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March 28, 2024, 10:38:26 pm

Author Topic: VCE Physics Question Thread!  (Read 603193 times)  Share 

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Adequace

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Re: VCE Physics Question Thread!
« Reply #960 on: April 30, 2015, 08:02:44 pm »
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Can someone help me with understanding questions? It seems like a broad question but I think that I understand the theory and concepts behind electronics but I can't seem to interpret the questions and understand what they want me to do.

It ranges from the different terminology used in questions and just sometimes just being confused..

Floatzel98

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Re: VCE Physics Question Thread!
« Reply #961 on: April 30, 2015, 08:21:24 pm »
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Can someone help me with understanding questions? It seems like a broad question but I think that I understand the theory and concepts behind electronics but I can't seem to interpret the questions and understand what they want me to do.

It ranges from the different terminology used in questions and just sometimes just being confused..
Keep doing the questions. If you get them wrong or don't understand what it was asking specifically, work it out by looking at the answers, asking somebody or even posting here. You said that you understand the theories and concepts, so even if you don't understand a certain question, once you have it explained you should try to relate it back to what you know.

Try to find patterns in questions. There is only a certain number of ways they can ask you things. Maybe it involves graphs of ohmic devices. Gather a whole heap of questions and attempt them. Find patterns in them and maybe even methods or steps of what to look for first when solving them.

Maybe you do actually have some gaps in your knowledge and you don't actually know what you think you should know. If this is the case, broaden your knowledge from more than just the textbook. Look at videos online about the concepts. Maybe they will fill in the unfamiliar terminology your textbook is using or will just give you a different perspective of the topic. 

I think you should start by just posting one of the questions you are having trouble with and getting help from us with it.
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Adequace

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Re: VCE Physics Question Thread!
« Reply #962 on: April 30, 2015, 09:40:20 pm »
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Keep doing the questions. If you get them wrong or don't understand what it was asking specifically, work it out by looking at the answers, asking somebody or even posting here. You said that you understand the theories and concepts, so even if you don't understand a certain question, once you have it explained you should try to relate it back to what you know.

Try to find patterns in questions. There is only a certain number of ways they can ask you things. Maybe it involves graphs of ohmic devices. Gather a whole heap of questions and attempt them. Find patterns in them and maybe even methods or steps of what to look for first when solving them.

Maybe you do actually have some gaps in your knowledge and you don't actually know what you think you should know. If this is the case, broaden your knowledge from more than just the textbook. Look at videos online about the concepts. Maybe they will fill in the unfamiliar terminology your textbook is using or will just give you a different perspective of the topic. 

I think you should start by just posting one of the questions you are having trouble with and getting help from us with it.
Cheers for the help, I agree I probably do have gaps in my knowledge without me knowing. I'll definitely post questions I have trouble with in the future.

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Re: VCE Physics Question Thread!
« Reply #963 on: April 30, 2015, 10:23:26 pm »
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A scientist has a 120g sample of the radioisotope polonium-218. The first three steps in the decay series of polonium-218 are an alpha emission followed by two beta particle emissions. These decays have half-lives of 3, 27 and 19 mins respectively. Calculate how many polonium-218 remains after 15 mins.

Realise this is year 11 physics. Should totally know but don't despite doing 3+4 this year. T.T

Please help?

silverpixeli

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Re: VCE Physics Question Thread!
« Reply #964 on: May 01, 2015, 08:26:21 am »
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A scientist has a 120g sample of the radioisotope polonium-218. The first three steps in the decay series of polonium-218 are an alpha emission followed by two beta particle emissions. These decays have half-lives of 3, 27 and 19 mins respectively. Calculate how many polonium-218 remains after 15 mins.

Realise this is year 11 physics. Should totally know but don't despite doing 3+4 this year. T.T

Please help?

there's some info we don't need here. The decay series it's talking about it this:



We see that after it decays for the first time, it's never going to be Polonium-218 again. So "Calculate how many polonium-218 remains after 15 mins" is just asking about the first decay, and the other halflives are meaningless to us.

