Physics marathon 2011 (3 Viewers)

[taeyang]

This is a thread for all you physicists out there who wish to cram before the externals or to just test their knowledge of the universe (at a highschool level that is ^.^)

There are only 3 rules of Physics club:

- The thread will not progress question wise unless the previous question has been answered
- The correct answer giver must post the next question in the same post as there answer.
- Looking up the answer is cheating, write your answers (or at least try ^.^) from memory

Note: It wouldn't be a good idea to set up a non-core related question unless you specifically say "this is for the ...... students"

Question Number 1.

The BCS Theory states that in superconducting materials electrons move through the lattice in "Cooper Pairs".

Discuss the BCS Theory by considering a material above and below its critical temperature (5 marks)



P.S. It would be a nice idea if you would post where the question is from, i.e. from this textbook, from this past paper... etc...


Good luck and HAVE FUN!

 

[jamesfirst]

Wouldn't it be better to just ask a direct question rather than a full marker question... as in, kinda like a multiple choice but you ask "what is _____________"


anyway, answer:


- The BCS theory was a theory proposed to explain the superconductivity of a ceramic material that has achieved it's superconducting state due to the achieved critical temperature.
- In the theory, the electrons travel in a pair called 'cooper-pairs' and will travel through the crystal structure unimpeded.

- The first electron of the pair travels through the structure and creates a vibration called a phonon which produces a dense positive area (i think that's what it's called ) which attracts the 2nd electron of the pair.
- The 2nd electron will travel through the disrupted structure to stabilize it again. Although the two electrons are both negatively charged, they are attracted by 'phonon-mediated attraction' (I think this is what it's called).

- Hence, the cooper pairs are able to travel within the lattice unimpeded.

Advantages:
- The BCS theory is successful in explaining Type 1 superconductors that has a critical temperature below 30K.

Disadvantages:
- However, it cannot explain the type 2 superconductors.



Next question: Why was scientists confused about the nature of cathode rays and who resolved this dispute and how did he ? - Question is from me. I just made it up.

 

[_deloso]

Wouldn't it be better to just ask a direct question rather than a full marker question... as in, kinda like a multiple choice but you ask "what is _____________"


anyway, answer:


- The BCS theory was a theory proposed to explain the superconductivity of a ceramic material that has achieved it's superconducting state due to the achieved critical temperature.
- In the theory, the electrons travel in a pair called 'cooper-pairs' and will travel through the crystal structure unimpeded.

- The first electron of the pair travels through the structure and creates a vibration called a phonon which produces a dense positive area (i think that's what it's called ) which attracts the 2nd electron of the pair.
- The 2nd electron will travel through the disrupted structure to stabilize it again. Although the two electrons are both negatively charged, they are attracted by 'phonon-mediated attraction' (I think this is what it's called).

- Hence, the cooper pairs are able to travel within the lattice unimpeded.

Advantages:
- The BCS theory is successful in explaining Type 1 superconductors that has a critical temperature below 30K.

Disadvantages:
- However, it cannot explain the type 2 superconductors.



Next question: Why was scientists confused about the nature of cathode rays and who resolved this dispute and how did he ? - Question is from me. I just made it up.

will you get full marks for that question? you haven't considered the material when it is above it's critical temperature.

 

[jamesfirst]

will you get full marks for that question? you haven't considered the material when it is above it's critical temperature.

idc.. haha it was at midnight. hehe

 

[Fizzy_Cyst]

Why was scientists confused about the nature of cathode rays and who resolved this dispute and how did he ? - Question is from me. I just made it up.

- Cathode Rays displayed both particle and wave characteristics

- British physicists believed them to be negatively charged particles, Ze Germans believed them to be EMR

- Ze Germans found they could pass thru thin gold foil without damaging it, seeing as the smallest unit of matter was the atom (at the time), they must be EMR, Hertz and his team were aware that moving charges create magnetic fields, yet there was no magnetic field detected, therefore could not be moving charged particles.

