# Electromagnetic induction between 2 coils problem.

1. Dec 5, 2012

### aditya23456

We know that
1)a change in current and voltage in a coil may result in induction in other stationary coil..this induction is directly related to frequency..ie The higher the frequency the more intense the induction effect.
2)we also know that there is no induction in case of DC current flow..
considering frequency to be infinite,we have a DC current flow and at same time we need to observe very high induction as per (1)..therefore where am I wrong.??

2. Dec 5, 2012

### Staff: Mentor

DC corresponds to a frequency of 0, not an "infinite frequency".

Depends on the setup and the definition of "intense".

3. Dec 5, 2012

### aditya23456

then what does frequency tending to infinite correspond to..? Is it anything which is not defined..
Intense defined as magnitude of field induced

4. Dec 5, 2012

### Staff: Mentor

I don't think there is a meaningful way to describe something as "infinite frequency". You can look how induction works for very high frequencies, but not for an infinite frequeny. In addition, you get many new effects if the speed of light becomes relevant.

5. Dec 5, 2012

### aditya23456

really thanks for info..I was reading abt telsa's wireless power transmission where i came across this..He used high frequency alternator for his tesla coil..I was wondering extreme case of this high frequency..I dint understand your last statement..did you mean speed of light relevant to frequency resulting in negligible wavelength..??what effects take place..?

6. Dec 5, 2012

### Staff: Mentor

In usual AC setups, you can assume that everything is in an equilibrium - the changes of current and voltage are small within the time electromagnetic fields need to propagate through your setup (about 1 nanosecond for 30cm, the speed of light). If that assumption is wrong, things can get tricky. A coil with 30cm diameter is not useful to convert GHz-signals (1 nanosecond per cycle), for example.

7. Dec 5, 2012

### Naty1

eletromagnetic induction occurs via electromagentic waves...light is the visible portion
of these waves...in many common inductions, like those of small coils at 60Hz, the speed of propagation...that is, the speed of light..... is immaterial...but at ultrahi frequencies and large coils or large coil separation, such time delay of propagation may have to be taken
into account.
all this is what mfb is taking about.

8. Dec 6, 2012

### aditya23456

what are these effects basically.? These effects can be found even at low(not highly low) frequency changes,,taking the other coil to be far enough..isn't it.?? Then we can find these effects to a coil near you,getting effected from some other inducing coil which is far enough for frequency to be comparible to time of propagation and magnitude being high enough that it can make necessary induction

9. Dec 6, 2012

### Staff: Mentor

You cannot build a transformer with a size of ~5000km (for 50 or 60 Hz). Everything beyond ~10m is impractical, and that corresponds to frequencies in the upper MHz range.

10. Dec 6, 2012

### Naty1

Replying to post #8: if I understand your points, 'yes'.....to them all.

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