How can there be a current flow when back emf equal and opposite of suply voltage? Isn't there a cancellation of voltage to zero volts? I have attached a video regarding this. Please answer this intuitively, I have already wasted enough time on this but couldn't find satisfactory answer.
Don't think of the back emf as being caused by the source voltage. Think of the back emf as being caused by the changing current.
The source produces a current. That (changing) current causes the inductor to produce a back emf. The back emf limits (but doesn't eliminate) the current produced by the source. If it eliminated the current, then there would be no back emf. The two things don't oppose each other so much as they balance each other.
By KVL, the back-emf of the inductor must be be equal and opposite to the voltage produced by the source, or else energy conservation would be broken.
Isn't there a cancellation of voltage to zero volts?
There is, in the sense that if you follow the path of the circuit and add up all the voltages you pass (the source and the inductor, in a simple case), then the sum of all those voltages will be zero once you've followed the complete circuit.
But of course that's just KVL, and it applies to any circuit elements (resistors, capacitors, or whatever) and not just to inductors.
I think what's confusing (at least to me) is the use of the word "opposite". In a normal drawing of this circuit, we draw the source with its positive terminal at the top of the page, and then the inductor will also produce a back emf that's positive at the top of the page. But if you consider the voltage in the direction of traversing the circuit (for example, always going in a clockwise direction around the circuit), then the two potential differences (source emf and inductor back-emf) will be opposite.
edit RE the video
I think the video is confusing because he never defines the sign convention he is using to define his voltages across the different elements.
In order for the description to be correct, he must have defined opposite conventions for the two types of elements, like this:
Keep in mind Kirchoff's voltage law. With these conventions, \$V_r\$ must be equal to the V1 source voltage (regardless of what type of resistor is used or even if you replaced the resistor with a different element like a capacitor or inductor). And \$V_l\$ must be the opposite of the V2 source voltage (again, it would still be opposite even if you replaced the inductor with a different element like a resistor or capacitor.
Again, this is simply because of KVL and the choice of sign convention for the voltage across the two elements. But in the video he never shows the chosen sign convention, so this makes it impossible to understand what he's trying to teach.
Either that or the guy in the video has no understanding of circuit theory and believes that a circuit with an inductor in it can violate KVL, which is simply not correct.
Think of current in an inductor as having inertia.
You need to apply a large force (voltage) to accelerate an inertial mass like a car (or increase current flow), and the only way to stop it once it's moving and has gained momentum, is to apply a large opposing force (voltage) like the brakes.
Attempting to stop it suddenly with a brick wall (or opening a switch) results in large and possibly destructive forces (or a high voltage spike).
- Linear solenoid DC fed,switched, Back-EMF(volts)=constant x di(amperes)/dt,negaive sign, so if dt tends to 0,Back-EMF tends to infinity.
- I saw this curve volts(EMF) vs dt in cents of a sec,seems to be of the form 1-e exp -kt,charge of a Capacitor form.I got a numerical real example 500 volts easy.
- Have we a switch time off controlable.Is so important as resonance.
I just upped the answer that said, "think of the back emf as being caused by the changing current." That's the correct view, I think.
Note: If you truly want to understand the details, you really need to read a book on physics. There are two really good sources I think you might consider, but which one is better will depend on your interests and background. One is Feynman's Lectures on Physics, available here (or just buy the set of three lecture books.) The second is a wonderful and more modern book on physics by Chabay and Sherwood, called "Matter & Interactions," 3rd edition (or later.) It's superb and provides a useful set of mental models to think with (more directly and better than any other introductory physics book I know about.)
As the current increases for example, then the non-Coulomb electric field curls around the region of changing flux in the inductor, opposing the increase in the current and polarizing the inductor. The new surface charges produce a Coulomb electric field that follows the wire and points opposite to the non-Coulomb field. (Note that the Coulomb voltage due to surface charges approximates the non-Coulomb voltage due to time-varying magnetic fields when the wire resistance is insignificant. In other cases where the wire resistance is significant, then those two values will no longer be so nearly close to each other.)