HSC Physics Marathon 2013-2015 Archive (3 Viewers)

[anomalousdecay]

re: HSC Physics Marathon Archive

Alright

Solid state drives are transistor based technology which uses the semiconductors as its basis of operation. By providing current, circuits can be switched on and off to store and manipulate large amounts of data.
Thermionic devices uses the property of cathode ray tubes (the ability to for the electron to carry current when heated from the cathode) to accomplish similarly tasks.

Thermionic devices however are large, bulky, fragile and takes time to 'warm up' before usage. Thus, they cost more to maintain, uses much more power and produce large amounts of waste heat. Transistors on the other hand are small, cheap, fast, reliable and produce little heat, making it a far more attractive option for circuits.

The second paragraph was great!

The first paragraph could use a bit of work. The tasks completed by what you stated are completely different in essence. Also, you should refer to them as solid state devices not "drives" lol. Also, can't really say "by providing current" just like that (I mean yeah transistors are effectively current sources) because they have different operation modes. Just refer to transistors being used as switches or amplifiers and that transistors can also be used to make circuits that store data.

Would give your answer 3 or 4/4.

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Next question:

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors.

 

[atargainz]

re: HSC Physics Marathon Archive

ye

next question

State the differences in the operation of solid state and thermionic devices.

or if you are up for writing up a bit more for me to criticise:

Describe differences between solid state and thermionic devices.

Thermionic devices involve two or more electrodes (heated filaments) enclosed in an evacuated glass tube. An electric current acts as a heating mechanism that heats the filament, effectively causing it to liberate electrons and hence act as a cathode -- this is the process of thermionic emission. These electrons are then accelerated by a high potential difference towards the positively charged anode.

Solid state devices on the other hand are made from semiconducting materials with the a junction between a p-type and n-type semiconductor acting as a diode (p-n junction). This allows for current to flow in only one direction, the free electrons from the n-type moving towards the p-type to neutralise the holes. The zone between the junction is called a 'depletion zone' and exerts a resistive force, preventing any electrons from further movement into the region, this creates a forward bias whereby the conventional current flow is confined to a single direction.... fuck dunno what i'm writing anymore so tired, only talked about operations cuz thats what you asked for

 

[sy37]

re: HSC Physics Marathon Archive

wtf is this question the answers showed 1 line of working for 4 marks, there was a 9.8 that came out of nowhere:

Just think about it in terms of forces...there is a magnetic field force (unknown) and a gravitational field force (this is known and equal to f = ma aka mass on balance * 9.8)

thus F(known) = Bil(B is unknown)

etc..

 

[godofindolence]

re: HSC Physics Marathon Archive

The second paragraph was great!

The first paragraph could use a bit of work. The tasks completed by what you stated are completely different in essence. Also, you should refer to them as solid state devices not "drives" lol. Also, can't really say "by providing current" just like that (I mean yeah transistors are effectively current sources) because they have different operation modes. Just refer to transistors being used as switches or amplifiers and that transistors can also be used to make circuits that store data.

Would give your answer 3 or 4/4.

haha oops!
Isn't the function of both essentially to act as switches in a circuit?

 

[godofindolence]

re: HSC Physics Marathon Archive

Solid state devices on the other hand are made from semiconducting materials with the a junction between a p-type and n-type semiconductor acting as a diode (p-n junction). This allows for current to flow in only one direction, the free electrons from the n-type moving towards the p-type to neutralise the holes. The zone between the junction is called a 'depletion zone' and exerts a resistive force, preventing any electrons from further movement into the region, this creates a forward bias whereby the conventional current flow is confined to a single direction.... fuck dunno what i'm writing anymore so tired, only talked about operations cuz thats what you asked for

Isn't this photovoltaic cells, does the same principle apply? thats pretty cool actually!

 

[malcolm21]

re: HSC Physics Marathon Archive

Thermionic devices involve two or more electrodes (heated filaments) enclosed in an evacuated glass tube. An electric current acts as a heating mechanism that heats the filament, effectively causing it to liberate electrons and hence act as a cathode -- this is the process of thermionic emission. These electrons are then accelerated by a high potential difference towards the positively charged anode.