The question becomes "how much of your 120g sample,half life 3min, is left after 15m?"

answer
15/3=5 half lives pass in the 15 minutes, so the sample size is halved 5 times.
120/2/2/2/2/2 = 120/32 = 3.75g

thus 3.75 grams remain after 15 mins.
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lolaishappy

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Re: VCE Physics Question Thread!
« Reply #965 on: May 02, 2015, 05:56:37 pm »
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When it comes to photonics, what theory is a must in knowing? Like modulation, demodulation, how about attenuation? Particularly from the Jacaranda text book.
Also How do I do sketch modulation graphs? :-[
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Cosec

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Re: VCE Physics Question Thread!
« Reply #966 on: May 02, 2015, 06:17:44 pm »
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When it comes to photonics, what theory is a must in knowing? Like modulation, demodulation, how about attenuation? Particularly from the Jacaranda text book.
Also How do I do sketch modulation graphs? :-[

A modulated graph, which im guessing is your just refeering to the graph of the wave itself, is a wave in which the information signal, normally one of low frequency, is super imposed on the carrier wave, one which has a much higher frequenc. producing a wave that is used to carry the information over a certain medium. Like this: http://www.sfu.ca/sonic-studio/handbook/Graphics/Amplitude_Modulation2.gif

I just had my sac, and learnt all of them which you described above. When i completed several practice sacs, a majority include a question about the modulation/demodulation process. So i would strongly advise you learnt them.

Adequace

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Re: VCE Physics Question Thread!
« Reply #967 on: May 02, 2015, 07:44:33 pm »
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We have our electricity SAC coming up in 2 weeks, we're allowed to bring a double sided cheat sheet. I've just finished mine but I have a lot of spare space - a whole page worth. I'm wondering if I should use that space to write questions I had trouble on or to use it for additional information about the less important things such as electric shocks and resistivity of your skin, I know it's going to be on the MCQ as well.

This is for unit 1/2 if you guys don't mind, thanks.

Floatzel98

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Re: VCE Physics Question Thread!
« Reply #968 on: May 02, 2015, 08:29:49 pm »
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We have our electricity SAC coming up in 2 weeks, we're allowed to bring a double sided cheat sheet. I've just finished mine but I have a lot of spare space - a whole page worth. I'm wondering if I should use that space to write questions I had trouble on or to use it for additional information about the less important things such as electric shocks and resistivity of your skin, I know it's going to be on the MCQ as well.

This is for unit 1/2 if you guys don't mind, thanks.
If you know what is coming up on the test for the most part and you know what you will struggle with, definitely try to fill it up with that. Specific examples of questions you struggled with. I think simple sentence definitions could suffice for the less important information or even just try to rote learn it. 

I know that we even get a cheat sheet to use in the 3/4 exam, but i would also recommend trying not to rely on it too much. It might seem counter-intuitive but the less you have to rely on a cheat sheet means that you know more of what you need to know already. You should / you usually are given a formula sheet with the standard equations you should know, so if you can try to dedicate more space on your cheat sheet to specifics of what you struggled on and not things you already know. This is not necessarily for this SAC now, but in the future as well. I know sometimes i just fill up my cheat sheets with all the equations that are already on the cheat sheet, sometimes with a few derived equations for myself. I spent time on it when i could have just been studying or i actually could have filled it up with useful information and examples that would aid me.

I know I'm just rambling on now and that my first sentence probably answers your question, but hopefully the rest helps as well if you can apply it at all. Anyway goodluck on your upcoming SAC and post any other questions you have! :)
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kk.08

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Re: VCE Physics Question Thread!
« Reply #969 on: May 02, 2015, 10:18:20 pm »
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Was just wondering, is it possible to find the elastic limit of a material without a stress-strain graph?

Thanks :)
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #970 on: May 03, 2015, 05:41:24 pm »
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If a resistor or even another diode is placed in series in front of a reversed biased diode (the current flows through the resistor before it hits the reverse biased diode), why doesn't  current still flow through. For example why isn't any power dissipated? The current has to go through the resistors and such to get to the reverse biased diode?

