# suddenly the minus pops up formula

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#### transgalactic

##### Banned
i know the formula for flux and inductance
$\phi=LI$

BUT
$\epsilon=-\dot{\phi}=-L\dot{I}$
but when i learned circuit theory
for inductor
$\epsilon=L\dot{I}$
and for capacitor
for inductor
$I=C\dot{V}$

there where no minus in the inductance formula

why in physics they add minus
??

#### skyhawk

##### New Member
why in physics they add minus
??

It's a sign convention to indicate that the induced emf opposes the current change.

#### transgalactic

##### Banned
so why in circuit theory we didnt add the minus sign?

Last edited:

#### skyhawk

##### New Member
Because it was connected with a certain convention for tracing around a loop. As you go around a loop you have rules for calculating the voltage across an element based on the direction you pass through relative to the assumed current flow. There can be various conventions for doing this. You must use a consistent set of rules.

ok i understand
thanks

#### birdman0_o

##### Active Member
It's called Lentz' law

#### transgalactic

##### Banned
ok thanks

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