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# What is back emf?

I am trying to understand what does back emf mean?

· Back emf:

The transformer core once voltage is applied one side will have +ve charges and -ve charges on the other side as shown on the first figure. This separation of charges by the applied voltage will cause the core to become polarized (positive charges will be separated from negative charges inside the core). So, as the second figure shows the core positive charges will be attracted to the applied voltage negative charges (opposite charges attract each other) and the core negative charges will be displaced towards the applied voltage positive charges. Thus the core will produce its own field E1 which opposes the applied electric field E0. E1 is what produces the back emf. So, the resultant field, E will be less than E0, But, in the same direction. The opposing electric field slows down the change of current.

Back emf changes instantly but the induced current in the core can’t change instantly since the current is di/dt (dt≠0).

the back emf is proportional to the amount of change of the current, the larger the current changes the larger the emf is.

emf’s can be generated in a wire in three different ways: by moving the wire (motional emf), by moving a magnet near the wire, or by changing a current in a nearby wire.

As it has a voltage opposite to the applied one, so, it goes back into the source to oppose the current thus the name back emf. Am I correct?

• Ah there is some confusion here. For inductors we don’t consider the electric field, which is for capacitors. For a inductor, say if you applied a voltage, current increases exponentially and after some time, to a almost steady current. Now, for this almost stead current flowing, there is a magnet field built up, which stores energy. Now the fun part: (1) If you try to switch off the voltage applied to the inductor, the almost steady current cannot continue flowing, so they would find another current path to flow, because the stored energy cannot disappear suddenly, / to continue, … – tlfong01 37 mins ago
• Comments are intended for seeking clarification on questions or answers. – Transistor 32 mins ago
• otherwise the Law of Conversation of Energy would be violated (and Mother Nature would not allow this to happen! :)). If you do not provide a path using say and “flyback” diode, which allows the flowing current to continue flowing “in the same original direction” , and dissipating the stored energy in the magnetic field as heat energy, thus the idoe get hot. If there is no flyback diode, you will see a “spark” which is the back EMF which is very high voltage, ionizing the air to become sort of conductor, allowing the current go through and dissipated the stored energy in the magnetic field. – tlfong01 31 mins ago

Ah, there is some confusion here. For inductors we don’t consider the electric field, which is for capacitors. We consider her twin/mirror sister, the magnetic field

For an inductor, say, if you apply a voltage across it, current increases exponentially and after some time, to a almost steady current.

For this almost steady current flowing, there is a magnet field built up, which stores energy, and let us call it magnetic energy.

Now the fun part: If at this time, you try to switch off the voltage applied to the inductor, the almost steady current can no longer flow, so it would try to find an alternative current path to flow. This is because the stored magnetic energy cannot disappear suddenly, otherwise the Law of Conversation of Energy would be violated (and Mother Nature would not allow this to happen! :)).

And if you don’t provide a alternative current flowing path using say, an “flyback” diode, which allows the flowing current to continue flowing “in the same original direction” , and dissipating the stored energy in the magnetic field as heat energy at the diode, which gets hot.

If there is no flyback diode, Mother Nature will create an alternative current path for the human, as summarized below.

A big voltage potential, called back EMF (ElectroMagnet Force) will let the flowing current to flow, by ionizing the nearby air to become sort of a electricity conductor (actually poor conductor, or high resistance), and the otherwise nowhere to go current would happily go through.

Since the ionized air is not very conductive, so the back EMF (“back” means “fighting back” the force to stop current flowing) must be very high, sometimes thousands of volts, to force the current to flow.

So you see the “Spark”, which is the current flowing in a big hurry.

Now energy has converted from magnetic to heat, thus conserved. Universe is in harmony, no scary Big Bang would happen again. Mother Nature is happy and goes to sleep, until another naughty human again switches off some flowing current has no where to go, and the fun repeats …