interference and diffraction - braggs experiment (1 Viewer)

[sasquatch]

In my edition of jacaranda hsc physics (edition 1), there are detailed explinations of diffraction and interference leading up to the Braggs experiment. It is necessary to know this (in such great detail) for the HSC?

Also is it necessary to know the formula, I = vqAp?

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Also i have another question im confused about:

When a metal has no potential applied to it, the electrons within still undergo random motion. Due to this random motion, the electrons collide with the atoms and other electrons. When the electrons collide, isnt their kinetic energy converted into heat energy? But obviously this does not actually happen...

Why is it that when a potential is applied across a metal, the collisions between electrons and atoms and other electrons, the kinetic energy of the electrons is converted to heat and when there is no potential it is not?

Well i dunno if i explained myself properly.. as i dont really understand it. Any help would be appreciated..

 

[alcalder]

Well, the HSC syllabus dot point says:

"define diffraction and identify that intereference occurs between waves that have been diffracted."

The HSC Supporting document says about these verbs:

define - state meaning and identify essential qualities
identify - recognise and name

HSC Online has this to say:

http://hsc.csu.edu.au/physics/options/quanta_quarks/3005/phy982.html

This says to me that you do not need to know diffraction and intereference in any great detail, nor any equations.

I would say, be able to give a good meaning of diffraction and pick our salient points about it eg, Huygen's Principle to explain why diffraction occurs, that noticeable diffraction only occurs when the wavelength is comparable or less than the slit width, that coherent light produces distinction diffraction patterns when passed through diffraction gratings and so on.

Know what Intereference is and why it happens.

Trying to think what that equation might be.

The old syllabus "Wave Properties of Light" used to go into the nitty gritty of both diffraction and intereference (it was way more involved - believe me, I tutored, taught and learnt it).

EDIT:

When a metal has no potential applied to it, the electrons within still undergo random motion. Due to this random motion, the electrons collide with the atoms and other electrons. When the electrons collide, isnt their kinetic energy converted into heat energy? But obviously this does not actually happen...

Why is it that when a potential is applied across a metal, the collisions between electrons and atoms and other electrons, the kinetic energy of the electrons is converted to heat and when there is no potential it is not?

When a potential is applied there is more energy to move the electrons and hence more motion and more collisions, hence more heat.

However, remember, electrons like to sit in in their shells and need a certain packet/quanta of energy before they can go visiting. If they were giving off heat all the time, they would be losing energy and spiralling into the nucleus - hence the need to think of an electron as a wave/particle that stays put unless it either absorbs or gives off a quanta of energy. But electrons do not lose energy when in their natural state. They just sit.

I wonder if that answered it? Or just made it more confusing. An interesting thought, though.

 

[sasquatch]

no wait wait..hehe i said "leading up to the braggs experiement". That dot point was from Quanta to Quarks. I guess i didnt make myself clear. I was refering to Superconductors within the module "From Ideas to Implementation".

Thanks though.

 

[alcalder]

OOPS, my mistake. Here we are in the Ideas to Implementation thread and I'm doing Q to Q. I guess mention dope at this point would be appropriate.

OK, so the syllabus dot point is:

"outline the methods used by the Braggs to determine crystal structure"

outline = sketch in general terms; indicate the main features of

HSC Online: http://hsc.csu.edu.au/physics/core/implementation/9_4_4/944net.html#net1


Once again, only a general description is needed. There is no mention of the diffraction equations in the syllabus (but they can be useful in descriptions - like a picture, sometimes an equations says a thousand words).

Oh and there is that equation you mentioned. I would say you don't need to know it BUT knowing and using equations like that can help in your explanation of superconductivity if you feel they will help when talking about drifty velocity of electrons.

And things do radiate heat. It may be room temperature, but it is not absolute zero. So even electrons in a metal, that are "bouncing" around in that valence shell are creating heat (if you like) by their very motion. But still, more potential, more movement, more collisions, more heat.

 

[sasquatch]

Cool thanks for all your help.

I also have another question which is confusing me.

Coul someone verify the following:

Its in reference to the dot point:

4. Identify that resistance in metals is increased by the presence of impurities and scattering of electrons by lattice vibrations.

When an electric field is applied across a metal, the electrons experience a net velocity in the opposite direction to the electric field. This increased motion causes an increase in the collosions existing among electrons and between electrons and the ions. This increased collisions causes the electrons to be scattered, and more kinetic energy of the electrons to be converted into heat energy. This heat energy causes the ions of the lattice to vibrate more, further increasing the chance of the electrons colliding with the ions of the lattice. Chemical impurities existing within the metal also cause the electrons to be scattered, and constitute to an increase in heat energy. As a result of these two factors, the passage of the current flow is greatly impeded, which offers resistance to the flow of current through the conductor.

Hope its not too much, but thanks!! And thanks again alcalder

 

[alcalder]

That all looks about right. Does it makes sense to you, because I think you described it nicely?

 

[sasquatch]

yeah it makes sense. i wrote that its not from a book or anything. just cuz like some books made it sound like an external source of heat was the reason why the ions in the lattice were to vibrate, it didnt describe the heat emanating from the actual collisions.