# Inductor polarity and induced current

#### abdulwahab.hajar

Joined Jun 14, 2016
93
Hello everyone
I was hoping you could help me with a concept that is mixing me up a bit.

it's a simple inductor circuit.. first current passes through the circuit till it's charged fully storing energy inside its magnetic field.... However when the switch is opened later.. the reaction of the inductor doesn't make sense to me....
We know that inductors resist change in the current through them... before the switch was opened current was passing through the inductor entering from the upper side... therefore when the switch is opened the inductor induces a voltage which will drive a current that should enter through the upper side as well right....
However looking at the polarity of the inductor when the switch is open.. the negative terminal is the upper side of the inductor and vice versa.... Now why would this drive a current entering through the upper side of the inductor... because there is a physical connection between both terminals... and we know that current goes from positive to negative... why doesn't the current just go upwards directly to the negative terminal (that is assuming of course that the inductor is connected to smth like a resistor which closes the circuit... Why would the current take the long route in a sense? In a battery it's because the only way to reach the negative terminal from the positive terminal is the load... but in an inductor's case it's a different..

Thank you
I hope my question is understood..

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#### abdulwahab.hajar

Joined Jun 14, 2016
93
never mind I just saw it xD

#### Jony130

Joined Feb 17, 2009
5,155
From the Ampere's law, we know that the current in the conductor product the Magnetic field. And this change in the Magnetic field "induced voltage" in the inductor (Faraday's law). And this voltage, by definition (Lenz Law), opposes any external effort to change the existing flux (or current) in an inductor.

When the switch is closed the current leaving the positive terminal of DC voltage source and the upper end of the inductor gets set to a higher voltage than its lower end. And the current is flowing from upper end of the inductor to its lower end. And during this phase, the inductor stored the energy in magnetic field (charging phase).

But when the switch is open the stored energy in the inductor during the previous phase cannot disappear instantly. So the magnetic field collapses and this change in the magnetic field "induced voltage" in the inductor (Faraday's law). And the polarity of this induced voltage by definition (Lenz Law), opposes any external effort to change the existing current. So the inductor "wants" to current to flow in the same direction as previously flowing. And this can only happen when the polarity of the induced voltage is reversed, you can mentally visualize the inductor as becoming a sort of “voltage source,” forcing the current to keep flowing in the same direction as before the switch was open.

#### MrAl

Joined Jun 17, 2014
7,477
Hello everyone
I was hoping you could help me with a concept that is mixing me up a bit.

it's a simple inductor circuit.. first current passes through the circuit till it's charged fully storing energy inside its magnetic field.... However when the switch is opened later.. the reaction of the inductor doesn't make sense to me....
We know that inductors resist change in the current through them... before the switch was opened current was passing through the inductor entering from the upper side... therefore when the switch is opened the inductor induces a voltage which will drive a current that should enter through the upper side as well right....
However looking at the polarity of the inductor when the switch is open.. the negative terminal is the upper side of the inductor and vice versa.... Now why would this drive a current entering through the upper side of the inductor... because there is a physical connection between both terminals... and we know that current goes from positive to negative... why doesn't the current just go upwards directly to the negative terminal (that is assuming of course that the inductor is connected to smth like a resistor which closes the circuit... Why would the current take the long route in a sense? In a battery it's because the only way to reach the negative terminal from the positive terminal is the load... but in an inductor's case it's a different..

Thank you
I hope my question is understood..
Hi,

That is one of the basic principles that allows a boost circuit using an inductor to work.