Solid state devices on the other hand are made from semiconducting materials with the a junction between a p-type and n-type semiconductor acting as a diode (p-n junction). This allows for current to flow in only one direction, the free electrons from the n-type moving towards the p-type to neutralise the holes. The zone between the junction is called a 'depletion zone' and exerts a resistive force, preventing any electrons from further movement into the region, this creates a forward bias whereby the conventional current flow is confined to a single direction.... fuck dunno what i'm writing anymore so tired, only talked about operations cuz thats what you asked for

that was sick it reads like a textbook

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Thermionic devices involve two or more electrodes (heated filaments) enclosed in an evacuated glass tube. An electric current acts as a heating mechanism that heats the filament, effectively causing it to liberate electrons and hence act as a cathode -- this is the process of thermionic emission. These electrons are then accelerated by a high potential difference towards the positively charged anode.

Solid state devices on the other hand are made from semiconducting materials with the a junction between a p-type and n-type semiconductor acting as a diode (p-n junction). This allows for current to flow in only one direction, the free electrons from the n-type moving towards the p-type to neutralise the holes. The zone between the junction is called a 'depletion zone' and exerts a resistive force, preventing any electrons from further movement into the region, this creates a forward bias whereby the conventional current flow is confined to a single direction.... fuck dunno what i'm writing anymore so tired, only talked about operations cuz thats what you asked for

You should mention a few examples for each as that is there practical applications (I should have made that a bit clearer in the question so I don't blame you) and need to say how they are different to each other and how they are different when they operate (as in difference between power consumption, size, etc).

haha oops!
Isn't the function of both essentially to act as switches in a circuit?

Both have multiple different functions. You can use a thermionic device as a switch, but it would have complimented your answer better if you just said the extra bit and gave an appropriate example. Also this sentence was nasty "Thermionic devices uses the property of cathode ray tubes (the ability to for the electron to carry current when heated from the cathode) to accomplish similarly tasks." Electron to carry current? The electron movement provides a current.

It's just the really simple and small incorrect things that I poked out of your answer

 

[godofindolence]

re: HSC Physics Marathon Archive

Next question:

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors.

Lenz's law states that a relative movement of a conductor in a magnetic field induces an EMF that creates a magnetic field opposing the initial change in flux. (formula) This is consistent with the conservation of energy as the induced current acts against the movement of the conductor, slowing it down. If this were not true, then a constant force will be provided by the induced EMF, accelerating the conductor without bound, and thus, violating the conservation of energy.

Back EMF in motors is created due to the movement of the coils(rotor) in the magnetic field (stator). This causes a change in flux, that according to Lenz's law, produces an induced EMF in the coil, that acts against the initial current, effectively negating part of it. This means that as the motor speeds up (rate of change in flux increases), the back EMF will increase, whilst supplied current decreases. In an ideal motor where there is no friction, back EMF = current as the motor approaches maximum speed. This is beneficial as it reduces the likelihood of a motor 'burning out'.

 

[malcolm21]

re: HSC Physics Marathon Archive

Lenz's law states that a relative movement of a conductor in a magnetic field induces an EMF that creates a magnetic field opposing the initial change in flux. (formula) This is consistent with the conservation of energy as the induced current acts against the movement of the conductor, slowing it down. If this were not true, then a constant force will be provided by the induced EMF, accelerating the conductor without bound, and thus, violating the conservation of energy.

Back EMF in motors is created due to the movement of the coils(rotor) in the magnetic field (stator). This causes a change in flux, that according to Lenz's law, produces an induced EMF in the coil, that acts against the initial current, effectively negating part of it. This means that as the motor speeds up (rate of change in flux increases), the back EMF will increase, whilst supplied current decreases. In an ideal motor where there is no friction, back EMF = current as the motor approaches maximum speed. This is beneficial as it reduces the likelihood of a motor 'burning out'.

lol that sounds like some theory of relativity stuff, think you should explicitly say "when a conductor experiences a change in magnetic flux" instead of that relative movement thing since you referred to "magnetic field opposing the initial change in flux" but sounded a bit vague in the first part of your sentence

 

[godofindolence]

re: HSC Physics Marathon Archive

lol that sounds like some theory of relativity stuff, think you should explicitly say "when a conductor experiences a change in magnetic flux" instead of that relative movement thing since you referred to "magnetic field opposing the initial change in flux" but sounded a bit vague in the first part of your sentence

Haha oh yea, that does sound a little silly -.-
Your version is so much better! thanks!

