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March 28, 2024, 08:58:56 pm

Author Topic: VCE Physics Question Thread!  (Read 603165 times)  Share 

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knightrider

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Re: VCE Physics Question Thread!
« Reply #945 on: April 21, 2015, 06:16:29 pm »
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Depends on the context. Usually, yeah, initial velocity refers to velocity at time t=0. But t=0 is kinda arbitrary, right? Otherwise t=0 might refer to the start of the universe or whatever absolute zero point you want to define. The idea is, we define a zero point that works best for us.

In many kinds of motion, we set t=0 to the start of that motion of interest, so 'initial velocity' relates directly to that starting instant.

If we're ever talking about a change in velocity, initial velocity will refer to the start of the change.

Sometimes, in projectile motion questions, we'll deal with different values of u, (different 'initial velocities') at different points in the question. For example if you want to find the time taken to fall from the peak of the projectile path to the landing point, you'd consider the start of this motion to be the peak of the path, and hence vertical u=0. In another part of the question, you might take u to be the launch speed because that's where you're measuring from (this is far more common).

Thanks silverpixeli  :)

knightrider

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Re: VCE Physics Question Thread!
« Reply #946 on: April 23, 2015, 08:45:42 pm »
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How would you draw a velocity time graph for the following situation. A particle  starts with a positive velocity  and  undergoes constant acceleration until the  end of the time period?

odeaa

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Re: VCE Physics Question Thread!
« Reply #947 on: April 23, 2015, 09:32:36 pm »
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It would just increase linearly because it has no friction
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #948 on: April 23, 2015, 10:00:26 pm »
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It would just increase linearly because it has no friction

The question hasn't actually said the acceleration is positive :P
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odeaa

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Re: VCE Physics Question Thread!
« Reply #949 on: April 23, 2015, 10:01:11 pm »
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The question hasn't actually said the acceleration is positive
Lzxnl 1 odeaa 0
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knightrider

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Re: VCE Physics Question Thread!
« Reply #950 on: April 24, 2015, 12:01:39 am »
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How would you draw a velocity time graph for the following situation. A particle  starts with a positive velocity  and  undergoes constant acceleration until the  end of the time period?

How would i draw this graph?

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Re: VCE Physics Question Thread!
« Reply #951 on: April 27, 2015, 08:06:31 pm »
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I'm interested to know what should I be doing in units 1/2 if I'm aiming for a 40SS for 3/4? I've been putting a significant amount of effort in to electricity and will be doing the some for motion but is there anything else?

silverpixeli

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Re: VCE Physics Question Thread!
« Reply #952 on: April 27, 2015, 09:56:32 pm »
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I'm interested to know what should I be doing in units 1/2 if I'm aiming for a 40SS for 3/4? I've been putting a significant amount of effort in to electricity and will be doing the some for motion but is there anything else?

Pretty sure only motion and electricity carry forward to 3/4, sounds like you're setting yourself up with some solid foundations for these topics in year 12. Place emphasis on developing really solid problem solving skills! You can of course work on them in year 12 as well, but you should ideally be able to tackle any practice question you see without any trouble.
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Kel9901

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Re: VCE Physics Question Thread!
« Reply #953 on: April 28, 2015, 01:58:31 pm »
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Pretty sure only motion and electricity carry forward to 3/4, sounds like you're setting yourself up with some solid foundations for these topics in year 12. Place emphasis on developing really solid problem solving skills! You can of course work on them in year 12 as well, but you should ideally be able to tackle any practice question you see without any trouble.

don't forget to still put work in to the other parts of the course though, even if they aren't that relevant, because it will help you develop a good work ethic.
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #954 on: April 29, 2015, 08:14:57 pm »
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I have a couple questions about diodes:

Firstly, I  don't really understand the purpose of diodes. What are they used for exactly and where? How does forward and reverse biased work? Like why does it have almost no resistance and then infinite resistance? I don't understand. Why do they have a switch on voltage? Before the switch on voltage is reached, my book says that the diode does not conduct. Does this mean, if it were in series for example with another resistor, the circuit would never actually turn on/be complete? Is this phase exactly the same as what happens in reverse bias?

