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Author Topic: VCE Physics Question Thread!  (Read 603342 times)  Share 

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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #975 on: May 03, 2015, 06:43:45 pm »
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Thanks for the fast reply. Additionally, could I find the current of the first arm which would be 2A then substitute to find the voltage at X? I did it like that, would that also be correct in terms of method?
If you mean find the total voltage of the series part of the circuit, then you would get a different answer to the voltage at X. You would get 6*2 = 12 V, which is the total voltage across the series arm. From there you could go and find the voltage across the 2 individual resistors, which is exactly the same as doing what i showed anyway.  Otherwise as long as you get the correct value for current and you use it to find the actual voltage across X and not the entire series arm, then yes. The method would be correct.
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silverpixeli

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Re: VCE Physics Question Thread!
« Reply #976 on: May 04, 2015, 09:15:48 am »
+1
Thank you for clearing everything up for me again. I really appreciate it :) For the part about the current flowing into a reverse biased diode: I know the answer stays the same, but does that mean that for the instant when it is turned on, current will flow up past everything to the diode? Also when you turn off the power source to a circuit, do the electrons that were currently in the circuit all leave the wire/circuit back into the power supply or can they get 'trapped' in there? Thanks again!

Actually, this is getting into the electromagnetism side, but there are always electrons distributed at all points along the wire, (conduction electrons bounce around fairly randomly when there's no potential difference affecting them)
When you 'turn on' the circuit, you're providing a 'potential difference' between one point and another. We think of this as a difference in energy, but the electrons are actually feeling a force, called 'electromotive force', because the battery is actually providing an electric field. The randomly bouncing electrons start to gradually experience a net movement with the force.
Compare this with the force of gravity, a uniform field when you're near the surface of the Earth. But sometimes we talk about potential energy (mgh). A grain of sand will fall downwards in a hollow tube, because there is a gravitational force pushing it that way. You could equivalently think of it as falling to the bottom of the tube, because then it will be at a lower gravitational potential energy (there is a 'potential difference' between the bottom and the top of the tube).
This is what's happening to electrons in your circuit. But we don't normally talk about it in terms of force in a circuit, we talk about the potential energy difference between the positive terminal and the negative terminal. In the case of the sand, we're talking about gravitational potential energy. In the case of the electron, we're talking about electrical potential energy.

'Electricity' travels at the speed of light, but this isn't the motion of the electrons, it's the speed that the field travels out and starts affecting the electrons that are further away. The electrons actually move relatively slowly :)
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #977 on: May 04, 2015, 03:47:44 pm »
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Thanks again Silverpixeli! But alas i have more questions again. This time about voltage amplifiers. I went over it in class today and we watched a few videos and i even watched the VTextbook video on it just then, but I'm still confused.

Firstly, i don't really understand how a transistor works in a circuit. From what i understood, it has a voltage input that goes through the collector and then one that comes through the base? If that's correct, are they actually 2 different power sources or do they come from the same power supply. If so, then how do you regulate how much voltage goes through it from the base input? From a picture i saw in class today, the base is apparently usually connected to a capacitor? I don't really know what a capacitor is either, but doesn't that store energy/voltage? Is that connected back to the same power supply of the circuit if that's the case. How do you regulate how much voltage goes through that anyway?

Next, from what i gathered, when no input voltage comes through the base, the resistance is infinite since it is basically 2 diodes facing each other and one of them will be in reverse bias? When there is a voltage input from the base that goes above the switch on voltage it will work and the resistance will basically be 0. How does the 'switch on voltage' allow the diode to work if it is approaching it in reverse bias? You would need like 50V to break it but it is still only about 0.7 V according to my book.

Even if that all makes sense to me, i still don't really know what it does. It amplifies the voltage but how? Is it because you are basically adding 2 voltages from 2 power sources together. Almost like as if you were adding ordinates of 2 sine graphs? I hardly understand what all the graphs are about either, but if someone can enlighten me on how all the above works, maybe i will be able to figure it out for myself. 

Thanks :)
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silverpixeli

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Re: VCE Physics Question Thread!
« Reply #978 on: May 04, 2015, 10:26:42 pm »
+1
Thanks again Silverpixeli! But alas i have more questions again. This time about voltage amplifiers. I went over it in class today and we watched a few videos and i even watched the VTextbook video on it just then, but I'm still confused.

Firstly, i don't really understand how a transistor works in a circuit. From what i understood, it has a voltage input that goes through the collector and then one that comes through the base? If that's correct, are they actually 2 different power sources or do they come from the same power supply. If so, then how do you regulate how much voltage goes through it from the base input? From a picture i saw in class today, the base is apparently usually connected to a capacitor? I don't really know what a capacitor is either, but doesn't that store energy/voltage? Is that connected back to the same power supply of the circuit if that's the case. How do you regulate how much voltage goes through that anyway?

Next, from what i gathered, when no input voltage comes through the base, the resistance is infinite since it is basically 2 diodes facing each other and one of them will be in reverse bias? When there is a voltage input from the base that goes above the switch on voltage it will work and the resistance will basically be 0. How does the 'switch on voltage' allow the diode to work if it is approaching it in reverse bias? You would need like 50V to break it but it is still only about 0.7 V according to my book.

