Self inductance (contiuous the inductor thread)

[allegro]

I read the next definition of self inductance:
Self-inductance, or simply inductance, is the property of a circuit whereby a change in current causes a change in voltage in the same circuit

Could you please explain it to me?
Couldnt this definition be said on ohms law?
Because even in a pure ohmic circuit, a change in current causes a change in voltage.

 

[dknguyen]

I am going to give you an example since you already seem to have read the difference between a resistor and inductor.

Resistors's voltage drop depends on the AMOUNT of current flowing through it. Inductor's voltage drop depends on the RATE of change of the current flowing in it- the actual amount of current flowing through the inductor does not influence the voltage drop in any way.

So we have a resistor and an inductor with a constant 1 billion amps flowing through it.
Resistor: Voltage drop is very large because lots of current is flowing through it.
Inductor: Voltage drop is ZERO even though 1 billion amps is flowing through it.
THis is because in an inductor, what matters for the voltage drop is not the amount of current flowing through it, but how fast the current is changing in it. In this case the 1 billion amps is constant so there is zero change so zero inductor voltage drop.

Now suddenly the current starts to fall at a rate of 1 million amps per second on both components.
Resistor: The voltage across the resistor will fall (towards zero) because the LEVEL of current across the resistor is dropping NOT because it is changing
Inductor: The voltage across the inductor will RISE (away from zero) from what it was before because the current in the inductor is CHANGING.

1000 seconds later, the current in both components has dropped to zero BUT the current is still falling (ie. the current is now starting to increase in the opposite direction, making current flow in the opposite direction through the components at an increasing rate.)
Resistor: Zero voltage drop because the current flowing through it is zero.
Inductor: Very large voltage drop even though the current flowing through it is zero.
Zero current in a resistor makes zero voltage drop. But in an inductor this does not matter. The current in the inductor is zero, but the current is still falling so there is still a voltage drop across the inductor.

In this example, I ignored the polarity of the voltage across the components. I only paid attention the magnitude of the voltage across the component (whether or not the voltage drop was zero or "very far away from zero). Rise meant the voltage moved away from zero and fall meant that the voltage moved towards zero (regardless of whether the move was positive or negative. In fact, when the current in an inductor falls, it has the voltage drop OPPOSITE polarity of a resistor and is actually a voltage rise (like a battery supplying power). When the current rises, the polarity of the voltage drop is the same as that of a resistor.

This is because one other thing an inductor does that a resistor does not do is store energy. When the current rises, it stores energy and when the current falls it releases this stored energy in the form of a huge voltage rise (a voltage spike) and produces current flowing in the same direction as the rest of the current in order to fight the current fall and keep the current level the same. But it can't do this forever since it can only store a finite amount of energy. The result is that the rate of current fall is slowed down and a huge voltage spike is generated in the process (faster the fall time the larger the voltage spike).

On the other hand, when current rises, the voltage drop in the inductor actually slows down the rise of current. So in short, inductors slow down the rate of change in current and the current in an inductor cannot instantly change from one level to (like a step function). The reason the current rates are slowed down is because when the current is increasing, the inductor steals some of this energy and stores it, and when the current decreases the inductor releases this stored energy.

DOes this example help?

 

[allegro]

Awesome explanation!
You really got me into understanding this.
So in what circuit, a change in current wont cause a change in voltage?
maybe only a circuit with inductor only.

 

[dknguyen]

Awesome explanation!
You really got me into understanding this.

I've been refining my first post over and over again to make it easier to understand. Hopefully if you read the latest one it will be even clearer, especially the extra section I added at the end about inductor energy storage which expands on some of the things an inductor does.

So in what circuit, a change in current wont cause a change in voltage?
maybe only a circuit with inductor only.

An ideal diode is the cloest thing to this. The voltage drop is independent of the current flowing through it (though in real life it does change a little bit).

 

[allegro]

I really thank you, This very very helped me.

Lots of greetings!