# Charging potential vs time graph.

1. Apr 30, 2009

### dE_logics

Why don't we simply take t as x? why is it t?

Where is the time constant.

If you're looking at a question mark instead of the symbol "tau"...you know its tau.

Last edited: May 1, 2009
2. May 1, 2009

### Andrew Mason

You might get an answer if you explained what the question is.

AM

3. May 1, 2009

### dE_logics

On one axis we have the charging current/voltage, while on the other instead of time (t) we have t ...why?

And yeah...latex is still giving problems?...I think I got a cache problem.

4. May 1, 2009

### Staff: Mentor

This picture shows what I'm seeing:

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5. May 1, 2009

### dE_logics

Actually I'm on linux, and so this website seems MS friendly.

So that 'tau' seems like a matrix in windows.

So if you encounter any sorta weird symbols, take it as 'tau' or time constant.

6. May 2, 2009

### Staff: Mentor

So are you trying to write $t / \tau$?

7. May 2, 2009

### dE_logics

ITs t*tau not t/tau.

8. May 2, 2009

### dE_logics

Latex just started working for me!

$$t \tau$$...this is what I mean.

Last edited: May 2, 2009
9. May 3, 2009

### Staff: Mentor

I've seen many capacitor charging/discharging graphs that use $t / \tau$ on one of the axes, but never one that uses $t \tau$. Are you sure the $\tau$ isn't a subscript, i.e. $t_{\tau}$?

10. May 3, 2009

### dE_logics

11. May 3, 2009

### Staff: Mentor

Are you referring to graphs like this one?

The horizontal scale markings indicate $t = \tau$, $t = 2 \tau$, $t = 3 \tau$, etc. This means the same thing as $t / \tau = 1$, $t / \tau = 2$, $t / \tau = 3$, etc.

The 1, 2, 3, etc. are not t's. t and $\tau$ both have units of time, so the numbers are dimensionless.

12. May 3, 2009

### dE_logics

Yeah...the same thing.

So why did this $$\tau$$ stuff pop by?...why not simply use time?

13. May 3, 2009

### Born2bwire

Normalization, the time constant will vary between different values of R and C but as long as you plot the time axis in terms of the time constant then the plots will all be the same (barring differences in the magnitude of the initial voltage).

14. May 3, 2009

### dE_logics

Oh...you mean to maintain the nature of the graph...right?

15. May 4, 2009

### Born2bwire

Yeah. Plotting in units of time constants allows the graph to be scale invariant with respect to the R and C of the circuit. It's the same reason why when we plot graphs of waves and such we use wavelengths as our units of space. It automatically scales the plots in such a way that the information of interest is readily seen.

16. May 4, 2009

### dE_logics

Humm...ok, thanks!