I'm trying to understand back emf & self inductance. This is what I know: let's say that you have a current around a loop. I picture this as a vector going around the loop. Then we decrease the current by a small amount, which equates to imagining a smaller vector pointing in the opposite direction. Lenz's Law tells me that emf is in such a direction as to oppose any change in current. Therefore, emf must be in the original current's direction.


  1. First of all, is my understanding correct?

  2. Should I think of emf as a vector, i.e. as something with a magnitude and a direction? I ask because my book always talks about its direction.

  3. What are the physical objects that actually create emf? Whenever I imagine a current, I imagine electrons moving through a wire. If this is correct, then what moves through the wire when there is back emf? Electrons?


2 Answers 2


The best way to think of it is: An inductor opposes any change in the current by generating back emf as the current increases, and forward emf if it is decreasing. Due to Lenz's Law, when the current is increasing, the increasing magnetic field produces a voltage (emf) the same as the applied (back emf) to oppose and slow the increase. When the current decreases, a forward emf (opposite to the applied) is generated to oppose and slow the decrease. This is used in switch mode power supplies. When the transistor is on, current builds up in an inductor, storing energy. When it turns off, the polarity across the inductor reverses, trying to keep the current flowing and the current flows through a diode to the output and the energy is fed out. In most SM power supplies, the inductor is a transformer, and the energy is fed out from a secondary low voltage winding. An inductor across AC, tries to average the current out to zero, reducing it. This is used to control the current through things like fluorescent lights.

DC electric motors control their current by acting as a generator, producing back emf (the same polarity as the applied) that opposes the current, but slightly less. As the motor is loaded, it slows down, increasing the difference between the applied emf pushing the current through, and the back emf pushing it back, so the current increases and supplies the extra torque deeded to drive the load. AC induction motors act like an inductor, but reduce their inductance when slowed down under load, increasing the current.


It's a reaction that the circuit gives for changing voltage or magnetic field. Its logic is like this:

  1. First you have a circuit without voltage let's say.
  2. When you start to increase the voltage, circuit wants to keep its original state which was when there was no voltage.
  3. Because of that circuit then creates voltage which would decrease the increase in voltage.

It's basically the circuit's way of protecting itself against voltage changes.

In case of a Magnetic field, the logic is same. When you increase the magnetic field around the bobin it creates a voltage opposite direction of the increasing magnetic field so it protects itself.

When high voltage or magnetic field decreases circuit supports the voltage so it can keep the same voltage around itself. Again the logic is to keep the state stable.

  • (-) in the Lenz Equation means that what you do to the circuit it will always act against it. You shouldn't think emf as vector it's scalar as far as i know but its reasons are vectoral just something to keep in mind.

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