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# A level physics resistance vs current graphs ? REPS watch

1. I was doing exam q's and it asked you to draw resistance vs current
one of the marking points was that it was a non zero graph... how can there be resistance with no current though?!?!?!?

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3. Can you link it
4. (Original post by Lola1244)
I was doing exam q's and it asked you to draw resistance vs current
one of the marking points was that it was a non zero graph... how can there be resistance with no current though?!?!?!?
When there is no current, the resistance is inifinte in a way because there is no current.

e.g.

V = IR
Lets just assume that the votlage is 16V
V / I = R
16 / 0 = ??

You would get an error when you try to unput that into the calculator as you cannot divide by zero, so we assume thta its infinte, or any other value which is not 0.

Kind of complex but I hope it makes sense
5. (Original post by Lola1244)
I was doing exam q's and it asked you to draw resistance vs current
one of the marking points was that it was a non zero graph... how can there be resistance with no current though?!?!?!?
(Original post by derpz)
When there is no current, the resistance is inifinte in a way because there is no current.

e.g.

V = IR
Lets just assume that the votlage is 16V
V / I = R
16 / 0 = ??

You would get an error when you try to unput that into the calculator as you cannot divide by zero, so we assume thta its infinte, or any other value which is not 0.

Kind of complex but I hope it makes sense
The graph would not cross the y-axis. It would tend to infinity as current tends to zero, the y axis would be the asymptote.
6. (Original post by Kyx)
The graph would not cross the y-axis. It would tend to infinity as current tends to zero, the y axis would be the asymptote.
Thanks for your replies but on a graph I saw the resistance started at a specific value and then went up in a straight diagonal line... I am assuming that is kinda the intrinsic resistance of the material??
Many thanks
7. This is thie link: http://www.cyberphysics.co.uk/Q&A/KS...harlampQ1.html
Is the y intercept just the resistance you would get if you immediately switched the circuit on
8. (Original post by Lola1244)
Thanks for your replies but on a graph I saw the resistance started at a specific value and then went up in a straight diagonal line... I am assuming that is kinda the intrinsic resistance of the material??
Many thanks
I have no idea?

maybe internal resistance of the cell?

Or resistivity?

EDIT: It must be resistivity
9. (Original post by Lola1244)
This is thie link: http://www.cyberphysics.co.uk/Q&A/KS...harlampQ1.html
Is the y intercept just the resistance you would get if you immediately switched the circuit on
You need to realise what the question is asking you - this question is not about v=IR, because as you can see from the graph it is non ohmic. If it was ohmic, you would get a straight horizontal line for the resistance - ohm's law states that v is proportional to I, and R is the constant of proportionality (the gradient of a graph of V against I) which for an ohmic conductor has a constant gradient i.e. the resistance is constant (since the resistance is the gradient).

Of course the wire still has a resistance when there is no current - you have probably done resistivity so should know that the resistance of a wire is equal to ro l/a , which is independent of current
10. (Original post by samb1234)
You need to realise what the question is asking you - this question is not about v=IR, because as you can see from the graph it is non ohmic. If it was ohmic, you would get a straight horizontal line for the resistance - ohm's law states that v is proportional to I, and R is the constant of proportionality (the gradient of a graph of V against I) which for an ohmic conductor has a constant gradient i.e. the resistance is constant (since the resistance is the gradient).

Of course the wire still has a resistance when there is no current - you have probably done resistivity so should know that the resistance of a wire is equal to ro l/a , which is independent of current
thanks! So if it was an ohmic conductor the gradient would be straight line and it would not start from origin as well
11. (Original post by Lola1244)
thanks! So if it was an ohmic conductor the gradient would be straight line and it would not start from origin as well
If it was ohmic it would be a straight horizontal line at some non zero value - r is the constant of proportionality so does not change with voltage or current (assuming we ignore heat, which by saying it is entirely ohmic we are)
12. (Original post by samb1234)
If it was ohmic it would be a straight horizontal line at some non zero value - r is the constant of proportionality so does not change with voltage or current (assuming we ignore heat, which by saying it is entirely ohmic we are)
Sorry to bug you lol! But say you have the resistance against current graph... Why is the y intercept the value for rho? Surely the non zero value on the y intercept multiplied by a and divided by length is rho? Because the y intercept is the resistance not resistivity??
Thanks for your help
13. (Original post by Lola1244)
Sorry to bug you lol! But say you have the resistance against current graph... Why is the y intercept the value for rho? Surely the non zero value on the y intercept multiplied by a and divided by length is rho? Because the y intercept is the resistance not resistivity??
Thanks for your help
I never said that the y axis intercept was rho - i used the idea of resistivity to try and explain why the wire has a non zero resistance and as you say the y axis intercept is the resistance
14. (Original post by samb1234)
I never said that the y axis intercept was rho - i used the idea of resistivity to try and explain why the wire has a non zero resistance and as you say the y axis intercept is the resistance
Oh yes!

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