- Hertz performed an experiment to show that cathode rays were not deflected by magnetic fields

- Crookes and the british believed they were -vely charged particles, as they were deflected in magnetic fields in a direction which negatively charged particles were expected to deflect, they caused a paddle wheel to turn, demonstrating they possessed energy and momentum (which waves do not have), they travelled much slower than the speed of light and the properties of the rays were the same regardless of the material making up the cathode (cathode rays seem to be present in all types of matter)

• J.J. Thomson observed that cathode rays were deflected in an electric field if the tube was evacuated (gas particles removed to create a partial vacuum)
o Cathode rays deflected towards the positive plate, proving that they were negatively charged particles

NEW QUESTION:
A piece of space junk of Mass M drops from rest from a position of 30,000km from the Earth's centre. Calculate the final speed it attains when it reaches an altitude of 1000km above the earth's surface. Assume air resistance is negligible. The radius of the earth is 6.37x10^6m

cant remember where i got this question from

 

[taeyang]

- Cathode Rays displayed both particle and wave characteristics

- British physicists believed them to be negatively charged particles, Ze Germans believed them to be EMR

- Ze Germans found they could pass thru thin gold foil without damaging it, seeing as the smallest unit of matter was the atom (at the time), they must be EMR, Hertz and his team were aware that moving charges create magnetic fields, yet there was no magnetic field detected, therefore could not be moving charged particles.

- Hertz performed an experiment to show that cathode rays were not deflected by magnetic fields

- Crookes and the british believed they were -vely charged particles, as they were deflected in magnetic fields in a direction which negatively charged particles were expected to deflect, they caused a paddle wheel to turn, demonstrating they possessed energy and momentum (which waves do not have), they travelled much slower than the speed of light and the properties of the rays were the same regardless of the material making up the cathode (cathode rays seem to be present in all types of matter)

• J.J. Thomson observed that cathode rays were deflected in an electric field if the tube was evacuated (gas particles removed to create a partial vacuum)
o Cathode rays deflected towards the positive plate, proving that they were negatively charged particles

NEW QUESTION:
A piece of space junk of Mass M drops from rest from a position of 30,000km from the Earth's centre. Calculate the final speed it attains when it reaches an altitude of 1000km above the earth's surface. Assume air resistance is negligible. The radius of the earth is 6.37x10^6m

cant remember where i got this question from

What a wacko question.. I sure hope it was as hard as I made it out to be

Answer - 22910.4022 m/s or km/h I can't really tell, so many units

I HIGHLY DOUBT it is right ^.^ But on the strange occasion that it is right....

Question Explain how Hertz witnessed the photoelectric effect in his experiment with radiowaves

 

[_deloso]

What a wacko question.. I sure hope it was as hard as I made it out to be

Answer - 22910.4022 m/s or km/h I can't really tell, so many units

I HIGHLY DOUBT it is right ^.^ But on the strange occasion that it is right....

Question Explain how Hertz witnessed the photoelectric effect in his experiment with radiowaves

Hertz used a spark gap with a parabolic reflector. This was connected to an induction coil. He observed a spark between the gap and as the induction coil constantly produced high voltage ac power, the spark remained in between the gap. the spark sets up a changing electric and magnetic field that propagates as a radio wave. A wire ring with a gap identical to the spark gap also induced a spark in between the gap due to the arriving radio wave. This was a clear indication that radio waves could induce current in a receiver because the receiver had no external connection to an electrical source.
As hertz shone ultraviolet light on the receiving loop, the spark intensified. As the gap was made larger, the spark still occured.
this event was known as the photoelectric effect but Hertz did not investigate this and did not provide any explantion

Next question: Astrophysics: define photometry and outline the advantages of photoelectric technology over photographic technology.
Modules: explain how the motor effect is applied in Galvanometers and in Loudspeakers?

 

[jamesfirst]

Modules: explain how the motor effect is applied in Galvanometers and in Loudspeakers?

-Galvanometer consists of a curved magnet that produces radial magnetic flux that is always parallel to the coils on an iron core in the centre.
-Thus, the torque will always be a max and a constant. When flows through the coils, it will be deflected due to motor effect.
-The spring in the centre of the iron core will counterbalance the torque created by the motor effect and cause it to rotate and move back to its original spot when there is no current flowing in the coil.

