Physics marathon (hsc) (1 Viewer)

[Keelan134]

fkn lol'ed

and whats with all the confusion over momentum?

Solar cell question has been answered I think.

Conservation of Momentum's been answered,
to put it simply Momentum = Mass x velocity
As a rocket burns fuel at a constant rate, its mass decreases proportionally to the increase of velocity, keeping momentum constant.

 

[nirukk]

The Jacaranda is pretty poor on the explanation of Solar Cells, so I'm unsure, when excited do photoelectrons move around the circuit to the p-type layer to create the potential difference, or across the junction?

This image is good.

 

[kiinto]

This image is good. View attachment 26896

Right. They move around the circuit. Cheers man.

Describe the occurrence of different striation patterns in discharge tubes with explicit reference to a practical you have performed. Include diagrams.

 

[someth1ng]

Momentum does remain constant. The exhaust gases are ejected at a constant speed and rate, thus the momentum of the exhaust gases is constant. The momentum of the rocket therefore also remains constant (in the other direction). As the mass of the rocket is decreasing however but the momentum remains the same, then its velocity must be increasing-> acceleration. This is also shown through F=ma, because the thrust of the rocket (F) remains constant but m decreases, therefore a must increase

People are getting confused here. Net momentum of the system remains constant (law of conservation of momentum) but momentum of the rocket is constantly changing.

/\=delta (change)

F(rocket)=-F(gases)
=> /\p(rocket)//\t=-/\p(gases)//\t
=> /\mv(rocket)//\t=-/\mv(gases)//\t

/\mv(rocket)=-/\mv(gases)=constant
.'. /\mv(rocket)=constant

The rate in which fuel is burnt is relatively constant, therefore, the RHS is relatively constant. However, in the left hand side, mass is decreasing and velocity is increasing meaning that there must be a very large change in velocity (logarithmic but looks exponential). Clearly, it is the CHANGE OF MOMENTUM of the rocket that is constant but obviously, the momentum is changing.

Only the net momentum of the system remains constant (p(rocket)+p(gases)=constant).

 

[J-Wang]

Right. They move around the circuit. Cheers man.

Describe the occurrence of different striation patterns in discharge tubes with explicit reference to a practical you have performed. Include diagrams.

different gas pressures within the tubes

 

[nirukk]

A 1.0 W beam of light transfers 1.0 J per second from one point to another. With reference to the particle model of light, contrast a 1.0 W beam of red light and a 1.0 W beam of blue light. 2 marks (2011)

 

[someth1ng]

A 1.0 W beam of light transfers 1.0 J per second from one point to another. With reference to the particle model of light, contrast a 1.0 W beam of red light and a 1.0 W beam of blue light. 2 marks (2011)

Not a hard question - I'm not going to go into too much depth but here's what I would include:

- Blue light has a shorter wavelength and hence, higher frequency than red light.
- As blue light has higher frequency, each photon of blue light has more energy than each photon of red light.
- Therefore, less photons of blue light are needed to produce 1J/s or 1W than red light.
- Hence, the 1W beam of red light is more intense than the 1W beam of blue light.

 

[ademb13]

is 85% raw enough for band 6, because i keep getting about that

 

[Danstar2]

is 85% raw enough for band 6, because i keep getting about that

Yes 75% onward is pretty much B6.


Doing Physics study for my exam, all was going well and I'm getting towards finished but there is one thing driving my CRAZY!

This source says:

"A typical optical fibre consists of a very pure glass core surrounded by a cladding of low refractive index."

Low R.I = low density, therefore the glass is surrounded by a lower density cladding.

BUT, only a few sentences on the source says: "As light travels from the inner glass core to the denser cladding, it bends so much that, instead of leaving the glass, it is reflected back into it. This process is known as total internal reflection."

So now it says it's going from lower density to higher. MAKE YOUR MIND UP! D:<

This is driving me nuts, someone explain this blasphemy.

 

[RishBonjour]

Yes 75% onward is pretty much B6.


Doing Physics study for my exam, all was going well and I'm getting towards finished but there is one thing driving my CRAZY!

This source says:

"A typical optical fibre consists of a very pure glass core surrounded by a cladding of low refractive index."

Low R.I = low density, therefore the glass is surrounded by a lower density cladding.

BUT, only a few sentences on the source says: "As light travels from the inner glass core to the denser cladding, it bends so much that, instead of leaving the glass, it is reflected back into it. This process is known as total internal reflection."

So now it says it's going from lower density to higher. MAKE YOUR MIND UP! D:<

This is driving me nuts, someone explain this blasphemy.

the source is wrong. I'm pretty sure cladding is less dense than the core. Is this for med physics?

when something travels from less dense to more dense it bends towards the normal :/

 

[Danstar2]

the source is wrong. I'm pretty sure cladding is less dense than the core. Is this for med physics?

when something travels from less dense to more dense it bends towards the normal :/

That's exactly what I believe to be correct, screw the books, I'm going with our idea. However, now I realize that over 3 sources are incorrect.

