# Thread: Current Voltage Filament Lamp

1. Consider a graph of V against I is given for a filament lamp. If one is asked to calculate the resistance of the filament lamp at a particular current value, can one simply work this by V/I, just one point or one have to find the gradient?

I am of the opinion that in this case to V/I ( just one point ) is the correct Maths to use.

Would like to know what others think.

Thanks.

2. Originally Posted by Maximus
Consider a graph of V against I is given for a filament lamp. If one is asked to calculate the resistance of the filament lamp at a particular current value, can one simply work this by V/I, just one point or one have to find the gradient?

I am of the opinion that in this case to V/I ( just one point ) is the correct Maths to use.

Would like to know what others think.

Thanks.
The resistance is not constant, it increases with temperature. So, you cannot "work with V/I", you need to plot the whole curve.

3. I know it is changing and I am not asking to plot if you have read carefully my question.

4. Originally Posted by Maximus
I know it is changing and I am not asking to plot if you have read carefully my question.
I read your question very carefully, you are wrong, you cannot use a single point. Is that clear?

5. Ok then... I am asking to find R from a given sketch.

but what makes me think if using gradient is appropriate here is due to the fact that V = IR,.....R is not the rate of change of something.. it is simply V/I.

Not like say Velocity .... the rate of change of displacement so the need of a gradient.

What do you think?

6. Originally Posted by Maximus
R is not the rate of change of something.. it is simply V/I.
False.

Not like say Velocity .... the rate of change of displacement so the need of a gradient.

What do you think?
ONLY IF the "sketch" is a STRAIGHT line you can use V/I. In real life, the "sketch" is never a straight line, so you NEED to use

Re: straight line to use V/I assuming it is passing through the origin.

8. Originally Posted by Maximus

Re: straight line to use V/I assuming it is passing through the origin.
yep

9. Originally Posted by Maximus
Consider a graph of V against I is given for a filament lamp. If one is asked to calculate the resistance of the filament lamp at a particular current value, can one simply work this by V/I, just one point or one have to find the gradient?

I am of the opinion that in this case to V/I ( just one point ) is the correct Maths to use.

Would like to know what others think.

Thanks.
Your question is ambiguous because you haven't specified for what purpose one is to use the answer. Without that additional information, there is no uniquely correct answer.

If, for example, the goal is to replace the lamp with an ordinary resistor and maintain the same power consumption, then one computes the value of that ordinary resistance with the simple ratio V/I at the current you specified.

If, on the other hand, one wishes to know how that dissipation changes for small changes in applied voltage, then one needs to know dV/dI, again evaluated at the current you specified.

10. Now this post opens all back to discussion...

I came across a question asking simply to find R at a particular voltage that is why I thought the answer is V/I.

11. Originally Posted by Maximus
Now this post opens all back to discussion...

I came across a question asking simply to find R at a particular voltage that is why I thought the answer is V/I.
Yes, and again, that question, as phrased without additional context, is too ambiguous to admit a unique answer. The writer of that question apparently did not understand that there are multiple ways in which the problem could be interpreted. It may have come from one of any number of poorly-written science textbooks (the same type that often conflate "speed of electricity" with "speed of electrons").

12. That was asked in an exam paper....

Well, in my opinion as R by definition is not the rate of change of something...I would think that the answer to a question like this should be V/I as it is asking for the resistance at a particular value of current.

If it was say, as I explained above, a displacement time graph, you would always need to find the gradient to calculate the velocity at a particular time t.

I think it all depends on the definition of the quantity being measured.

13. Originally Posted by Maximus
That was asked in an exam paper....

Well, in my opinion as R by definition is not the rate of change of something...I would think that the answer to a question like this should be V/I as it is asking for the resistance at a particular value of current.
Your opinion is wrong, as explained to you with reasoning by me and AndrewC.

A standard linearisation technique in science is to compute the first derivative of a nonlinear quantity. That quantity relates changes in one variable to that of another. As you rudely ignored, if the question to be answered is "How does the current change if the voltage changes," you use the derivative, not V/I. These things are not matters of opinion.

14. ' if the question to be answered is "How does the current change if the voltage changes," you use the derivative, not V/I.'

Completely agree!

The question is asking to find R at a particular value of I as you explained below:

'If, for example, the goal is to replace the lamp with an ordinary resistor and maintain the same power consumption, then one computes the value of that ordinary resistance with the simple ratio V/I at the current you specified.'

So why not V/I in this case?

15. Originally Posted by Maximus
' if the question to be answered is "How does the current change if the voltage changes," you use the derivative, not V/I.'

Completely agree!

The question is asking to find R at a particular value of I as you explained below:

'If, for example, the goal is to replace the lamp with an ordinary resistor and maintain the same power consumption, then one computes the value of that ordinary resistance with the simple ratio V/I at the current you specified.'

So why not V/I in this case?
I've explained it twice now, and you seem to be determined to ignore it.

Both V/I and dV/dI are resistances. There is therefore not a unique R. You seem to think there is only one definition of resistance, despite being shown multiple times that there are at least two. And despite additional explanations of what one uses these for, you seem unwilling to think about what has been demonstrated to you, and instead prefer an uninformed opinion (yours) over that which has been logically and scientifically demonstrated.

That's as clear as I can make it, so I will now exit. Good luck with your studies.

16. Ok thanks...