# Thread: need help in transposition

1. ## need help in transposition

Hi,
i need help in transposing this fomula

u=e-(i x r)
where,
u=terminal voltage
e=E.M.F
i=current
r=internal resistance

i want to caculate r (internal resistance). i cant transpose this formula to the extend as it confuses me.
i have ended up doing in this way and it may be right or may be wrong but please explain me.

(u-e) divided by i = r

therefore

r = u-e divided by i

please explain me.
thankyou.

2. I'm assuming that you want to express r in terms of the other variables. Is this correct?

If you indeed want this (I'm changing the variables to those most commonly used to the equation I think it is.

$V = \varepsilon - Ri$

Now you sum $Ri - V$ in BOTH sides of the equations:

$
V + Ri - V = \varepsilon - Ri + Ri - V
$

EDIT: it was wrong before:

$Ri = \varepsilon - V$

and finally,

$R = \frac{\varepsilon - V}{i}$

So your calculations are, indeed, correct. Is this clear why it works?

3. ## Negative problem

The problem here is that the product has a minus in front. Easiest way to solve this is to X by -1 it get -u=-e + ir, which can then be rearranged to give e-u = ir, so e-u/i = r.

4. $Ri = \varepsilon - (-V)$

There is no need for both - here, the first disappears when you cancel the [tex]Ri

5. Originally Posted by Here to Help
$Ri = \varepsilon - (-V)$

There is no need for both - here, the first disappears when you cancel the [tex]Ri
Surely not, I just left this to increase detail level.

6. e (- Ri + Ri) - V simplifies to e - V not e + V, the correct transposition it e-V/i.

7. Originally Posted by Here to Help
e (- Ri + Ri) - V simplifies to e - V not e + V, the correct transposition it e-V/i.
True. Edited my previous post to make it right, thanks for pointing it out. I should have noticed this before, specially because the electromotive force $\varepsilon$ should be greater than tension $V$