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exam question help

ocr physics a g482 june 2013 question 1c

why can't you say that 1/r is increasing so r must be decreasing for 1 mark? the mark scheme says don't allow it but i don't see why not



and also can someone please explain to me question 5bii and 3cii
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jamesbird18
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Original post by jamesbird18
ocr physics a g482 june 2013 question 1c

why can't you say that 1/r is increasing so r must be decreasing for 1 mark? the mark scheme says don't allow it but i don't see why not



and also can someone please explain to me question 5bii and 3cii



paper attached
G482 Jun13.pdf220.6KB
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jamesbird18
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Original post by jamesbird18
paper attached



mark scheme
G482_MS_June13.pdf170.0KB
Original post by jamesbird18
ocr physics a g482 june 2013 question 1c

why can't you say that 1/r is increasing so r must be decreasing for 1 mark? the mark scheme says don't allow it but i don't see why not



and also can someone please explain to me question 5bii and 3cii


The question is asking you to recognise that for a semiconductor, the rate of change of current w.r.t. voltage is increasing as the pd across the slice increases. .i.e. resistance is a function of the gradient at any point on the I-V curve and since the gradient increases with increasing pd, then resistance is falling.

Power is a function of I2R

As the resistance falls, the current increases, the power dissipated also increases and so the temperature of the semiconductor must also be increasing.
(edited 9 years ago)
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jamesbird18
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yeah but if i increases and r decreases how do you know which one changes the most?
Original post by jamesbird18
yeah but if i increases and r decreases how do you know which one changes the most?


The mark schemes are intended for examiners use and not model answers for students.

Current, voltage and resistance are all interrelated. Even so, I'm not sure why the question specifically excludes mention of the gradient. Perhaps they specifically want the the student to mention the inverse relationship because resistivity falls as a function of temperature in a semiconductor. That's the only reason I can think of.

Current will only flow if there is a motive force pushing or pulling on the electrons in the slice.

i.e. an charge imbalance (emf) must be present across the slice.

That will create a current (flow of electrons) which will then give up potential energy as they are captured and released by the atoms of the semiconductor. work must be done (power expended) as they do so. So temperature is increasing as the emf increases, the rate of change of current is increasing and so the resistivity is falling.

From the I-V curve, as V is increased, the rate of change of current is also increasing. (i.e. gradient = I/V = 1/R) therefore resistance is falling.

The mark scheme does not really make this explicitly clear.

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