Physics marathon (hsc) (3 Viewers)

[Danstar2]

Whoever answers this correctly deserves an award.
Identify differences in p-type and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes.(4-marks)

It's fairly straight forward,

p-type and n-type semiconductors have an introduced 'impurity' atom which changes their conductivity.

p-type have an introduced impurity atom with one less electron (e.g. 3 valence electrons instead of 4) than that of the silicon atoms. This leaves a positive hole in the lattice, thus making the material a p-type semiconductor.

n-type also have an introduced impurity atom but with one more electron (e.g. 5 valence electrons instead of 4) than that of the silicon atoms. This increases the overall amount of charge carriers in the material, thus making it an n-type semiconductor.

Also I believe to make a p or n type conductor it's something like only 1 impurity atom per 1million silicon that is actually required, but I'm not sure..

 

[lolcakes52]

I Read this:
A superconductor will not allow a magnetic field to penetrate its interior.

An external magnetic field causes currents to flow inside the super conductor. These currents generate a magnetic field inside the superconductor that just balances the field that would have otherwise penetrated the material.

seems to contradict

 

[nerdasdasd]

So what's a general definition of a superconductor ?

 

[Danstar2]

I Read this:
A superconductor will not allow a magnetic field to penetrate its interior.

An external magnetic field causes currents to flow inside the super conductor. These currents generate a magnetic field inside the superconductor that just balances the field that would have otherwise penetrated the material.

seems to contradict

They don't contradict, both statements establish the idea that an external B field cannot penetrate that of a superconducting material. The second sentence just explains it. An external magnetic field is stopped by the superconductor as it causes currents to flow in the conductor which will effectively produce an adequately strong B field to balance the external field. Thus the two fields balance, the external does not penetrate the superconductor, if you get me?

 

[lolcakes52]

So what's a general definition of a superconductor ?

A Material, which below a certain critical temperature, has zero resistance due to a population of electron pairs which experience net attraction.

 

[nucgaek]

How would you describe rectifier/LEDs

 

[lolcakes52]

They don't contradict, both statements establish the idea that an external B field cannot penetrate that of a superconducting material. The second sentence just explains it. An external magnetic field is stopped by the superconductor as it causes currents to flow in the conductor which will effectively produce an adequately strong B field to balance the external field. Thus the two fields balance, the external does not penetrate the superconductor, if you get me?

What I find confusing is that for a current to be induced doesn't that magnet's field have to penetrate the interior for a brief amount of time? or is the interaction between magnetic fields which creates the current.

 

[someth1ng]

A Material, which below a certain critical temperature, has zero resistance due to a population of electron pairs which experience net attraction.

Actually, you need more than this. A superconductor is a material, when below a certain temperature, has zero electrical resistance and expels all magnetic flux.

Make sure you include the Meissner effect in the definition because when it is only zero electrical resistance, it is called an ideal conductor.

 

[Danstar2]

What I find confusing is that for a current to be induced doesn't that magnet's field have to penetrate the interior for a brief amount of time? or is the interaction between magnetic fields which creates the current.

When you attempt to place a magnet onto a superconductor, during a superconductor's 'superconducting' state, it will set up an electrical current flow near it's surface called 'persistent currents' and will effectively operate as an electromagnet. Thus, the superconductor will expel all magnetic flux (given it isn't too strong).

In other words, no the Ext. B field will not induce a current, or need to help create the current. The explanation is beyond the scope of this course.

 

[dawooddaman]

Plan Finish ITI till 10pm, space rev 11pm, MAG rev 12pm, Med rev 1am, ITI rev 2am

Wake up at 9am hit the exam, get back sleep until 6pm then start chemistry PP, 1 each 3 hour =14 overall.

Sad thing is I've only done 6 PP for physics only and don't know how to properly do projectile motion yet =(

 

[freeeeee]

Question: Discuss advantages and disadvantages of AC and DC generators and relate these to their uses

 

[dawooddaman]

Question: Discuss advantages and disadvantages of AC and DC generators and relate these to their uses

All ready been done.

 

[freeeeee]

Question: Outline how transmisson lines are insulated from supporting structures and protected from lightning strikes (4marks)

Question: Explain the role of transformers in electricity substations (4marks)

 

[Danstar2]

Question: Outline how transmisson lines are insulated from supporting structures and protected from lightning strikes (4marks)

Question: Explain the role of transformers in electricity substations (4marks)

1. There's a cable to earth run along the very top of transmission towers. Transmission towers are also located away from any trees, this further lowers the risk of lightning strikes. Transmission towers are insulated from the lines through the use of a series of cup shaped ceramic insulators, these act to break any probability of an electrical connection between the tower to the lines, e.g. they stop dust build up, and disallow rain to short across from the lines to the tower etc. Some other transmission lines use chain insulators, to limit the likelihood of a discharge. Chain insulators are around 2m in length.

2. Transformers in substations work to step up or step down power AC transmission. For example, they may step down 330kv transmission or 33kv transmission down to 11kv for suburban distribution. This 11kv's is then further stepped down by pole mounted transformers to either 3 phase 415V AC or single phase 240v. The AC transformer allows for high voltage transmission of AC at low currents across a large distance. Without the transformer there would be a large loss of electricity due to resistance in transmission lines as a result of high currents.

 

[dawooddaman]

discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor?

 

[freeeeee]

discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor?

Prior to 1947, the invention of the world's first transistor, a big problem in communication technology was amplification, the received signals were extremely weak and could not produce a loud sound without being amplified. Another problem was the short comings with valves (unreliable, high power consumption, size and warm up time), therefore the need to replace valves were brought about, not only because of the limitations of vlaves but also because of the high demand for communication technology. Therefore research into the properties of semiconductors rose in attention.

Pretty much a remember it history dotpoint

 

[freeeeee]

Question: In your studies at school, you have performed an investigation to model the behaviour of semiconductors, outline this experiment (5marks)

 

[dawooddaman]

Question: In your studies at school, you have performed an investigation to model the behaviour of semiconductors, outline this experiment (5marks)

Creation of positive holes and movement of positive holes and electrons in the presence of an external electric field:
A draughts board can be used, leave a space in one of the squares. As the playing pieces are moved into the hole, and the pieces behind are moved too, it is observed that pieces (representing electrons) move in one direction, but the space (representing the positive hole) moves in the opposite direction.

Don't think it will be a 5 marker =/

 

[lemon1]

Question: In your studies at school, you have performed an investigation to model the behaviour of semiconductors, outline this experiment (5marks)

Could we just re-state the Meisner effect for this experiment as it uses semiconductors?

 

[GoldyOrNugget]

Could we just re-state the Meisner effect for this experiment as it uses semiconductors?

Nooooooooo. You have to talk about the movement of electrons and holes.