it is important what is the phase velocity etc..?.

Please explain me these concepts..

Any help would be appreciated..

[Danijel Gorupec]

You can plan to do any job in phases. Somebody can ask you ‘in what phase of your job you are now’ and you will tell him ‘I am just finishing the first phase’.

 

 

Similarly, an oscillator does a ‘planned job’ – it oscillates. You can ask ‘in what phase is it now?’, and the answer could be ‘it almost reached the upper point’. So, the phase tells us about where on its ever-repeating path the oscillator exactly is.

 

 

As oscillators exhibits ‘circular’ behavior patterns (they just oscillate round and round) we like to describe its phase in radians (or degrees of arc). Zero radians means that it is just at the beginning of its cycle, pi radians means that is in the middle, and 2*pi radians means that it just closed its cycle (and is beginning the new one). Of course, this is possible only if the starting point is assumed.

 

 

Two similar oscillators can have same frequency, but can differ in phase. One of them can be just ahead of the other one, for example. We say that there exist a phase difference between them. The phase difference is also mostly stated in radians, of course.

 

 

The problem is that people sometime use word ‘phase’ when they actually mean ‘phase difference’. You may hear sentences like ‘first oscillator has phase of zero, while second one has phase of pi/4’. It actually means that there is pi/4 phase difference between them.... In equations you will find phi and this phi is about phase difference (in respect to some reference).

 

 

 

Waves are a bit more complicated than oscillations because have spatial dimension. You may imagine a wave as an array of dots that all oscillate in orchestrated manner. Neighborhood dots are oscillating with just a slight phase difference. In one moment one dot is in some phase, just a bit latter the neighborhood dot reaches the same phase, just another bit later the next dot reaches the same phase and so on... The ‘phase velocity’ tells you how fast is this progressing through space.

 

 

The phase velocity sounds fancy, but it is not (people use it every time when they talk about ‘how fast are waves on the sea’). It tells about propagation velocity of energy/disturbance in a wave.