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March 29, 2024, 06:44:40 am

Author Topic: VCE Physics Question Thread!  (Read 603344 times)  Share 

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barydos

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Re: VCE Physics Question Thread!
« Reply #240 on: October 17, 2013, 09:54:22 pm »
0
If asked what RMS voltage was, what would you respond with?
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #241 on: October 17, 2013, 10:04:56 pm »
+1
The corresponding constant DC voltage that provides the same average power output.
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barydos

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Re: VCE Physics Question Thread!
« Reply #242 on: October 17, 2013, 10:08:56 pm »
0
The corresponding constant DC voltage that provides the same average power output.

Thanks!
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #243 on: October 17, 2013, 10:14:00 pm »
+3
what is a good answer to the following:
-what is modulation and demodulation?

Modulation is the conversion of an electric signal into a light signal. In the VCE course, we only look at amplitude modulation. In this form of modulation, the shape of the electric field signal is embedded in the shape of the carrier wave light signal, which generally has a very high frequency. When the electric field is negative, the light signal's amplitude is just lower than its average amplitude, which corresponds to the zero of the electric field signal. The purpose of the modulation is to allow the electric signal to be literally carried by the carrier way across a distance to where it is demodulated, or converted back into an electric signal, for use.

As an example, let's take two circuits, one with an LED and one with a photodiode. In the circuit with the LED, the current through the LED determines brightness of the light emitted. This light contains information about the current of the LED circuit; it is modulated. When it hits the photodiode, the intensity of the light then determines the current of the photodiode circuit; the light becomes demodulated. That's the example I use to remember what modulation does.
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Robert123

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Re: VCE Physics Question Thread!
« Reply #244 on: October 18, 2013, 09:26:58 pm »
0
VCAA 2010 Q16:
Basically a moving trolley with a spring on its front collide into a stationary trolley. The spring compress then uncompressed in the collision which  is elastic. How would the total momentum of the system varies with time. The answers says that momentum is always conserved in a collision so it will be a straight line with no variation when the springs come into play. But, my intuition tells me that some of the momentum will sort of be temporarily stored within the spring and decrease, could someone please explain to me why I'm wrong. Cheers :)

barydos

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Re: VCE Physics Question Thread!
« Reply #245 on: October 18, 2013, 10:36:28 pm »
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This was a VCAA 2006 question:
Spoiler

Answer:
Spoiler

So for max induced voltage, we would use that -NBA/t formula. The difficult for me was finding 't'
If the square was moving at 4.0 cms^-1, I thought the max change in flux would have been when it reaches the middle of the face of the magnet (aka 6cm in?) so t would be 6/4 = 1.5 seconds. But the answer suggests that it just takes 0.5 seconds meaning the square only just goes completely in the field.
      Could someone just clarify if I'm being silly or not, and whether or not the square requires to be in the middle or not?
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #246 on: October 18, 2013, 10:44:59 pm »
+1
The square doesn't need to be in the middle; it just needs to be in a position such that the entire square is in the magnetic field. This is because the magnetic field is uniform.
Remember, the formula flux = BA refers to the area that is exposed to the magnetic field.
Now, the maximum voltage induced would then be due to the change in flux from when the square first enters the field to when it is fully immersed in the field. The side length is 2 cm, and the speed of motion is 4 cm/s, so the time we want is 0.5 s.
Just one loop only.
The change in flux is 3.7*10^-3 T * (2 cm)^2 = 14.8 * 10^-7 T m^2 (note change in units)
So dividing by the time, our max voltage is 29.6* 10^-7 V, so 2.96 uV which is the unrounded form of the answer given.

The trick is identifying what the time means. The time is the period of time over which the change in flux occurs. Note that when the square is in the middle of the magnetic field, the entire square will have remained in the field for a period of time, so the flux isn't changing; the voltage is zero in that case.

Honestly mate, I don't fully understand half the crap we're learning in physics and I got 96% on that sample VCAA 2013 exam.

All you need to do is sub in values.

We know speed = distance/time so therefore time = distance/speed = 0.02/0.04 = 0.5, all values directly from the question subbed into our formulas from our A3 cheat sheet.

Then sub time = 0.5 into the EMF equation and wallah, you get the answer.

Another example of how physics is becoming ridiculous; you don't even need any understanding of the mechanics of the formulas to get the marks.
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SocialRhubarb

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Re: VCE Physics Question Thread!
« Reply #247 on: October 18, 2013, 10:49:44 pm »
+1
VCAA 2010 Q16:
Basically a moving trolley with a spring on its front collide into a stationary trolley.
Spoiler
I think this is quite a difficult question to grasp conceptually, and difficult to answer without simply parroting the line that momentum is always conserved. But essentially what is happening is that forces are acting on the spring in both directions, and the spring is exerting a force on both trolleys. Because momentum is a vector quantity, that is, it has direction, exerting equal forces in opposite directions results in no change in momentum. The problem is it is a little more complicated than that, as the spring would not exert equal forces on both the trolleys, but essentially what would happen is that it would exert a slightly greater force on the stationary trolley than the moving trolley behind it, and the 'unbalanced' force actually contributes to the change in momentum of the spring itself. I expect that VCAA might expect you to briefly touch on some of these points and simply mention that momentum is always conserved.