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Lenz's law states that a relative movement of a conductor in a magnetic field induces an EMF that creates a magnetic field opposing the initial change in flux. (formula) This is consistent with the conservation of energy as the induced current acts against the movement of the conductor, slowing it down. If this were not true, then a constant force will be provided by the induced EMF, accelerating the conductor without bound, and thus, violating the conservation of energy.

Back EMF in motors is created due to the movement of the coils(rotor) in the magnetic field (stator). This causes a change in flux, that according to Lenz's law, produces an induced EMF in the coil, that acts against the initial current, effectively negating part of it. This means that as the motor speeds up (rate of change in flux increases), the back EMF will increase, whilst supplied current decreases. In an ideal motor where there is no friction, back EMF = current as the motor approaches maximum speed. This is beneficial as it reduces the likelihood of a motor 'burning out'.

Good answer. Would give it 4/4. You could also briefly say why it would burn out otherwise, due to a large friction/inertia meaning that a larger current must be supplied.

However, what about the case of different types of motors? Just be a bit careful when generalising your answer. It's fine in this case, but in some cases you might be required to distinguish between different motor set ups and explain it in terms of different sorts of motor configurations.

lol that sounds like some theory of relativity stuff, think you should explicitly say "when a conductor experiences a change in magnetic flux" instead of that relative movement thing since you referred to "magnetic field opposing the initial change in flux" but sounded a bit vague in the first part of your sentence

No I think it's fine. You pretty much just said the same thing as godofindolence lol.

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Btw I'm literally just copying and pasting syllabus dot points as questions here.

It's good practice to ensure you know everything in the syllabus and have a solid understanding.

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Next question:

Explain how eddy currents can be utilised in electromagnetic braking.

 

[godofindolence]

re: HSC Physics Marathon Archive

However, what about the case of different types of motors? Just be a bit careful when generalising your answer. It's fine in this case, but in some cases you might be required to distinguish between different motor set ups and explain it in terms of different sorts of motor configurations.
.

Ahh yes thank you. I forgot about the AC induction, those wouldn't have any back EMF.

 

[malcolm21]

re: HSC Physics Marathon Archive

Good answer. Would give it 4/4. You could also briefly say why it would burn out otherwise, due to a large friction/inertia meaning that a larger current must be supplied.

However, what about the case of different types of motors? Just be a bit careful when generalising your answer. It's fine in this case, but in some cases you might be required to distinguish between different motor set ups and explain it in terms of different sorts of motor configurations.



No I think it's fine. You pretty much just said the same thing as godofindolence lol.

But if the conductor experiences 'relative movement to the magnetic field', it's still possible theres no change in magnetic flux if the conductor is still fully immersed inside the magnetic field isnt it

like this:

 

[godofindolence]

re: HSC Physics Marathon Archive

Next question:

Explain how eddy currents can be utilised in electromagnetic braking.

Eddy currents are formed as an application of Lenz's law, wherein circular currents are induced in sheets of metal due to a change in flux. According to Lenz's law, the eddy currents would provide a magnetic field that opposes the initial motion of the conductor. When attached to a moving object, like a train or roller-coaster, the eddy currents induced will therefore provide a breaking force, known as electromagnetic breaking. As the object slows, the rate at which the metal cuts the flux decreases, resulting in smaller eddy currents and smaller resistive forces, thus providing smooth braking.

 

[anomalousdecay]

re: HSC Physics Marathon Archive

But if the conductor experiences 'relative movement to the magnetic field', it's still possible theres no change in magnetic flux if the conductor is still fully immersed inside the magnetic field isnt it

like this:

I do agree with this case actually haha.

However, he said "which opposes an initial change in magnetic flux" which covered his answer in the end. It would be nice to change that wording though. I like the use of "relative movement", so maybe "if relative movement between a conductor and magnetic field applies a change in magnetic flux to the conductor then a back EMF is induced into the conductor to oppose this change in flux" would be a nice answer.

 

[godofindolence]

re: HSC Physics Marathon Archive

But if the conductor experiences 'relative movement to the magnetic field', it's still possible theres no change in magnetic flux if the conductor is still fully immersed inside the magnetic field isnt it

like this:

Is this related to this question: http://puu.sh/kVO51/bf1e392ee2.png
If so please explain

 

[godofindolence]

re: HSC Physics Marathon Archive

Ok so just to clarify, when a conductor is moving at a constant velocity through a magnetic field with an uniform flux density, then there is no change in magnetic flux?