Electricity AS

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Cruckshank
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#1
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http://papers.xtremepapers.com/AQA/P...W-QP-Jan02.pdf

Question 1a on this paper is a bit weird. I haven't come across the idea that if you have a resistor and wire (assume no resistance?) in parallel, the current will totally avoid the resistor, and just flow through the wire.

I have two problems with this:

1) Why, in alternate circuits, does the current simply not 'avoid' the higher resistance parallel components. I.e. opt to flow through a 10ohm resistor rather than a 20ohm resistor (when in parallel).

2) Has this concept been removed from the specification? I have completed all Physics A AQA unit 1 past papers on electricity, and have never seen anything of the sort - I'm simply worried that this type of question (linked above) will appear in June, and take me by surprise.
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uberteknik
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(Original post by Cruckshank)
Question 1a on this paper is a bit weird. I haven't come across the idea that if you have a resistor and wire (assume no resistance?) in parallel, the current will totally avoid the resistor, and just flow through the wire.

I have two problems with this:

1) Why, in alternate circuits, does the current simply not 'avoid' the higher resistance parallel components. I.e. opt to flow through a 10ohm resistor rather than a 20ohm resistor (when in parallel).
This is a common 'assumption' so not sure why you have not had this explained.

Short circuits are 'idealised' zero resistances which makes qualitative analysis and close approximation calculation a lot easier. In reality, even short-circuits have a small finite resistance.

I = V/R

As resistance tends to zero, current approaches infinity and clearly this cannot happen. Current is limited by the conductors finite resistance.

In a parallel circuit, the pd across all parallel paths is the same. The current through each path is still determined by the I = V/R relationship through individual paths and the total current entering and leaving the common parallel junctions is the sum of currents in the individual paths.

With a short circuit in the real world, the short circuit resistance means that the current through the s/c path is far greater than the current through the adjacent high resistance path/s. The parallel currents still sum nevertheless.

It's just easier for a qualitative analysis to 'assume' all the current flows down the s/c path. In reality this cannot happen.


(Original post by Cruckshank)
2) Has this concept been removed from the specification? I have completed all Physics A AQA unit 1 past papers on electricity, and have never seen anything of the sort - I'm simply worried that this type of question (linked above) will appear in June, and take me by surprise.
Questions are graded in level of difficulty. You may find there will be one or two questions which forces you to analyse from first principles. These are designed to assess your ability to use your knowledge and understanding to think 'outside the box'.

This question is not one of them.
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