What happens to the current in the diode when its in reverse biased. Does it send it back the other way? Does it just store it? Or does it get used 100% as heat energy?

For diode labeling conventions. Are we supposed to always change the polarity of the circuit or can we just draw the diode facing the other way? Does it matter at all?  Because sometimes I just don't look at which way the circuit is and assume a diode is in forward bias when it is really in reverse.

What exactly determines the switch on voltage of  a diode. I know the material but what are the differences in material. Is a diode technically still a resistor? Are there other non-ohmic resistor type things as well?

Just some question i thought of while doing my homework today. Thanks to anyone that can answer them :)
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Adequace

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Re: VCE Physics Question Thread!
« Reply #971 on: May 03, 2015, 06:02:05 pm »
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http://imgur.com/wPXnz5R

In this circuit, what is voltage at X? I got the answer but I'm still confused on the method of getting the answer since I just guessed. We found the total resistance in the question above.

silverpixeli

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Re: VCE Physics Question Thread!
« Reply #972 on: May 03, 2015, 06:14:09 pm »
+2
Was just wondering, is it possible to find the elastic limit of a material without a stress-strain graph?

Thanks :)

I'm not sure if it's possible without SOME knowledge of the behaviour (a graph, a table of stress and strain values, a force/extension graph), like there's certainly no VCE level formula for calculating the elastic limit of a material based on other quantities. There may be some formula that involves a lot of other properties of the material (but I doubt it).

If a resistor or even another diode is placed in series in front of a reversed biased diode (the current flows through the resistor before it hits the reverse biased diode), why doesn't  current still flow through. For example why isn't any power dissipated? The current has to go through the resistors and such to get to the reverse biased diode?

Good question. Current in series has to be the same everywhere. Think of a road with a fixed amount of lanes and lots and lots of cars. The rate of the slowest moving cars determines the rate of all cars because otherwise fast moving cars would catch up and crash into slower cars. This analogy has plenty of holes but the basic idea is that current cant be high in some sections and low in others if those sections are in series.

So, even when current wants to leave the battery, the electrons can't physically go anywhere because there's a blockage all the way along the wires at the diode. (a reverse biased diode is just like a hole in the circuit - no current flows around then either)
Thus, no current flows through the wires, or the resistor, and that's why P=VI=V*0=0.

What happens to the current in the diode when its in reverse biased. Does it send it back the other way? Does it just store it? Or does it get used 100% as heat energy?

Same explanation as first question, there is no current in this circuit. Nothing happens with it. If a small amount of current were to flow (all real reverse bias diodes let a tiny leakage current through, i think), it would just go round the circuit like normal - the diode acting like a very high-resistance resistor.

For diode labeling conventions. Are we supposed to always change the polarity of the circuit or can we just draw the diode facing the other way? Does it matter at all?  Because sometimes I just don't look at which way the circuit is and assume a diode is in forward bias when it is really in reverse.

Conventional current flows out of the bigger terminal of the battery (the positive one) and if that direction around the circuit points the same way that the triangle points, the diode is forward biased.

Electrons actually flow the other way in a real circuit, but the convention still holds.

Always watch out for trick questions 'what is the current in this circuit' when they sneakily put it in reverse mode.

If you're asked to draw a circuit with the diode in the other mode, it's probably easiest to reverse the diode direction rather then the battery direction: it's a lot easier to tell the difference for the diode. An assessor might not see that you changed the battery.

What exactly determines the switch on voltage of  a diode. I know the material but what are the differences in material. Is a diode technically still a resistor? Are there other non-ohmic resistor type things as well?

This is to do with the magical chemistry that makes diodes function so I'll let someone else try to answer it.

BUT for the other bit, a diode can be thought of as a resistor but it has no well defined resistance because it is non ohmic (resistance is not the same for different voltages/currents, V-I graph not linear, etc)

You can calculate R=V/I for any given situation but your result is meaningless for the same diode in a different circuit. It's merely the 'effective resistance' of the diode, if you replaced it with a resistor of that same resistance you would get the same behaviour for that circuit.

