Nuvertion
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#1
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#1
I always drop marks when it comes to written answers (not covering the points they want even though they didn't say they have to be covered) but I'm completely baffled by the mark scheme of this question.

http://i.imgur.com/nrZo1dx.png
http://i.imgur.com/m2cTnfL.png

Of course I went on talking about the gradient (gradient increases so r decreases) yet because of that I've instantly lost 2/4 marks. Is this another one of OCR's quirks or have I failed to understand the topic? Surely saying I increases faster than V is the same as saying the gradient increases?

Also about using P=I^2R (which I wouldn't have guessed). Are we meant to assume all of the power used by the component is heat (therefore a P is proportion to I^2 so heat increases even though resistance decreases)? Does resistance/current cause higher temperature or is it the other way round?
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phoenixsilver
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#2
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(Original post by Nuvertion)
I always drop marks when it comes to written answers (not covering the points they want even though they didn't say they have to be covered) but I'm completely baffled by the mark scheme of this question.

http://i.imgur.com/nrZo1dx.png
http://i.imgur.com/m2cTnfL.png

Of course I went on talking about the gradient (gradient increases so r decreases) yet because of that I've instantly lost 2/4 marks. Is this another one of OCR's quirks or have I failed to understand the topic? Surely saying I increases faster than V is the same as saying the gradient increases?

Also about using P=I^2R (which I wouldn't have guessed). Are we meant to assume all of the power used by the component is heat (therefore a P is proportion to I^2 so heat increases even though resistance decreases)? Does resistance/current cause higher temperature or is it the other way round?
Concerning the gradient, I have researched this and basically, you cannot talk about resistance in terms of the gradient. This is because the resistance is a "by-point" value and is not represented by the gradient. You will get marked down for talking about gradient. You can google "i-v resistance gradient" to find out more about this.

The increase in resistance is caused by the increase in temperature hence current will decrease (sorry silly mistake, edited, it will decrease!)
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Nuvertion
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(Original post by phoenixsilver)
Concerning the gradient, I have researched this and basically, you cannot talk about resistance in terms of the gradient. This is because the resistance is a "by-point" value and is not represented by the gradient. You will get marked down for talking about gradient.
But differentiating the curve will find the resistance at any point and the differential is also known as the gradient function. Also I was talking about the change in resistance, if the gradient changes the resistance has to change. All seems rather silly, there is a clear link between resistance and gradient.
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phoenixsilver
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(Original post by Nuvertion)
But differentiating the curve will find the resistance at any point and the differential is also known as the gradient function... Sounds silly to me.
ik it doesn't make much sense, luckily you don't have to explain why.

also note that temperature increase causes increase in resistance causing a DECREASE in current (contrary to my original answer which I edited).
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Nuvertion
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(Original post by phoenixsilver)
ik it doesn't make much sense, luckily you don't have to explain why.

also note that temperature increase causes increase in resistance causing a DECREASE in current (contrary to my original answer which I edited).
So for semiconductors:

A higher current causes a higher temperature which causes a lower resistance (as more conduction electrons are freed).

But for conductors:

A higher current causes a higher temperature which causes a higher resistance (as more collisions) which causes a lower current (infinite loop?).
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phoenixsilver
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(Original post by Nuvertion)
So for semiconductors:

A higher current causes a higher temperature which causes a lower resistance (as more conduction electrons are freed).

But for conductors:

A higher current causes a higher temperature which causes a higher resistance (as more collisions) which causes a lower current (infinite loop?).
In the semiconductor, isn't the current affected by the temperature/resistance? or is it the current that effects them.

I mean, if you think about an NTC thermistor, as temp increases, resistance decreases so current increases.
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Nuvertion
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(Original post by phoenixsilver)
In the semiconductor, isn't the current affected by the temperature/resistance? or is it the current that effects them.

I mean, if you think about an NTC thermistor, as temp increases, resistance decreases so current increases.
The mark scheme stated that the current caused the heating effect (P=I^2R).
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phoenixsilver
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#8
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(Original post by Nuvertion)
The mark scheme stated that the current caused the heating effect (P=I^2R).

I guess it just depends on what effect you're talking about.
If you're talking about heating an NTC thermistor, then what I said previously is correct.
However what you also say is correct, it just depends on which variable you change.
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