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[dolbinau]

Ok, a rip off the Bio thread. We need an official 'study' thread.

I'll start,

Describe the net forces acting on a satellite orbiting the earth?

(It's a genuine question though, hehe)

 

[aussiechick007]

erm...

the satellite is experiencing a centripetal force towards the centre of its orbit, which in this case is gravity pulling it towards the earth, and its velocity is at right angles to this force, which produces a net circular orbital motion around the earth. (my wording is not too good there, lol)

Outline ONE advantage of using superconductors, with reference to TWO applications.

 

[juhe]

erm...

the satellite is experiencing a centripetal force towards the centre of its orbit, which in this case is gravity pulling it towards the earth, and its velocity is at right angles to this force, which produces a net circular orbital motion around the earth. (my wording is not too good there, lol)

Outline ONE advantage of using superconductors, with reference to TWO applications.

tangential velocity

yeah so superconductors are able to carry current with no resistance. Applications where this is useful include in particle accelerators/cyclotrons where they are used to make super strong magnets to bend accelerated particles, and also in MRI magnets, and of course maglev trains.

summarise the use of the photoelectric effect in photocells, and the effect of light on semiconductors in solar cells

 

[SkimDawg]

summarise the use of the photoelectric effect in photocells, and the effect of light on semiconductors in solar cells

A photocell is a device that uses the photoelectric effect. These devices include photovoltaic, or solar cells, which convert electromagnetic energy, such as sunlight, into electrical energy. Other examples are photoconductive cells and phototubes.
Modern solar cells use silicon and gallium arsenide. they use focusing devices, such as lenses, to acheive efficiencies of greater than 37% in converting light energy falling onto the cell is converted into electricity. The work function of most materials requires the electromagnetic energy to have a frequency near that of ultraviolet light to allow electrons to be emitted. This limits the application of photocells using the photoelectric effect. Devices that use p-n junctions are more commonly found in the generation of power and in the detection and measurement of light.
In a solar cell, light energy is applied to the junction region of a semiconductor diode where p-type silicon is in contact wit n-type silicon. Electrons are released from the silicon crystal lattice because of the photoelectric effect. This has the effect of raising the junction voltage. For the solar cell to work, the n-type lyaer is exposed to light and the p-type layer is not. On the light-exposed side of the solar cell a fine grid of metal provides electrical contacts. These contacts are able to collect the photoelectrons emitted from the light-exposed n-type silicon surface.

next qu: Scientists tried to explain observations of black-body radiation using classical wave theory and then quantum theory. How does quantum theory satisfactorily explain black-body radiation?

 

[jozza80]

tangential velocity

yeah so superconductors are able to carry current with no resistance. Applications where this is useful include in particle accelerators/cyclotrons where they are used to make super strong magnets to bend accelerated particles, and also in MRI magnets, and of course maglev trains.

summarise the use of the photoelectric effect in photocells, and the effect of light on semiconductors in solar cells

A photovoltaic cells (solar cells) operation is based on the formation of a junction (pn-junction). When light strikes the cell, some is absorbed within the semi conductor material. Light photons knock th electrons from the valence band to the conduction band allowing electrons to become free moving; with a complete circuit (diagram), current can be used for external purposes. Electrons flow easily from p-type to n-type, therefore they move through the junction and into the n-type semiconductor, then because they cannot reverse (because of diode) the electron flow through the external circuit - creating a current for a light globe etc. - and returning to p-type, for process to continue...

Bit wordy i know. Any extra info needed?

Outline Thomson's experiement to measure charge/mass ration of an electron...

 

[SkimDawg]

Outline Thomson's experiement to measure charge/mass ration of an electron...

Thomson built a cathode ray tube with charged parallel plates to provide a uniform electric field and a source for uniform magnetic field. Thomsons experiment involved two stages:
- Varying the magnetic field and electric fields until their opposing forces cancelled, leaving the cathode rays undeflected. By equating the magnetic and electric force equations Thomson was able to determine the velocity of the cathode ray particles.
- Applying the same strength magnetic field and determining the radius of the circle path travelled by the charged particles in the magnetic field. Thomson combined the results and obtained the magnitude of the charge to mass ratio for the charged particles that constituted cathode rays.

 

[dolbinau]

I think the SuccessOne answer is wrong for this so just to make sure:

An alarm clock is set to go off in one hour when it is at rest. The alarm clock is set in motion at .95 times the speed of light

a) Calculate how long it would take the alarm clock to go off for stationary observers watching it move away at .95c

 

[juhe]

I think the SuccessOne answer is wrong for this so just to make sure:

An alarm clock is set to go off in one hour when it is at rest. The alarm clock is set in motion at .95 times the speed of light

a) Calculate how long it would take the alarm clock to go off for stationary observers watching it move away at .95c

t0=1 v=0.95c

tv= 1/(1-(0.95)^2)^0.5
= 3.2hrs

 

[juhe]

next qu: Scientists tried to explain observations of black-body radiation using classical wave theory and then quantum theory. How does quantum theory satisfactorily explain black-body radiation?

The predictions of classical physics suggested an exponential increase of energy as the wavelength approached zero. experimental results did not support this, it is know as the UV catastrophe.

Planck developed the quantum theory as to explain these results. He suggested that energy, ie the radiation is quantized, made of distinct packets rather than a continuous wave. The individual units are characterized by the frequency of the radiation and these individual units explain the behavior of the observed results.

explain how eddy currents have been utilised in electromagnetic braking

 

[aussiechick007]

explain how eddy currents have been utilised in electromagnetic braking

Eddy currents are currents which are induced in a conductor cutting a magnetic flux that oppose the change in the flux that caused them. In electromagnetic braking, the conductor on the moving object is brought into a magnetic field, and therefore has a current opposing the change that caused it induced in it, producing a magnetic field opposing the original one, and thus slowing greatly.