Secondly, in class we went over how they work, a lot of the chemistry behind them, but I really didn't understand most of it. Do we need to know this at all for any reason? Would someone be able to explain how they work anyway because I still want to understand it. I looked at a couple videos online but i still don't really understand.

Thanks :)
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Adequace

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Re: VCE Physics Question Thread!
« Reply #955 on: April 29, 2015, 09:15:11 pm »
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I started doing Checkpoints Physics 1/2's electric circuit chapter, I'm getting most of the questions wrong or don't understand the terminology...killed my self-esteem.

alchemy

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Re: VCE Physics Question Thread!
« Reply #956 on: April 29, 2015, 09:30:13 pm »
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I started doing Checkpoints Physics 1/2's electric circuit chapter, I'm getting most of the questions wrong or don't understand the terminology...killed my self-esteem.

Which questions exactly? Maybe not post them on this thread, but feel free to start your own thread so we can help you! :)


silverpixeli

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Re: VCE Physics Question Thread!
« Reply #957 on: April 29, 2015, 11:39:23 pm »
+3
I have a couple questions about diodes:

Firstly, I  don't really understand the purpose of diodes. What are they used for exactly and where? How does forward and reverse biased work? Like why does it have almost no resistance and then infinite resistance? I don't understand. Why do they have a switch on voltage? Before the switch on voltage is reached, my book says that the diode does not conduct. Does this mean, if it were in series for example with another resistor, the circuit would never actually turn on/be complete? Is this phase exactly the same as what happens in reverse bias?

Secondly, in class we went over how they work, a lot of the chemistry behind them, but I really didn't understand most of it. Do we need to know this at all for any reason? Would someone be able to explain how they work anyway because I still want to understand it. I looked at a couple videos online but i still don't really understand.

Thanks :)

Can answer some of your questions but can't explain how diodes actually work... it's not assessable so I didn't bother :)



forward or reverse bias?

first of all, forward bias refers to when conventional current in a circuit flows in the same direction that the triangle in the diode symbol points. this is opposite to the flow of electrons, but it's not like the physical diode is actually an arrow, it's just chemically a one-way thing and so we chose to line the triangle up with conventional current in our diagrams.

reverse bias is obviously the other way. if you see your conventional current hitting the point side of a triangle, your diode will not conduct because it's in reverse bias mode.



why wont it conduct below the switch on voltage?

a diode is not made of material that can inherently conduct electricity. however, because of the magical chemistry going on under the hood, the diode can ---  in one direction --- 'carry' electrons across. this is different to electrons being pushed through a resistor because of potential difference from one side to the other. electrons are unable to push themselves through a diode, but they can hitch a ride through and so the diode appears to be conducting, just like a resistor does.

if there isn't enough potential across the diode, however, this 'carrying' service wont actually start and the diode wont conduct. likewise, if the circuit is pushing the other way, the diode is gonna be a dead end because electrons can't push themselves through, and the diode can only carry them in the forward direction.

i'm thinking about this 'carrying' analogy now, and it seems appropriate because of the one-way property and it's close to how diodes actually work, but theres another property of diodes that doesnt really sit well with it. When you're an electron and you push your way through a resistor, you can spend as much of your energy as you like (and arrive at the other side with less potential, hence potential 'difference' across the resistor). but in a diode, assuming we're above the switch on voltage so that the diode conducts, we'll never spend more than the switch on voltage.

because of this, maybe you should think of diodes as a one-way toll gate. you can go through as fast as you like (indeed, diodes don't influence current in a circuit -- that's decided by the elements in the rest of the circuit) but you always have to pay the same amount (the voltage across the diode will always be the switch on voltage, or less, never more). if you don't have enough potential to pay the toll, you wont even be allowed through! (this is the case where the diode has a potential difference below the switch on voltage across it) and of course, if you try to go back, you won't be allowed, because this gate is on a one way street (no conduction in reverse bias mode)



diodes dont influence current?

