# Why do we need frequency of changing current for induced EMF?

I know that a constant current doesn't create induced emf, however, I do not know why a changing one does. Are there any ways to understand it briefly/easily, or is it just the way it is?

• Physics doesn't ever really answer "why" things are. It mostly tries to describe what we observe in the universe around us. In this case, we observe a back-emf when we try to change the current through a loop, and we can describe the effect with Faraday's Law. – The Photon Dec 22 '18 at 18:26
• Okay, thank you! I do not want to be like memorizing equations without understanding what's really happening, but I guess I can do it without knowing the why for most cases. – Vyun Dec 22 '18 at 20:55

$$\varepsilon = - \frac{\Delta \Phi_B}{\Delta t}$$
Here $$\varepsilon$$ is the EMF, $$\Phi_B$$ is the magnetic flux which equals the multiplication of a magnetic field $$\bf B$$ and the area $$\bf A$$ crossed by $$\bf B$$, whereas $$t$$ is time, and $$\Delta$$ represents changes. So the equation just says that the EMF can be produced by the magnetic flux that changes over time.
Furthermore, in the special case you mentioned $$\bf B$$ is produced by the electric current. So assuming there's no change in $$\bf A$$, the only way to produce $$\varepsilon$$, i.e. the EMF, is by having $$\bf B$$ that's changing over time, and in order to get $$\bf B$$ that's changing over time you need an electric current that's changing over time too. So that's how it's explained!