In terms of band structures and relative electrical resistance, describe the differences between a conductor, an insulator, and a semiconductor.

 

[SkimDawg]

explain how eddy currents have been utilised in electromagnetic braking

Consider a metal disk that has a part of it influenced by an external magnetic field. As the disk is made of metal, the movement of the metal through the region of magnetic field causes eddy currents to flow. Using the right-hand push rule, it can be shown that the eddy current within the magnetic field will be upwards. The current follows a downward return path through the metal outside the region of magnetic influence.
The magnetic field exerts a force on the induced eddy current. This can be shown to oppose the motion of the disk by applying the right-hand push rule. In this way eddy currents can be utilised in smooth braking devices in trams and trains. An electromagnet is switched on so that an external magnetic field affects part of a metal wheel or the steel rail below the vehicle. Eddy currents are established in the part of the metal that is influenced by the magnetic field. These currents inside the magnetic field experience a force that acts in the opposite direction to the relative motion of the train or tram. In the case of the wheel, the wheel is slowed down. In the case of the rail, the force acts in a forward direction on the rail and there is an equal and opposite force that acts on the train or tram.

 

[lost1]

In terms of band structures and relative electrical resistance, describe the differences between a conductor, an insulator, and a semiconductor.

Electrically, a conductor has the least resistance, followed by semiconductors and insulators having the highest resistance. This is a direct consequence of their relevant band structures:

-Conductors have their valence band and conduction band very close to each other (almost touching)
-Semiconductors have their band slightly apart at a distance close enough that small amount of energy (thermal energy) will allow electrons to 'jump' to conduction band and hence produce a current flow.
- Insulators have a much larger gap between said bands and thus requires a large amount of energy for current to flow (hence is of high resistance).

I know We're meant to make up our own Questions but this one from 2005 HSC Q20 is bothering me (worth 6marks!!):

In your course you had to gather information to explain how induction is used in certain applications.
With reference to TWO appplications, describe how you assessed the reliability of information you found.

 

[JamesTockuss]

In your course you had to gather information to explain how induction is used in certain applications.
With reference to TWO appplications, describe how you assessed the reliability of information you found.


The principle of induction is used in:
(a) Induction Cooktops - AC current is fed into a coil of wire placed beneath the metal saucepan. The changing flux of the alternating current induces eddy currents within the pan to establish a magnetic field to oppose the change that created it (Lenz's Law). The eddy currents generate heat through resistance which cooks the food.
(b) Electromagnetic braking - A pair of electromagnets are established on either side of the wheel. When a reduction in speed is needed the magnets are turned on. The changing flux through the wheel as it rotates generates eddy currents which oppose the motion (Lenz's Law) and so the wheel is slowed.

To assess the reliability we:
- Checked other sources of consistancy
- Assessed the appropriatness of the publisher (e.g. Science journals)
- Assessed the publication date to ensure up to date information
- Conferred with our teacher to ensure validity.


Next Question: Describe the Slingshot Effect and the principles behind it.

 

[HuntingSpark]

Next Question: Describe the Slingshot Effect and the principles behind it.

The slingshot effect is used to accelerate a spacecraft in a planetary flyby. it is the result of entering the gravitational field of a massive orbiting body. The spacecraft's velocity relative to the planet is the same outbound as inbound. However, relative to the Sun, the objects exchange momentum the spacecraft gains momentum, and the planet loses some. Because the planet is so much more massive than the spacecraft, however, the spacecraft gains a LOT of velocity, while the planet loses very little.

Next: Explain the BCS theory proposed to explain superconductivity.

 

[Glenjamin]

The slingshot effect is when space probes use a planet's gravitational pull as a velocity boost.

the probe accelerates as its gets pulled by a planet's gravitational pull and then as it passes it reduces its speed relative to the planet but relative to the sun it has increased

Describe Hertz's observation of the photoelectric effect

 

[old.skool.kid]

When Hertz was experimenting with spark gaps he noticed whan UV light was shone onto a spark gap, the spark was much brighter. Hertz failed to further investigate this. Later studies showed this was due to UV light causing electrons to be ejected from the surface. No electrons were released if the frequncy was below a certain threshold frequency.

Next: Explain how the principle of induction can be used to heat a conductor.

 

[Glenjamin]

i think that q has been asked already oldskoolkid

 

[what else?]

The principle of induction is used by having coils beneath a conductor and appying a potential difference to the coils. The magnetic feild produced by the coil is a changing magnetic feild due to the current being used is Alternating current. When a object is placed within the feild there is a change in flux and current is induced. The resistance within the material produces heat.

--Discuss the impact of the development of transformers on society.

 

[SkimDawg]

--Discuss the impact of the development of transformers on society.

AC generators allow for the production of AC electricity which can be supplied to the public. This affects society greatly allowing for lights in houses and streets, and many other electrical household appliances. It has affected the environment because of the required infrastructure that has been built to support, generate and supply the electricity to homes. It has also affected the environment as the main source of the torque in AC generators comes from the burning of fossil fuels which contributes to the greenhouse effect. In contrast to that AC generators allow for production of energy from ‘green’ sources such as hydroelectric and wind power plants. AC generators compared to DC generators have allowed for transformers to be used to change the voltage of electricity. This has allowed for different appliances to be used from the same source of electricity, making the use of electricity on a household level much simplified.

--HSC Q19 2005 lol, cant be bothered to type it up atm, its bugging the shit outta me

 

[Glenjamin]

Compare the model for the conduction of electricity in metals at room temperature with the model of the conduction of electricity in superconductors below the critical temperature