one final thing about the properties of a diode is the current through them. I said that diodes allow any current just like a toll gate may allow things to move through at any speed - as long as they pay their toll of potential.

think of a circuit where the battery provides the electrons with 6V of potential at the start, and they have to pass through a 0.8V diode and a 520 ohm resistor, in series. the electrons have the job of deciding how to spend their potential across the circuit before they get back to the battery - where they must have zero. if the circuit were two identical resistors, they would decide to spend their voltage evenly.
But because there's now a 'toll gate', their first priority will be to pay the toll to move through the diode. but they wont spend any more than the toll. then, they each have 6-0.8=5.2V of potential left to spend on the rest of the circuit. of course, there's only the resistor left, so they will splurge their remaining 5.2V on the 520 ohm resistor and move through it as fast as they can: I=V/R=5.2/520=0.01A. Their current through this resistor must be their current everywhere (series circuit) so they must also speed through the diode at 0.1A as well.

The point i'm getting at here is that it's the REST of the circuit that determines the current, you can't tell anything about currents just from a diode because they dont have a well defined resistance, but you can use them to tell you about voltage drops elsewhere because they have a well defined cost in crossing them.

in your example, a diode in series with a resistor, assuming the electrons have enough potential to switch on the diode, they will spend that potential on the diode and spend the rest on getting through the resistor with whatever current they can afford.
but if they weren't given enough potential to even pay the diode toll (less than switch on voltage), then none of them could get around the circuit and it wont conduct, just like if the diode was n reverse bias mode which is effectively the same as cutting one of the wires - no electrons can push through because diodes are a one-way toll gate.


non ideal diodes

in reality, diodes aren't perfectly on or off components. they will begin to conduct a little before the switch on voltage and they will charge slightly more for high current electrons - the I V graph doesn't have infinite gradient at V=switch on voltage.

also, they aren't perfectly impossible to pass in reverse bias mode. if there's enough voltage across a reverse bias diode, the materials inside will stop blocking electrons from flowing and it will begin to conduct freely (well, not freely, in this case you'd be paying a huge voltage toll to keep the gates open the wrong way)




what are they used for?

the one-way thing is actually hugely advantageous. there are many applications of regular diodes in circuitry, for example you can use them and a clever circuit layout called a bridge rectifier to turn AC current into positive-only current. (not studied in core electricity).

in photonics you'll also look at light-emitting diodes (LEDs) which I am sure you are familiar with. probably a bunch of lights in your home, school or any building are LED these days, meaning instead of a hot, glowing globe filament used in old light bulbs they just fire current through a particular type of diode and you get that blue/white light due to the emission of photons. computers, torches, toys, LOTS of things have little LED lights because it's an efficient way to illuminate stuff.
aside from looking really cool, LEDs can be used in optical applications like flashing on and off really fast sending pulsating light signals down an optic fibre for speed-of-light, highly efficient signal transmission.

there are also 'photodiodes' which you'll look at in photonics as well. These again have slightly different chemistry behind them which i dont really understand, but they basically leak a small amount of current that depends on the amount of light that illuminates their surface. In this way, they can capture changing lighting conditions and translate them into changing electrical current - we call this an opto-electric transducer, or something (i forget the technical terminology), and it's useful for doing these sorts of signal conversions

there are probably heaps more examples of applications of diodes! I've probably only scratched the surface.
« Last Edit: April 29, 2015, 11:41:07 pm by silverpixeli »
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #958 on: April 30, 2015, 07:29:56 am »
+1
^Holy crap, thanks so much for all that information. That toll-gate analogy actually helps a lot :)
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silverpixeli

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Re: VCE Physics Question Thread!
« Reply #959 on: April 30, 2015, 09:52:39 am »
+1
^Holy crap, thanks so much for all that information. That toll-gate analogy actually helps a lot :)

I know, I'm very proud of it, I understand diodes a lot better now after writing that :P
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