
#1
Nov812, 12:29 PM

P: 47

Hi, i have a graph with voltage (y axis) plotted against current (x axis) and I need to calculate the resistance of a resistor and a lamp from this.
My physics teacher has told me that by Ohm's law, R = V/I, and he has also told me that on the graph, the resistance is the gradient of the graph. The graph for the resistor is linear, so V/I is the same as the gradient, I have no problem with this. However, the graph for the lamp is non linear, so V/I isn't the same as the gradient. What's the correct answer here? Am I missing something blindingly obvious? Thanks for any help 



#2
Nov812, 12:47 PM

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P: 39,611





#3
Nov812, 01:07 PM

P: 47

It makes perfect sense to me :D but what doesn't is that if I calculate the gradient at that point, it isn't the same as V/I. surely this contradicts Ohm's law?




#4
Nov812, 02:30 PM

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P: 39,611

Calculating resistance from a graph 



#5
Nov812, 02:42 PM

P: 47

Well if the resistance of the lamp is ΔV/ΔI at any point on the graph, then given that the relationship between the voltage and current for the lamp is nonlinear, this will not be V/I which by Ohm's law is the formula for resistance.




#6
Nov812, 02:51 PM

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P: 39,611





#7
Nov812, 04:40 PM

Sci Advisor
PF Gold
P: 11,352

Ohm's Law only describes the behaviour of a metal at constant temperature. That's all. The Resistance of a component is defined as V/I but, of course, as V changes, this WILL change for a nonlinear component (by definition).
There is no violation of any law here because Ohm's Law is not really a physical 'Law', in any case  it is just an observation of how a certain material tends to behave under given conditions. For 'small signals', many nonlinear components behave as if they have a 'resistance' because ∂V/∂I happens to be more or less constant over a particular range of V. So, under certain circumstances, we can assign a value for R and work with it. EE does this all the time. We use 'current sources' and 'voltage sources' all the time and it works very well in practical terms and it's the same sort of thing. You must not let it spoil your day  just be glad that the system works so well. 



#8
Nov912, 11:31 AM

P: 47

Thank you guys, I'm pretty sure I understand that now :)



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