Physics marathon (hsc) (5 Viewers)

[someth1ng]

solid state:
- small size--> thermionic devices involve vacuum tubes and are bulky--> limits their use on more complex electrical systems.
- efficient--> thermionic devices have significant heat dissipation thus electrical energy is lost as heat where as solid state devices lose only a fraction of this. Also, thermionic devices require large voltages to run, solid state devices such as transistors, run on much smaller voltages
- No starting time lag---> Thermionic devices need to be heated up first thus slow processing speed
- relatively tough--> solid state devices can endure more physical impact while thermionic devices --> lol--> made of glass and stuff
- Last longer--> thermionic devices have several parts need to be replaced often e.g. the cathode
- cheaper to produce + maintain.



edit:
ohh, advantages of thermionic devices? hmm not sure

1. Guitarists prefer thermionic amplifiers for their aesthetic appeal and warmer sound.
2. Thermionic devices are less susceptible to electromagnetic pulse effects than solid state devices and hence, are often used in military instead of solid state.

 

[RishBonjour]

can create a warmer/more vintage sound than solid state devices which is an advantage for musicians.

LOL I was going to say, it lights up your room.

btw for a given power supply, which motor is more powerful, AC or DC and why?

 

[barbernator]

LOL I was going to say, it lights up your room.

btw for a given power supply, which motor is more powerful, AC or DC and why?

DC motors will be more powerful for a given power supply.

Power is the rate at which energy is converted into other forms, and for this application the more powerful motor is the one which converts input electrical energy most efficiently into mechanical energy.

AC motors are more inefficient than DC motors.
Synchronous: The continual change in magnetic flux due to the AC current causes heating loss as eddy currents form in the iron core.
Induction: Energy is lost through the magnetising of the squirrel cage and eddy currents that are induced that are not within the magnetic field and hence do not provide a torque.

DC motors have a small loss through the changing of direction of current through the coil every 180 degrees, yet this is not as great as the losses in AC motors.

Hence DC are more powerful for a given power supply.

not sure about the quality of my answer in this one... :/

 

[someth1ng]

LOL I was going to say, it lights up your room.

btw for a given power supply, which motor is more powerful, AC or DC and why?

By AC, I'm assuming that's specific to induction motors and DC, that shitty one.

It's difficult to say which is more powerful but I believe it would be DC because induction motors aren't exactly efficient.

 

[RishBonjour]

By AC, I'm assuming that's specific to induction motors and DC, that shitty one.

It's difficult to say which is more powerful but I believe it would be DC because induction motors aren't exactly efficient.

Say AC electric motor (not induction), since the current constantly changes, hence changing flux, it would lead to induction of emf in the wires right? :/
Which would mean its less efficient than DC for a given power which will have no emf induced. Not 100% sure though, but was thinking about this for a while.

 

[someth1ng]

On second though, RishBonjour's question is much more difficult to answer because we don't know show strong the DC's magnetic field is. If the current is equal but there's no magnetic field then obviously, AC is stronger so it all depends.

 

[barbernator]

Say AC electric motor (not induction), since the current constantly changes, hence changing flux, it would lead to induction of emf in the wires right? :/
Which would mean its less efficient than DC for a given power which will have no emf induced. Not 100% sure though, but was thinking about this for a while.

this wouldn't happen because an EMF is induced when an external change in magnetic flux acts through a coil. If the coil is changing flux due to current through it itself, the flux wouldn't cause a consequent emf that opposes the current in the wire because that would result in the current in the wire being close to 0 at all times!
But yes I do think DC are more powerful just because the constant change in current direction could lead to heat losses in the iron core more consistently and possibly just some more resistive heating in the wires.

 

[someth1ng]

Actually, define "more powerful"

In a DC motor, there is little torque because of back emf - does this make it "weak"? In AC, you'd have less torque but there would be slip speed etc.

 

[RishBonjour]

this wouldn't happen because an EMF is induced when an external change in magnetic flux acts through a coil. If the coil is changing flux due to current through it itself, the flux wouldn't cause a consequent emf that opposes the current in the wire because that would result in the current in the wire being close to 0 at all times!
But yes I do think DC are more powerful just because the constant change in current direction could lead to heat losses in the iron core more consistently and possibly just some more resistive heating in the wires.

Ahh thanks for clearing that up

Any good/nice questions ?

 

[barbernator]

Actually, define "more powerful"

In a DC motor, there is little torque because of back emf - does this make it "weak"? In AC, you'd have less torque but there would be slip speed etc.

Powerful. percentage conversion of electrical energy into mechanical energy.

and yeh I think the question is a bit difficult because it depends on many factors.


Ok new question!

The physics syllabus and many people say that direct current CANNOT be stepped up or down. Explain why DC current CAN be stepped up and down except to an extremely inefficient extent (assume the DC current is a single coil so single phase)

 

[RishBonjour]

Powerful. percentage conversion of electrical energy into mechanical energy.

and yeh I think the question is a bit difficult because it depends on many factors.


Ok new question!

The physics syllabus and many people say that direct current CANNOT be stepped up or down. Explain why DC current CAN be stepped up and down except to an extremely inefficient extent (assume the DC current is a single coil so single phase)

hmm.
I know that DC can be stepped up or down but it is difficult and inefficient to do so.
I would think something like this:
- Ovbiously, there needs to be a changing magnetic flux
- so if you have a turn on coil on one side and 2 turns on another side of an iron pole, you could turn on the DC in the "primary coil". In that time as it reaches max, there will be a changing B field and hence the current should be induced in the other. And then you turn it off (this parts gets tricky because the current induced will be the other way--> hence AC) so I assume we have to skip this by moving the secondary coil away? And then turn on DC again. And process repeats?