Other non ohmic devices studied in VCE include temperature dependant resistors, light dependent resistors, and the other types of diodes (LED's and photodiodes)
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Floatzel98

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Re: VCE Physics Question Thread!
« Reply #973 on: May 03, 2015, 06:19:53 pm »
+1
http://imgur.com/wPXnz5R

In this circuit, what is voltage at X? I got the answer but I'm still confused on the method of getting the answer since I just guessed. We found the total resistance in the question above.
Well first you can find the total resistance which should be

Then the total current should be

The current though 12 ohm resistor should be since votlage is constant in parallel.

That leaves 2 amps to be found in the series part of the circuit and since current is constant in series they both have the 2 amps through them.

That leaves the voltage across the 2 ohm resistor to be

The voltage in the 4 ohm resistor is

You can verify this because voltage is constant in parallel and the 2 series resistors voltage add up to 12 (4 + 8 )

I'm pretty sure this is right. I'm notorious for forgetting at least something which stuffs up all the other calculations. It might be a long working but i hoped it highlighted everything for you and answered the question :)

Spoiler
I'm not sure if it's possible without SOME knowledge of the behaviour (a graph, a table of stress and strain values, a force/extension graph), like there's certainly no VCE level formula for calculating the elastic limit of a material based on other quantities. There may be some formula that involves a lot of other properties of the material (but I doubt it).

Good question. Current in series has to be the same everywhere. Think of a road with a fixed amount of lanes and lots and lots of cars. The rate of the slowest moving cars determines the rate of all cars because otherwise fast moving cars would catch up and crash into slower cars. This analogy has plenty of holes but the basic idea is that current cant be high in some sections and low in others if those sections are in series.

So, even when current wants to leave the battery, the electrons can't physically go anywhere because there's a blockage all the way along the wires at the diode. (a reverse biased diode is just like a hole in the circuit - no current flows around then either)
Thus, no current flows through the wires, or the resistor, and that's why P=VI=V*0=0.

Same explanation as first question, there is no current in this circuit. Nothing happens with it. If a small amount of current were to flow (all real reverse bias diodes let a tiny leakage current through, i think), it would just go round the circuit like normal - the diode acting like a very high-resistance resistor.

Conventional current flows out of the bigger terminal of the battery (the positive one) and if that direction around the circuit points the same way that the triangle points, the diode is forward biased.

Electrons actually flow the other way in a real circuit, but the convention still holds.

Always watch out for trick questions 'what is the current in this circuit' when they sneakily put it in reverse mode.

If you're asked to draw a circuit with the diode in the other mode, it's probably easiest to reverse the diode direction rather then the battery direction: it's a lot easier to tell the difference for the diode. An assessor might not see that you changed the battery.

This is to do with the magical chemistry that makes diodes function so I'll let someone else try to answer it.

BUT for the other bit, a diode can be thought of as a resistor but it has no well defined resistance because it is non ohmic (resistance is not the same for different voltages/currents, V-I graph not linear, etc)

You can calculate R=V/I for any given situation but your result is meaningless for the same diode in a different circuit. It's merely the 'effective resistance' of the diode, if you replaced it with a resistor of that same resistance you would get the same behaviour for that circuit.

Other non ohmic devices studied in VCE include temperature dependant resistors, light dependent resistors, and the other types of diodes (LED's and photodiodes)
EDIT: 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!
« Last Edit: May 03, 2015, 06:32:45 pm by Floatzel98 »
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Adequace

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Re: VCE Physics Question Thread!
« Reply #974 on: May 03, 2015, 06:27:59 pm »
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Well first you can find the total resistance which should be

Then the total current should be

The current though 12 ohm resistor should be since votlage is constant in parallel.

That leaves 2 amps to be found in the series part of the circuit and since current is constant in series they both have the 2 amps through them.

That leaves the voltage across the 2 ohm resistor to be

The voltage in the 4 ohm resistor is

You can verify this because voltage is constant in parallel and the 2 series resistors voltage add up to 12 (4 + 8 )

I'm pretty sure this is right. I'm notorious for forgetting at least something which stuffs up all the other calculations. It might be a long working but i hoped it highlighted everything for you and answered the question :)
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?