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Electricity questions

So I have a few queries about some content....
1) Should you always presume batteries have a negligible internal resistance if not mentioned?
2) do we need to know about contact resistance/ what exactly is it/ any formulas?
3) in an emf graph with voltage on y and current in x, emf is the y intercept and -ir is the gradient, to get the ir, do you take the negative reciprocal of -ir?

Cheers in advance!


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Original post by Audi
So I have a few queries about some content....
1) Should you always presume batteries have a negligible internal resistance if not mentioned?


The question will tell you whether it can be ignored or not. If it does nit say anything and no circuit symbol representing internal resistance is shown, then you can safely assume the value can be ignored.

Original post by Audi
2) do we need to know about contact resistance/ what exactly is it/ any formulas?


Contact resistance is the inherrent resistance across the junction between two touching conductors. This refers to switch contacts, relay contacts, lamp holders - anything where two conductors (not soldered) are joined.

Unless the question states contact resistance and gives values, then these too can be ignored.

Original post by Audi

3) in an emf graph with voltage on y and current in x, emf is the y intercept and -ir is the gradient, to get the ir, do you take the negative reciprocal of -ir?


The V vs I plotted line is referred to as a load-line.

gradient = (rise in Y) / (distance X)

i.e. R = V/I at any given point on the load-line.





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