I've talked about both forces on the spring and forces on the trolley, and used these interchangeably as they'll be of the same magnitude as they're Newton third law pairs, but it can make it a bit confusing.
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #248 on: October 18, 2013, 11:01:33 pm »
+1
Spoiler
I think this is quite a difficult question to grasp conceptually, and difficult to answer without simply parroting the line that momentum is always conserved. But essentially what is happening is that forces are acting on the spring in both directions, and the spring is exerting a force on both trolleys. Because momentum is a vector quantity, that is, it has direction, exerting equal forces in opposite directions results in no change in momentum. The problem is it is a little more complicated than that, as the spring would not exert equal forces on both the trolleys, but essentially what would happen is that it would exert a slightly greater force on the stationary trolley than the moving trolley behind it, and the 'unbalanced' force actually contributes to the change in momentum of the spring itself. I expect that VCAA might expect you to briefly touch on some of these points and simply mention that momentum is always conserved.

I've talked about both forces on the spring and forces on the trolley, and used these interchangeably as they'll be of the same magnitude as they're Newton third law pairs, but it can make it a bit confusing.

Why don't we look at a different reference frame, that of the "stationary trolley", so that the situation becomes a moving trolley collides with a trolley with a spring on it?
Let the now moving trolley be trolley A and the stationary trolley with a spring be trolley B.
Now, as trolley A runs into the spring, A exerts a force on the spring, which compresses it. Similarly, the spring exerts a retarding force on A which slows it down. The spring, meanwhile, also exerts a force on trolley B, which reduces the compression of the spring, effectively transferring momentum from A to B. As the spring compresses, it slows A down more and more. When it can be compressed no further, it pushes A backwards and this pushes B forwards. Even when the spring isn't being compressed and is 'stationary', the situation becomes similar to having two blocks; here, A effectively pushes directly onto B. When the spring unwinds itself, it pushes A backwards and B forwards. Momentum is conserved, but there are a number of forces at play here.

Note; conservation of momentum can be seen to be a consequence of Newton's third law in classical mechanics, which is partly the argument I'm making here.
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SocialRhubarb

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Re: VCE Physics Question Thread!
« Reply #249 on: October 18, 2013, 11:12:05 pm »
0
I think the spring's on the other trolley in the question, although obviously it's essentially the same scenario anyway. I think what Robert was interested in though was how the compression of the spring affected the momentum of the trolleys.

Also, how does the spring exerting a force on trolley B reduce the compression of the spring? Surely at that stage of your argument the spring is still being compressed.
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #250 on: October 18, 2013, 11:21:38 pm »
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The spring is on the other trolley to begin with, but I decided to just switch things around for simplicity in my own mind. You're right, it doesn't make a difference.

The point about the force on trolley B is that the compression of the spring is occurring because trolley A is pushing onto the spring. Trolley B is in the other direction of the applied force, so pushing on that, and trolley B's resultant motion, relaxes the spring. It's probably most easily demonstrated with a wall. If you ran a trolley with a spring on it against a wall and the wall doesn't move, the spring is compressed a lot, because although it pushes on the wall, the wall doesn't budge. In my scenario, the spring pushes on trolley A and makes it move, and as the trolley is attached to the spring, the spring is stretched in the opposite direction it is being compressed.
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barydos

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Re: VCE Physics Question Thread!
« Reply #251 on: October 18, 2013, 11:28:31 pm »
0
The square doesn't need to be in the middle; it just needs to be in a position such that the entire square is in the magnetic field. This is because the magnetic field is uniform.
Remember, the formula flux = BA refers to the area that is exposed to the magnetic field.
Now, the maximum voltage induced would then be due to the change in flux from when the square first enters the field to when it is fully immersed in the field. The side length is 2 cm, and the speed of motion is 4 cm/s, so the time we want is 0.5 s.
Just one loop only.
The change in flux is 3.7*10^-3 T * (2 cm)^2 = 14.8 * 10^-7 T m^2 (note change in units)
So dividing by the time, our max voltage is 29.6* 10^-7 V, so 2.96 uV which is the unrounded form of the answer given.

The trick is identifying what the time means. The time is the period of time over which the change in flux occurs. Note that when the square is in the middle of the magnetic field, the entire square will have remained in the field for a period of time, so the flux isn't changing; the voltage is zero in that case.

Another example of how physics is becoming ridiculous; you don't even need any understanding of the mechanics of the formulas to get the marks.

Ahhh that makes so much sense. Thanks for the clarification!
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SocialRhubarb

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Re: VCE Physics Question Thread!
« Reply #252 on: October 18, 2013, 11:29:01 pm »
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Oh, I think I see what you're saying, but surely you should state that it's the acceleration of trolley B that's important, not the force.

In your example of the wall, the spring is still exerting a force. In fact, it's probably exerting an even greater force. But the force doesn't 'decrease the compression'. The force increases the compression of the spring. You can imagine that if you had no force you'd have no compression. Which is why it's strange to say that the "force on trolley B ... reduces the compression of the spring".

But of course, the force accelerates the trolley, which increases its velocity, reducing its speed relative to the spring, reducing its compression, which I think is what you're trying to say?
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lzxnl

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Re: VCE Physics Question Thread!
« Reply #253 on: October 18, 2013, 11:37:08 pm »
+1
But of course, the force accelerates the trolley, which increases its velocity, reducing its speed relative to the spring, reducing its compression, which I think is what you're trying to say?

Yes, that was my point that I failed to articulate clearly. Thank you for clarifying it :D
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barydos

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Re: VCE Physics Question Thread!
« Reply #254 on: October 19, 2013, 11:07:01 am »
0
How do you do this question?
Spoiler

What happens between the two?
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