OCR Physics A G482, Electrons, Waves and Photons, 25th May 2012

any unofficial mark scheme?

Straight line through the origin idea is irrelevant. Whenever there is a straight line (or near to as in the question) between current and voltage the relationship after that point will be linear so constant resistance. In my opinion, all they were looking for was identification of the straight line, as they said 'features' of the graph, not a string of calculations.

well im sort of relieved but my mind wont be at rest until I know why lol. At first I thought straight line and tested the values but saw R was decreasing so put that.

Can you explain why resistance is not 1/gradient?

I thought I was smart but you sir, are a genius. Would I have to say that in the paper? I just said as voltage increases, current decreases, did the calculations at 1.8V, 1.9V and 2.0V showing R decreasing and said its non ohmic.

I think I did the one where you have to draw the wave on the graph right, but talk me through it

yeah i said because of the straight line but i didn't say anything about it obeying ohms law because a diode doesn't fml i hope we get at least one mark for stating it's a straight line.

For the LED question for V > 1.8v the line actually becomes straight. There is a past exam question which was similar, in which the mark scheme said it becomes straight for V > a certain value. The whole point of the question was to spot the fact that it had become straight for V > 1.8v, as for V < 1.8v, it was obviously curved. So 1.8v was like a limit for which the resistance no longer decreases.

I nearly messed the question up my self.
I had to change it in the last 10 minutes and scribbled down the calculations for 2V to justify it. It was only 2 marks + a QWC so that is probably enough I'd imagine?

For the graph each half second segment was 0,+.25,-0.12,+0.08,0 with those value being what your wave should go through at the midpoint of each segment.
Those values might be slightly off.



And everyone.
A straight line does not means a constant resistance unless said lines passes through the origin

However, from basic graph drawing, a straight line does represent a linear relationship, the constant being the resistance.

However, from basic graph drawing, a straight line does represent a linear relationship, the constant being the resistance. What this fuss about the origin is I do not know.

any unofficial mark scheme?

If it passes through origin its directly proportional.

Again I'll use a simple example.
a straight line on a V,I graph
10,0 to 20,10
At 10V and below infinite resistance, that fits with it being a 0 gradient line
at just over 10V very high resistance
at 20V 2ohms
at 40V 4/3
it keeps decreasing toward the gradient of one but will never actually equal one

I nearly messed the question up my self.
I had to change it in the last 10 minutes and scribbled down the calculations for 2V to justify it. It was only 2 marks + a QWC so that is probably enough I'd imagine?

For the graph each half second segment was 0,+.25,-0.12,+0.08,0 with those value being what your wave should go through at the midpoint of each segment.
Those values might be slightly off.



And everyone.
A straight line does not means a constant resistance unless said lines passes through the origin

In your example, however, one of the points is at 0 current. So this doesn't compare to our question, at 1.8V the LED had a definite resistance.

again i'll use a simple example.
A straight line on a v,i graph
10,0 to 20,10
at 10v and below infinite resistance, that fits with it being a 0 gradient line
at just over 10v very high resistance
at 20v 2ohms
at 40v 4/3
it keeps decreasing toward the gradient of one but will never actually equal one