-Loud speaker consists of a circular magnets attached to a speaker cone. The coil is wrapped around the inner magnet and due to motor effect, the magnet will vibrate and cause the speaker cone to move in and out.
-This vibration will produce sound, where the electrical energy is converted into kinetic energy and hence sound energy.


Next question:

When is the torque maximum on a motor?

 

[_deloso]

when parallel to the magnetic field.

question: outline the advantages of solid state devices over thermionic devices.

 

[whatsmyname]

when parallel to the magnetic field.

question: outline the advantages of solid state devices over thermionic devices.

Solid state devices use diodes and transistors. Thermionic devices use valves and glass which makes it much more fragile then SSD.
SSD devices are more efficient (i.e less heat loss, more energy efficiency) compared to TD.
SSD have a short start up time then TD
SSD are smaller and cheaper then TD.

Question: Explain Thomson's experiment in working out the charge to mass ratio.

 

[jamesfirst]

Solid state devices use diodes and transistors. Thermionic devices use valves and glass which makes it much more fragile then SSD.
SSD devices are more efficient (i.e less heat loss, more energy efficiency) compared to TD.
SSD have a short start up time then TD
SSD are smaller and cheaper then TD.

Question: Explain Thomson's experiment in working out the charge to mass ratio.

-Previous scientists were confused about the nature of cathode rays. It was until Thomson determined its charge/mass ratio, that made it pretty obvious that it was a negatively charged electron that was different from a proton's charge to mass ratio.

-Thomson use an evacuated tube consisting of an electron gun, magnetic and electric field inside a discharge tube with no air.
-He balanced out the electric and magnetic field so that the cathode rays would travel straight. Thus, the velocity of the cathode rays could be determined by using:

F = bvq and F= Eq

-Thomson then realised that if the electric field was removed and only the magnetic field was exerting a force on a cathode ray, the cathode ray would travel in an arc of a circle - otherwise: centripetal force due to the continuous motor effect applied on a moving charged particle (it will go in a spiral motion).
-Thomson did so by removing the electric field and by equating:

F = bvq and F = mv^2/r, he found the charge to mass ratio. He could do this by deriving the formula and subbing in the velocity of these cathode rays that was previously derived above.

-He has found that the charge to mass ratio was different from a proton and has discovered an electron.


Next question:

Describe the procedure of determining the value of 'g' and identify the reason why the value of 'g' varies.

 

[Uzername]

Describe the procedure of determining the value of 'g' and identify the reason why the value of 'g' varies.

'g' can be determined in a number of ways.

Firstly, a pendulum can be attached to a non-elastic string to a retort stand. The mass of the pendulum is irrelevant. The pendulum is then deflected to a maximum of 30 degrees (any more, and the string will lose tension) from the perpendicular of the retort stand, and let free to swing. Using a stopwatch, time how long it takes the pendulum to complete 10+ full cycles (the larger the cycles, the greater the accuracy of the experiment). Then, divide the number of cycles by the time to obtain the period, T. Then, using the formula 2pi(length/g)^(1/2) = T, the value of g can be determined.

Alternatively, a mass can be attached to a ticker tape and is then dropped from a height.
Using v = (range/time), the velocity at various points can be determined.
Then, using (final v - initial v)/time = average velocity, the acceleration 'g' can be determined.

The value of 'g' varies for mainly 3 reasons:
- the equator is furthest from the centre of the earth, hence at this point g is smaller than at the poles, where the Earth 'flattens'. The poles are closer to the centre of the earth --> greater g.
- The density of the geography at the location alters g. A greater density will mean a greater attraction, hence a larger g. If the surrounding geography is water or low-density rock, then the value of g decreases. If you're near mountains/dense rock, there will be a higher g.
- The altitude affects g. Standing on top of a mountain will obviously mean less g compared to places below sea level. FUN FACT: During the Mexico Olympics, records in the sprint/long jump/high jump were broken because the location of Mexico on Earth meant that the value of 'g' was relatively smaller than other places.

Next Question:
Analyse the changing acceleration of a rocket during launch and the forces experienced by astronauts. You must make reference to the Law of Conservation of Momentum.