 

[Keelan134]

Yeah i notice the same thing with my notes, it contradicts. I'm going with good ol Snell's Law to tell me that that the external has to be less dense for it to totally internally reflect

 

[zeebobDD]

Explain how induction is used in certain applications. Refer to two applications. (5marks)

 

[kiinto]

Explain how induction is used in certain applications. Refer to two applications. (5marks)

Induction Cooktops:
In an induction cooktop, AC current is passed through a solenoid in order to produce a changing magnetic field. This then induces an emf in the base of a saucepan, in accordance with Lenz's law, which causes eddy currents to flow. The electrical resistance of the saucepan produces heat, which cooks the food. It is more efficient than conventional cooktops because energy is directly applied to the saucepan, rather than indirectly.

Induction Breaking in trains:
An electromagnet is fitted to the bottom of a train. When switched on it provides a magnetic field. As the train moves, it causes the tracks below it to experience a change in magnetic flux. According to Lenz's law, this induces an emf within the tracks, which produces a magnetic field which opposes the change which caused it (i.e. the movement of the train). This causes the train to experience a breaking force. The benefit of induction breaking is that the breaking force is proportional to the rate of change of flux, this means that at high velocities, a greater breaking force will be experienced, which is just when it is required.

Asses the various problems involved in communication with orbiting satellites (6mks)

 

[J-Wang]

Induction Cooktops:
In an induction cooktop, AC current is passed through a solenoid in order to produce a changing magnetic field. This then induces an emf in the base of a saucepan, in accordance with Lenz's law, which causes eddy currents to flow. The electrical resistance of the saucepan produces heat, which cooks the food. It is more efficient than conventional cooktops because energy is directly applied to the saucepan, rather than indirectly.

Induction Breaking in trains.
An electromagnet is fitted to the bottom of a train. When switched on it provides a magnetic field. As the train moves, it causes the tracks below it to experience a change in magnetic flux. According to Lenz's law, this induces an emf within the tracks, which produces a magnetic field which opposes the change which caused it (i.e. the movement of the train). This causes the train to experience a breaking force. The benefit of induction breaking is that the breaking force is proportional to the rate of change of flux, this means that at high velocities, a greater breaking force will be experienced, which is just when it is required.

Asses the various problems involved in communication with orbiting satellites (6mks)

With your answer, it's all good except when you mention Lenz's Law, state that the induced current opposes the change in mag. flux and hence the induced currents occur in the opposite direction to the motion of the current in the magnet. Otherwise, your answer does not makes sense. You are simply quoting Faraday's Law

 

[Doomader]

Hey guys so I have a quick question,

Why is it that the majority of the questions asked here seem to be on Space? I mean motors and generators is just 4 main points so thats easy, but seriously I know pretty much nothing on Ideas to Implementation but heaps on Space.

Anyways actual question time:
By referring to the doping of the layers, contrast the electrical properties of the p-type and n=type layers in device B. (note device B is just an anode->p-type silicon->n-type silicon->cathode straight line diagram [a diode] )

And thats 3 marks

(for reference its from the 2011 Catholic Trials)

 

[kiinto]

Hey guys so I have a quick question,

Why is it that the majority of the questions asked here seem to be on Space? I mean motors and generators is just 4 main points so thats easy, but seriously I know pretty much nothing on Ideas to Implementation but heaps on Space.

Anyways actual question time:
By referring to the doping of the layers, contrast the electrical properties of the p-type and n=type layers in device B. (not device B is just an anode->p-type silicon->n-type silicon->cathode straight line diagram)

And thats 3 marks

(for reference its from the 2011 Catholic Trials)

Space is a pretty cool guy. Eh kills aliens and doesn't afraid of anything.

p-type silicon is doped with GIII elements to produce extra positive holes, thereby increasing electrical conductivity.
n-type silicon is doped with GV elements to produce extra free electrons, thereby increasing electrical conductivity.
At the p-n junction (i.e. where the two layers meet) there is a diffusion of electrons and holes which causes the p-type silicon to become negative and the n-type silicon to become positive. This only occurs very close to the junction, in what is called the depletion zone. It produces an electric field.

 

[someth1ng]

Induction Cooktops:
In an induction cooktop, AC current is passed through a solenoid in order to produce a changing magnetic field. This then induces an emf in the base of a saucepan, in accordance with Lenz's law, which causes eddy currents to flow. The electrical resistance of the saucepan produces heat, which cooks the food. It is more efficient than conventional cooktops because energy is directly applied to the saucepan, rather than indirectly.

Induction Breaking in trains:
An electromagnet is fitted to the bottom of a train. When switched on it provides a magnetic field. As the train moves, it causes the tracks below it to experience a change in magnetic flux. According to Lenz's law, this induces an emf within the tracks, which produces a magnetic field which opposes the change which caused it (i.e. the movement of the train). This causes the train to experience a breaking force. The benefit of induction breaking is that the breaking force is proportional to the rate of change of flux, this means that at high velocities, a greater breaking force will be experienced, which is just when it is required.

Asses the various problems involved in communication with orbiting satellites (6mks)

EDIT: sorry, misread.

 

[dawooddaman]

Explain who solar cell work, including a diagram (6 marker)

 

[Danstar2]

Explain who solar cell work, including a diagram (6 marker)

Junction between P and N type semiconductors, is subject to high frequency light from the sun. This allows electron-hole flow across the p and n type semiconductors. Holes move to the Positive side, electrons move to the negative side. Electrons are gathered by an outer metal grid which is connected to a circuit and allows current to flow.