No idea, but nice question.

 

[tarce94]

Someone find a question on galvanometers or loudspeakers. I never really learnt about them properly

 

[someth1ng]

DC current is not purely constant and smooth when supplied from a single phase DC generator.

DC current supplied from a single phase motor appears as: http://media.smashingmagazine.com/wp-content/uploads/2011/09/msine.png

This means that there is a constantly changing current and hence a somewhat changing magnetic flux and hence, if connected to a normal transformer, can be stepped up or down.

 

[barbernator]

hmm.
I know that DC can be stepped up or down but it is difficult and inefficient to do so.
I would think something like this:
- Ovbiously, there needs to be a changing magnetic flux
- so if you have a turn on coil on one side and 2 turns on another side of an iron pole, you could turn on the DC in the "primary coil". In that time as it reaches max, there will be a changing B field and hence the current should be induced in the other. And then you turn it off (this parts gets tricky because the current induced will be the other way--> hence AC) so I assume we have to skip this by moving the secondary coil away? And then turn on DC again. And process repeats?

No idea, but nice question.

spoiler alert (highlight below)
Well within a DC generator output, the current is not constant, it changes from 0 to maximum and back to 0 recursively (for a single phase motor) so the output looks like the graph |sin(x)|. Hence, the magnetic flux in the coil will be constantly changing and it will be able to be stepped up and down.

A hand cranked AC generator is spun at a constant speed and produces an output EMF. The coil is then spun 4 times as fast. Explain the change in the curve in relation to its amplitude and period.

 

[freeeeee]

Powerful. percentage conversion of electrical energy into mechanical energy.

and yeh I think the question is a bit difficult because it depends on many factors.


Ok new question!

The physics syllabus and many people say that direct current CANNOT be stepped up or down. Explain why DC current CAN be stepped up and down except to an extremely inefficient extent (assume the DC current is a single coil so single phase)

This is because for a transformer to work it requries a changing magnetic field fed by the supply current from the primary coil to induce an emf in the secondary coil.
DC current produces a steady magnetic field and current flows in one direction, therefore the direction or magnitude of the DC current will have to be altered first so that it fluctuates before it can be alternately stepped up or stepped down, this is achieved very inefficiently so "people say that DC cannot be stepped up or down."

a sidenote: can you use the three dots for therefore in Physics?

 

[deswa1]

spoiler alert (highlight below)
Well within a DC generator output, the EMF is not constant, it changes from 0 to maximum and back to 0 recursively (for a single phase motor) so the output looks like the graph |sin(x)|. Hence, the magnetic flux in the coil will be constantly changing and it will be able to be stepped up and down.

A hand cranked AC generator is spun at a constant speed and produces an output EMF. The coil is then spun 4 times as fast. Explain the change in the curve in relation to its amplitude and period.

The amplitude becomes 4 times bigger and the period becomes one quarter of what it was originally. This is because EMF is equal to the negative of the rate of change of magnetic flux

 

[someth1ng]

spoiler alert (highlight below)
Well within a DC generator output, the EMF is not constant, it changes from 0 to maximum and back to 0 recursively (for a single phase motor) so the output looks like the graph |sin(x)|. Hence, the magnetic flux in the coil will be constantly changing and it will be able to be stepped up and down.

A hand cranked AC generator is spun at a constant speed and produces an output EMF. The coil is then spun 4 times as fast. Explain the change in the curve in relation to its amplitude and period.

As it is spun faster, the change of flux is also higher as the change of time is lower and hence, amplitude is higher. If it is spun 4 times as fast, it means that frequency is increased by a factor of 4 meaning that period is decreased by a factor of 4.

 

[barbernator]

A beam of blue light is shone upon a platinum cathode within a vacuum tube. This circuit contains a galvanometer, and no current flows in the circuit. The platinum cathode is replaced with a zinc cathode and blue light of the same intensity shone upon it. A current flows.

Explain this phenomenon and why it occurs.

What change to the light would have to be made for a current to flow from the platinum cathode?

 

[deswa1]

A beam of blue light is shone upon a platinum cathode within a vacuum tube. This circuit contains a galvanometer, and no current flows in the circuit. The platinum cathode is replaced with a zinc cathode and blue light of the same intensity shone upon it. A current flows.

Explain this phenomenon and why it occurs.

What change to the light would have to be made for a current to flow from the platinum cathode?

I won't write a full answer but these are the main ideas.

- Photoelectric effect causes emission of electrons
- Each metal has a unique work function which is basically the energy required to liberate electrons from its surface -> the energy of incident photons (E=hf) must be above this in order to create a current
- Platinum has a higher work function than zinc so when the light is shone on the zinc, the energy of the photon is greater than the work function and photoemission occurs

Part B: The light would have to be of a higher frequency (and hence energy). For example, violet light might work or UV may be required, depending on the work function of platinum.

Question: Explain how maglev trains operate

 

[Ichiii]

The work function of the different metals is why this phenomenon occurs. The platinum has a higher KEmax Work function compared to Zinc. (KEmax = hf - work function)
To increase the KE energy of the light to overcome the work function of the metal you would have to increase the frequency of light until
the Energy of the light > Work Function of the metal (Photoeectric Effect)