cant make sense of current, voltage, resistance relationship, graph

[brownt]

I can't make sense of this graph




I understand the relationship between V, I and R in written terms as in proportionality and the inverse. But I can't see how the graph relates. Can someone explain it please.

 

[d123]

Hi,

You're not the only one. I feel like I'm walking into a trick question...

It looks like the relationship has been drawn upside down. I've yet to see a circuit where less resistance equals less current...

 

[brownt]

well thanks. It's not a trick. I can't make sense of it, and I guess it is wrong. But, what do i know? That is why I am asking. Is the graph upside down, or is it not possible to represent what its trying to represent with a single line?

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what about these graphs. I think the first one makes sense, but not the second. correct?

 

[rmanalo]

I understand the relationship between V, I and R in written terms as in proportionality and the inverse. But I can't see how the graph relates. Can someone explain it please.

Hi. This looks pretty normal to me. If you take the voltage constant, say a constant voltage source with a resistor attached to ground then the current going through the resistor decreases as the resistance increases. Maybe we're used to seeing the current in the y axis but if you look closely, it does follow the V=IR relationship. if current increases (going left) resistance decreases (going down). But to control the current we need to adjust resistance that is why it is better to put current in the y-axis. the same applies to the other graphs.

 

[KlausST]

Hi,

Post#1
I agree the graph doesn't make much sense.

There is Ohm's Law. R = V / I
And it says "constant voltage"

Let's calculate some values:
Let's assume V = 12V
Then: if
I = 1A --> R = 12 Ohms
I = 2A --> R = 6 Ohms
I = 4A --> R = 3 Ohms
I = 6A --> R = 2 Ohms
I = 12A --> R = 1 Ohms
You see: the higher the current, the lower the resistance.
This is what the graph shows.
Mind: the graph has no X values and no Y values. In most cases the axis are linear --> but there is no need for this.
Thus one may expect that the graph shows a hyperbola --> but there is no need for this. With non linear axis the graph may look like a straight line.

Post#3:
Both graphs show the correct relationship:
Constant R: the higher the current, the higher the voltage
Constant current: The higher R, the giher V.

Both are directly proportional. In both casess: with both linear axis you will get a straight line graph.

I recommend to do some example calculations and graphs with Excel to get a feel.

Klaus

 

[brownt]

I think i have it then. Are the comments correct on the graphs attached?

Basically the centre one is upside down, and all in all its a weird way to represent ohms law.

 

[KlausST]

Hi,

You are correct with figure 1.21 and figure 1.23.

But you are not correct with figure 1.22.
See my example of post #5.

Mathematical solution:
V = constant, thus V1 = V2
V1 = R1 x I1
V2 = R2 x I2
because V1 = V2 we can say:
R1 x I1 = R2 x I2

Now we increase R2. Lets say R2 = 2 x R1
Then we get
R1 x I1 = 2 x R1 x I2 ; we divide both sides by R1
--> I1 = 2 x I2
I2 = 1/2 x I1
we see that I2 is decreased.
This is named "indirect proportionality"
The higher R the lower I

Klaus

Added:
Chart with non linear "R" axis.