 

[InVinsanity]

The Law of Conservation applies in the launch of a rocket in space, which states in a closed system the initial momenta is equivalent to the final momenta. In a rocket launch the momentum of the gases released, is equivalent to the momentum of the rocket going forward. mv = -mv. As the rocket is propelled, each fuel tank provides a constant thrust, and the with usage of the gases the mass of the fuel decreases. So by F=ma by rearranging it to make the acceleration the subject, the thrust remains constant and the mass decreasing this increases the acceleration.
The maximum g-forces experienced, in launch will be just prior to the release of the fuel tank. Astronauts will experience g-forces

Discuss the advantages and disadvantages of superconductors?

 

[jamesfirst]

Discuss the advantages and disadvantages of superconductors?

Superconductors are ceramic materials that reaches it's superconducting state below critical temperatures.

It can be used in certain applications:

- Power storage/transmission
- Maglev train
- Particle accelerators
- Microchips

It is very advantageous because:

- Has 0 resistance and electrical energy can travel unimpeded, hence power loss is very low resulting in a further transmission and cheaper cost of electricity
- Since it has 0 resistance, in applications such as microchips it can send data incredible fast
- In maglev trains, it provides a 0 carbon emission transportation that has no friction between the wheels and the tracks (which means wheels don't have to be replaced)
- In particle accelerators, it can be used as electromagnets in cyclotron or synchrotron that will provide a very high powered electrical current, producing a much strong magnetic field (I think).

Disadvantages:

- Too damn expensive. It would cost a fortune to provide every application with superconductors.
- Too damn hard to maintain the critical temperature
- When ceramics reaches it's critical temperature, it becomes very brittle due to the incredibly cold temperature.


Next question:

Evaluate Michelson and Morley's experiment and discuss how it has helped with Einstein's Special relativity.

 

[InVinsanity]

Evaluate Michelson and Morley's experiment and discuss how it has helped with Einstein's Special relativity?

The Michelson and Morleys experiment was to determine the velocity of the earth relative to the ether, they thought that this would different in different directions because of interference patterns due to aether. After gazillion of attempts they were left with null results, as both beams of light arrived at the same time on the interferometer. These null results "questioned" the validity of the aether, with some scientists supporting that maybe the aether did not exist whilst others saying the the equipment utilised was not sensitive enough. It was not until Einstein in the early 1900s that claimed that the speed of light is constant in all frames of references.
The Michelson & Morley experiment, illustrates how new theories are introduced whilst others are abandoned, it has quite significance despite the null results.

How does an AC Induction motor work?

 

[RANK 1]

How does an AC Induction motor work?

ac current goes through the stator to create a rotating magnetic field, and due to lenz's law and faraday's law a current will be induced in the squirrel cage thing that will oppose the change causing the cage to spin with the magnetic field. and some other shit about 'slipping'

not the best answer but thats the general drift of things

 

[jamesfirst]

where's the next question dude...

 

[RANK 1]

oh right, umm...

describe what happens to a superconductor when cooled below critical temperature

 

[InVinsanity]

When superconductor is cooled below or at critical temperature there is minimal lattice vibrations to halt the movement of electrons through the crystal lattice. When it is cooled, a magnetic field does not permeate through, which leads to the Meissner effect, and this there is no loss of energy when it below its critical temperature.

Explain how the Braggs measured the crystal structure of metals, and what is a requirement for diffraction to occur?

 

[jamesfirst]

Explain how the Braggs measured the crystal structure of metals, and what is a requirement for diffraction to occur?

- Braggs carried out an experiment to measure the crystal lattice by firing an x-ray towards it and by recording the diffraction.
- They setup an x-ray source, collimator, a crystal lattice and x-ray spectrometer.

- When they fired the collimated x-ray towards the crystal lattice, it was diffracted and gave an interference pattern on the spectrometer. Diffraction occurs when the light wave bends around an object, whereby the corner of the object acts as a new point source from which new wavefronts are formed.
- The two waves will constructively and destructively interfere to give a region of maxima and minima (superposition).

- They noticed that the spacings between the crystal lattice is similar to the angle at the which the x has been diffracted and the wavelength of these x-rays:

" nㅅ = 2dsin∮ "


Discuss the principle of relativity