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Let L be an inductance, then you find the following formula in textbooks (high school level):

U_ind = - L dI/dt

When you actually do calculations for circuits, you see that the minus sign is wrong.

So why do they put it there?

- Thread starter arcnets2
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- #1

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Let L be an inductance, then you find the following formula in textbooks (high school level):

U_ind = - L dI/dt

When you actually do calculations for circuits, you see that the minus sign is wrong.

So why do they put it there?

- #2

marcusl

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- #3

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I think it is definitely wrong. Yes marcusl, the field inside the coil opposes that in the source. But you could as well say that of a resistor. But I have never seen a minus sign in R = U/I.The minus sign is certainly not wrong, ...

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marcusl

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Edit: removed this post, not relevant to the solution. (Sorry!)

Last edited:

- #5

berkeman

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In all of these, the top of the inductor is connected to the + power supply rail, and the bottom of the inductor is connected to ground through a transistor operating as a saturated switch. When the transistor is turned on, a current builds up in the inductor (or transformer primary winding). The current may end up being limited by the coil resistance, as in a relay, or it may keep on building until the switch is turned off.

When you turn off that low-side switch, you get a positive spike in voltage at the bottom of the inductor. That's how I remember the polarity of the L di/dt kickback.

- #6

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sorry, no offence meant! Just trying to understand.

Well imagine you have L and R in series. You switch on the source. Then the current is

I(t) = U/R (1 - exp (-R/L t)).

OK?

I can only derive this using U_ind = + L dI/dt, with a plus sign instead of minus. But the minus sign is there in the textbook, so my question is why.

- #7

berkeman

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[tex]V = L \frac{di}{dt}[/tex]

- #8

marcusl

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For an inductive circuit let's consider an example of a battery, switch, R and L in series. The "counter-emf" in the inductor seems to be in the same direction as in the R, as you point out, BUT it's considered by convention to be a

1

There's a very clear discussion in Reitz and Milford, Foundations of EM Theory, p. 246 (1960), if you can find a copy in your U library.

- #9

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There's the negative sign in many standard textbooks and even websites such as wikipedia, so I guess it's not a typo. Rather they treat the coil as a source.

Still seems strange to me ...

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