# Does change in current cause voltage across an inductor or vise versa?

I am currently studying power electronics and trying to truly understand the operation of buck and boost converters. From the Wikipedia page for a buck converter, it states, "When the switch is first closed (on-state), the current will begin to increase, and the inductor will produce an opposing voltage across its terminals in response to the changing current". This sentence really puzzled me because I had always been under the impression that a change in voltage across the inductor caused the current to change.

Even in my power electronics class, my professor discussed how the difference in the voltage across the inductor from input voltage to output voltage, causes the inductor current to ripple. We calculated the inductor ripple by using the voltage across the inductor for the on time of the active switch and solving the differential equation $v&space;=&space;L\frac{\mathrm{d}&space;i}{\mathrm{d}&space;t}$. However, from this Wikipedia article, it seems that the ripple in inductor current actually is what is causing the change in voltage from input to output.

So which one is it? Does a voltage differential across the inductor cause the current to start ramping up, or does a change in current induce a voltage differential?

• electronics.stackexchange.com/questions/470171/… – G36 Feb 11 at 18:14
• What causes the voltage across a resistor; current. What causes the current in a resistor; voltage. It's a circular argument and is as pointless for a resistor as it is for an inductor. Don't get hung up on it. – Andy aka Feb 11 at 18:49
• The inductor enforces the equation V = Ldi/dt. Well, that is the simplified version of the equation. But the point is that cause and effect cannot be separated. So if you are trying to produce a current ramp, you can apply a voltage. And if you want to produce a voltage you can ramp the current to get it. It works both ways all the time as needed to make the equation true. – mkeith Feb 11 at 18:54

The answer is "yes". You can't have a change in current without a voltage appearing across the terminals. And if a voltage appears across the terminals there will be a change in current.

If you're driving the inductor with an ideal voltage source, it's easiest to think of the voltage causing the current to change.

If you're driving the inductor with an ideal current source, it's easiest to think of the change in current causing a voltage to appear across the terminals.

It depends what you mean by 'cause'.

If you kick a football at a window, and the window breaks, then you can say the football caused the window to break, and you can say the broken window did not cause the football to move. It's this ability to say that one did not cause the other that establishes an unambiguous cause and effect chain.

With inductance, and many other things in physics, the two things happen at the same time. The current changes, and there's a voltage at the terminals. You can say either caused the other, depending on how you set up the experiment. You cannot say that either did not cause the other.

If you have hold of the battery and the switch, then you'd tend to say that the voltage caused the change of current. But there is a back EMF which prevents a huge current flowing when you close the switch. If you're holding on to the wires of the inductor when you break the current, you will get a shock, and you'd tend to say that the drop in current caused the high voltage. However, that high voltage will be dropping the current down to match the increase of resistance in the switch.

A more familiar example would be force and acceleration. Those two happen at the same time. Again, it depends whether you are the carpenter or the nail when thinking about acceleration, deceleration and force around the hammer.