# Log-Log graphs

Discussion in 'Electronics Homework Help' started by Integrator741, Oct 11, 2013.

1. ### Integrator741

125
4
Jun 16, 2013
Hello, I need help to understand how to sketch log-log graphs.

The question is " Show from the following results of voltage V and admittance Y of an electric circuit that the law connecting the quantities is of the form V = kY^2"

I need to plot the graph on logarithmic scale to find k and n. (I know how to find k and n.) what I don't know is how to start...

For instance i got numbers for Voltage V: 2.88; 2.05; 1.60 etc. and then Admittance Y: 0.52; 0.73 and etc..

I tried to take logs out of them but that didn't give me much..

I tried to look for information online but i didn't find much, maybe somebody can help me to get a grip of it??

Thank you 2. ### Laplace

1,252
185
Apr 4, 2010
Taking the logs would give you a log-log data series that you could plot on graph paper. No need for log-scribed graph paper here.

A log-log plot of the given equation would be a straight line. From the straight line you can get a slope and intercept. The slope and intercept provide the K and n values for the equation.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
OK, are you planning to use graph paper?

If so, do you have linear, log/linear, or log/log graph paper?

Are both axis going to be logarithmic?

(Says Steve rather too long after Laplace has already covered it)

4. ### Integrator741

125
4
Jun 16, 2013
That's the paper I am using:
http://polymer.bu.edu/ogaf/image/fig22.gif

I don't know how to answer your second question. What do you mean by that?

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
OK, that's log-log paper (each axis is logarithmic)

You just plot your actual values onto that paper. For example, I will plot the values (given as (x,y) below)

(1,10), (2, 15), (5, 35), (10,110), (20,420), (30, 930) Note that the point (1,10) was drawn at 1 on the X axis, and 10 on the Y.

Note that the point (2,15) was drawn at 2 on the X axis, and between 10 and 20 on the Y. Note that it is above 1/2 way. If you look along the X axis at where 5 is with respect to 1 and 10 you'll see the numbers are bunched up toward the higher numbers, so 15 is a bit higher than half way.

Hopefully your points will make a prettier picture than mine.

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6. ### Integrator741

125
4
Jun 16, 2013
Okay, but for example how can I plot (2.88;0.52)? 7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
In order to do that, you need to add another order of magnitude to your Y axis.

Divide everything by 10.

So, from the bottom, the numbers go 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2, 4, 6, 8, 10, ...  