- #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?

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In summary, the minus sign is conventionally used in textbooks to indicate that the current in an inductor opposes the current in the source. However, this is not always true, as it can be difficult to draw the voltages and currents properly.f

- #1

- 7

- 0

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

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- #3

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The minus sign is certainly not wrong, ...

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.

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

Last edited:

- #5

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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

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

- #8

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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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