Even if that all makes sense to me, i still don't really know what it does. It amplifies the voltage but how? Is it because you are basically adding 2 voltages from 2 power sources together. Almost like as if you were adding ordinates of 2 sine graphs? I hardly understand what all the graphs are about either, but if someone can enlighten me on how all the above works, maybe i will be able to figure it out for myself. 

Thanks :)

Transistor workings are also not really part of the course. Capacitors, also, are left for the detailed study 'further electronics'.
Voltage amplification questions range from interpreting graphs, talking about cutoff/saturation, voltage gain and converting from input signals to output signals and back.

A transistor definitely has the two source things going on, but I'm not sure what you're talking about when you mention two diodes and one in reverse bias mode.
Whatever voltage applies at the base is like the voltage across a diode, in that if it's lower than some switch on value, there will be no conduction. A transistor is different in that when the diode IS switched on, the majority of the current to the emitter doesn't come through the base, it comes through the collector. Basically, the voltage across the base regulates how much current can come through the collector. Typically, you have the collector hooked up to some more powerful supply that can provide as much current to the collector as the transistor will allow through it.

This is useful for signal amplification as follows: Typically you have a signal captured as small variations in voltage/current in a circuit. If you feed this varying voltage into the base of a transistor, it will allow proportionate amounts of current through from the high-power collector. The result will be a higher power version of your original signal coming out of the emitter!

This assumes that your input signal is always within the working range of the transistor. Too low, and you won't open the gate for current from the collector. We refer to this as 'cutoff'. Too high, and you wont be able to get any more current through the transistor, and all higher signals will output that same max. We call this saturation. Both are examples of 'clipping' which result in the extremes of a signal being 'chopped off' from signals that dont lie within this range.

The complicated circuits that actually make this work are not studied and I have no understanding of them personally other than the high-level sketch above. Sorry :P


As for the amplifier graphs, Vin - Vout graphs map input voltages to output voltages. If you put in an input signal (often represented as a Vin - time graph) you can use the amplifier's characteristic graph to map it to an output signal v. time graph. This process, and the reverse, are semi-common exam questions.
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Re: VCE Physics Question Thread!
« Reply #979 on: May 06, 2015, 03:53:08 pm »
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Hi guys, just wondering, is it possible to make a black (light emitting) LED/Torch? For example, if you were in a brightly lit room (such as an art gallery) with lots of big blank white walls and you were to shine this torch, it would make a dark patch..?

silverpixeli

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Re: VCE Physics Question Thread!
« Reply #980 on: May 06, 2015, 04:23:18 pm »
+1
Hi guys, just wondering, is it possible to make a black (light emitting) LED/Torch? For example, if you were in a brightly lit room (such as an art gallery) with lots of big blank white walls and you were to shine this torch, it would make a dark patch..?

Good question. afaik a 'black light' is impossible.

When we look at something black, it looks black because there are no photons (light particles) reaching our eyes from that place. If there were some red photons, it would look red. If there were some red+green+blue photons (or another combination of lots of colours) it might look white.
To make it look black, we'd have to stop it from reflecting photons towards our eyes. We can't do this with a torch.
I guess we could alternatively try to stop the photons from reaching our eyes once they have been reflected, but I don't see how this could be done either.

In short, black things look black because there is no light coming from there. There are no black photons that you could shine on something.
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Re: VCE Physics Question Thread!
« Reply #981 on: May 06, 2015, 08:52:31 pm »
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Hi guys, just wondering, is it possible to make a black (light emitting) LED/Torch? For example, if you were in a brightly lit room (such as an art gallery) with lots of big blank white walls and you were to shine this torch, it would make a dark patch..?

Hmmm....

I wonder if a molecule exists that absorbs essentially completely in the visible range and has such a low fluorescent quantum yield that all of the light absorbed is re-radiated as heat. In theory, if you could make a molecule like that, said molecule would block out all light hitting it, so you would have a dark trail in the air. But I can't imagine what on earth would have such a property.


But yeah, no black photons though.
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Re: VCE Physics Question Thread!
« Reply #982 on: May 10, 2015, 02:10:32 pm »
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Hey Guys, I've got my Electronics and Photonics SAC this week and was wondering if anyone had some past Sacs or resources they'd be kind enough to share. Thanks :D
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #983 on: May 10, 2015, 08:15:37 pm »
+1
I'm having a bit of trouble with the attached question. I have looked at the worked solutions for question 3 and i understand the steps but i don't understand how they thought the steps up and how they got to them. I find that a lot of these ratio questions stump me, especially when they were used in Gravity and Satellites. I can kind of see how it wants me to answer it but i can never actually go through with it and get the correct answer. Can someone show me how they went through the steps to solve this? Thanks :)

EDIT: Another question. There is a circuit with 3 identical resistors of 100 Ohms that are connected with one in series and then the other 2 in parallel and the max power from any one resistor is 25W.

