/0's physics phread

[TrueTears]

I am having lots of problems with the Insight 2009 Trial Exam...
http://vcenotes.com/forum/index.php/topic,8214.15.html

Question 6 - Electronics and Photonics:
If you look at the transfer characteristic of the amplifier, a voltage of should give an output voltage of something like , and a voltage of should give , because that is well within the saturation region of the amplifier.
But in the answer they just apply the linear gain to the max and min of the input graph... How is this correct if it defies the characteristic graph??
I would expect an output graph that is mostly constant at 10V but which drops down every now and then.

Question 11 - Electronics and Photonics:
Are the answers wrong with this!?? Should the modulated signal be modulated at its minimums as well? They just have it at constant minimum!

Question 5 - Structures and Materials:
Isn't D as acceptable an answer as A?

I remember Q 6 and thought exactly as you did, but if you look closely the graph for they give you begins at 0. A suitable graph would have started at around 5 as the mid point for the graph and then oscillate to 10 and down to 1.4. So basically you take 0 on the graph as 5, and the highest you go up is around 4 which corresponds to around 10 if you had the midpoint around 5.

[/0]

I am having lots of problems with the Insight 2009 Trial Exam...
http://vcenotes.com/forum/index.php/topic,8214.15.html

Question 6 - Electronics and Photonics:
If you look at the transfer characteristic of the amplifier, a voltage of should give an output voltage of something like , and a voltage of should give , because that is well within the saturation region of the amplifier.
But in the answer they just apply the linear gain to the max and min of the input graph... How is this correct if it defies the characteristic graph??
I would expect an output graph that is mostly constant at 10V but which drops down every now and then.

Question 11 - Electronics and Photonics:
Are the answers wrong with this!?? Should the modulated signal be modulated at its minimums as well? They just have it at constant minimum!

Question 5 - Structures and Materials:
Isn't D as acceptable an answer as A?

I remember Q 6 and thought exactly as you did, but if you look closely the graph for they give you begins at 0. A suitable graph would have started at around 5 as the mid point for the graph and then oscillate to 10 and down to 1.4. So basically you take 0 on the graph as 5, and the highest you go up is around 4 which corresponds to around 10 if you had the midpoint around 5.

Why would start at 5? Shouldn't it start at 10? If you graph it like that doesn't that defy the characteristic graph?

[TrueTears]

I am having lots of problems with the Insight 2009 Trial Exam...
http://vcenotes.com/forum/index.php/topic,8214.15.html

Question 6 - Electronics and Photonics:
If you look at the transfer characteristic of the amplifier, a voltage of should give an output voltage of something like , and a voltage of should give , because that is well within the saturation region of the amplifier.
But in the answer they just apply the linear gain to the max and min of the input graph... How is this correct if it defies the characteristic graph??
I would expect an output graph that is mostly constant at 10V but which drops down every now and then.

Question 11 - Electronics and Photonics:
Are the answers wrong with this!?? Should the modulated signal be modulated at its minimums as well? They just have it at constant minimum!

Question 5 - Structures and Materials:
Isn't D as acceptable an answer as A?

I remember Q 6 and thought exactly as you did, but if you look closely the graph for they give you begins at 0. A suitable graph would have started at around 5 as the mid point for the graph and then oscillate to 10 and down to 1.4. So basically you take 0 on the graph as 5, and the highest you go up is around 4 which corresponds to around 10 if you had the midpoint around 5.

Why would start at 5? Shouldn't it start at 10? If you graph it like that doesn't that defy the characteristic graph?

No, I'm saying if you had to draw a graph without the axes and scales given in the exam. If you had to draw on yourself you'd have mid point of the sin curve as y = 5V, then the highest would go up to around 10V ish and lowest point be would be around 1.4V.

Now just consider everything being moved down 5, your mid point would be lowered to 0, your highest value would be around 4-5ish (depending what value you took before) and your lowest point(1.4V) would be lowered to around -4ish. Now it fits the scales and axes given to you.

[/0]

lol true, are we allowed to do that though?

[TrueTears]

lol true, are we allowed to do that though?

Yeah, just transforming the whole graph down by 5 units, the amplitude and frequency stays the same.

It's the same for characteristics graphs, where the midpoint doesn't always have to start at (0,0).

[/0]

thxxx

[dejan91]

But in this case, it doesn't apply because they've already given you a scale. Which is stupid because their answer doesn't fit with the graph or scale provided....

[/0]

For this LED, what is the threshold voltage?

[TrueTears]

Correct me if I'm wrong, but I don't think you can determine it. They'd have to give you a certain current and you'd have to read the voltage off the graph.

[/0]

If you wanna check this question up it's from question 7 in the electronics section of Physics TSFX 2009

http://vcenotes.com/forum/index.php/topic,8214.30.html

[dejan91]

I didn't get that one either. Realistically, it could be anythnig from 1.5-2.but in the answers, it says anything from 1.8-1.9 is acceptable. Yeah right...

[/0]

Is it possible to express a magnetic field as a function like for a paritcular magnet strength... or perhaps if it's not a function then can the strength of the field be found at any particular point mathematically in general?Nevermind:


Also, say... if you were a north pole and you were momentarily at on the south pole a distance away from the north pole of another magnet. Is there an 'escape velocity' at which you can escape the attraction of the other magnet, or would it require infinite energy to do that?

[/0]

How does AC current actually work? If the current goes back and forth then how does it get anywhere?

[/0]

Calculate the maximum magnetic flux passing through a coil of 50 turns, 12cm^2 in area in a magnetic field of strength 0.025T.

I just got . Shouldn't flux be independent of the number of coils? Doesn't it just depend on area?

[TrueTears]

No, you must consider how many turns there are.

Consider a solenoid, if you just have 1 "turn" of coil, let the area be A

But if you have more "turns" the coil gets bigger (or rather longer) right? So you are exposed to more area. Say you got number of turns, your area would now be