The question was to find the max voltage the can be applied. I used and had already found the total resistance and went for the 3 resistors, but my answer seems to be wrong.
« Last Edit: May 10, 2015, 09:03:11 pm by Floatzel98 »
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Re: VCE Physics Question Thread!
« Reply #984 on: May 10, 2015, 08:34:09 pm »
+2
I'm having a bit of trouble with the attached question. I have looked at the worked solutions for question 3 and i understand the steps but i don't understand how they thought the steps up and how they got to them. I find that a lot of these ratio questions stump me, especially when they were used in Gravity and Satellites. I can kind of see how it wants me to answer it but i can never actually go through with it and get the correct answer. Can someone show me how they went through the steps to solve this? Thanks :)

Q3) Total resistance
Using the voltage divider formula,
Alternatively, we can just use ratios- because is only 1/3 the size of , it will only have a third of the voltage drop across it (from here you can just figure it out using basic maths)
Q4)

Then, remembering that  is 1/4 of the total and is the other 3/4, they will be 15 and 45 ohms respectively
b)


I hope thats right

edit: I suck at latex, I think its all good now
« Last Edit: May 10, 2015, 08:39:28 pm by odeaa »
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silverpixeli

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Re: VCE Physics Question Thread!
« Reply #985 on: May 10, 2015, 08:54:55 pm »
+1
Another question. There is a circuit with 3 identical resistors of 100 Ohms that are connected with one in series and then the other 2 in parallel and the max power from any one resistor is 25W.

The question was to find the max voltage the can be applied. I used and had already found the total resistance and went for the 3 resistors, but my answer seems to be wrong.

Why are you adding ohms and watts together? I assume typo?

Okay so just because the maximum power of one resistor is 25 Watts doesnt mean it's operating at that power. That just means that as you increase the voltage applied, you have to be careful not to take ANY of them over the max. In particular, With more resistance and twice the current, the resistor OUTSIDe of the parallel section will probably have the most power at any given time. So you should solve for the total voltage in the circuit when all you know is that the series resistor has 25W, 100Ohms.

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Re: VCE Physics Question Thread!
« Reply #986 on: May 10, 2015, 09:02:43 pm »
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Thanks, using the voltage divider formula makes more sense :)

Plus i have another question now. If you have a voltage divider over a resistor that is in parallel, do we take voltage divider resistors parallel voltage or it's individual voltage? From the questions I've done it's apparently the total parallel resistance but i don't think that makes sense for some reason. Is it because the voltage in parallel is constant? That's my only reasoning. If i turn the parallel resistor into a single resistor ( and put it in series with another resistor) is that still the same thing.

While typing that i think i've answered my own question. But does that make sense still? If i have a voltage divider over one resistor in parallel, i can just simplify that into a single resistor and everything will still work the same?

Thanks :)
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Re: VCE Physics Question Thread!
« Reply #987 on: May 10, 2015, 09:11:30 pm »
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Why are you adding ohms and watts together? I assume typo?

Okay so just because the maximum power of one resistor is 25 Watts doesnt mean it's operating at that power. That just means that as you increase the voltage applied, you have to be careful not to take ANY of them over the max. In particular, With more resistance and twice the current, the resistor OUTSIDe of the parallel section will probably have the most power at any given time. So you should solve for the total voltage in the circuit when all you know is that the series resistor has 25W, 100Ohms.
Yes, that was a typo. My original workings were and solve for V such that . The total resistance is 150 ohms and if each resistor can have a max output of 25 W, then the total power output could be 75 W, so . Is the total power across the parallel resistors not 50W? Because i'm pretty sure my answer is wrong because of my watts working out. If the total voltage is the same and the resistors are identical, the the current is exactly split across them, which would then give 12.5W + 12.5W each, changing the equation to  , but that also doesn't give the correct answer. The question was what is the total allowed voltage that can pass across them. I might still be interpreting it wrongly though. Should i try to find the voltage across the parallel and series separately and then  add them together?
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silverpixeli

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Re: VCE Physics Question Thread!
« Reply #988 on: May 10, 2015, 09:23:29 pm »
+1
Yes, that was a typo. My original workings were and solve for V such that . The total resistance is 150 ohms and if each resistor can have a max output of 25 W, then the total power output could be 75 W, so . Is the total power across the parallel resistors not 50W? Because i'm pretty sure my answer is wrong because of my watts working out. If the total voltage is the same and the resistors are identical, the the current is exactly split across them, which would then give 12.5W + 12.5W each, changing the equation to  , but that also doesn't give the correct answer. The question was what is the total allowed voltage that can pass across them. I might still be interpreting it wrongly though. Should i try to find the voltage across the parallel and series separately and then  add them together?

Actually because the resistance is down over the parallel branch, they will actually not have as much voltage as across the series resistor.
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Re: VCE Physics Question Thread!
« Reply #989 on: May 10, 2015, 09:45:25 pm »
+1
What does it mean to have a negative voltage? Is it only when you have a photodiode or something in reverse bias and hence a negative current?
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