Thursday, November 2, 2017

electromagnetism - Induction and electromagnetic fields


I've got a few questions on induction and electromagnetic fields. My current understanding of induction and electromagnetic fields is that, when electricity/current flows through a wire, it creates an electromagnetic field. If you twist that wire into a coil then you can concentrate that field. You can then induce current into a different coil by placing it inside of the electromagnetic field created by the first coil.


So here are my questions:


Q1. Does the electromagnetic field contain any energy?


EDITED QUESTION

Q2. Electromagnetic fields are the same as waves, so does that mean that when current is induced, they get the energy from electromagnetic waves being exchanged, or do they induce current directly from the electromagnetic field?


Q3. If electromagnetic fields and waves are the same, then doesn't that mean that you can induce current from waves? Therefore transferring electricity over long distances?


Q4. What spectrum of light are the electromagnetic waves that are in a field created by electricity?


Q5. Finally, why is it not possible to induce current from the Earth's magnetic field? I thought Maxwell said that electric fields and magnetic fields were the same thing.



Thank you to anyone who answers.


Edited Q2: So my original question (although asked poorly) still stands. Where does the current that gets induced in the secondary coil come from? How does the electricity transfer from the primary coil to the secondary? For example, is it taken from the stored energy in the e.m field? Does the field just act as a bridge between the 2 coils? Or do the 2 coils exchange e.m waves which carry the electricity that is being transferred from coil 1 to coil 2? Or is it none of these things and something completely different?



Answer



Q1: yes, it does.


Q2: "Electromagnetic fields are the same as waves"


Not always, e.m. fields may be static - static electric field around charges and static magnetic fields around magnets or (DC) currents, or waves - e.g as emitted by an antenna.


"so does that mean that when current is induced, they get the energy from electromagnetic waves being exchanged, or do they induce current directly from the electromagnetic field?"


In your example you don't have waves, you have a static magnetic field.


Q3: "If electromagnetic fields and waves are the same,"


As I said, they are not always the same - see my answer to question Q2.



"doesn't that mean that you can induce current from waves? Therefore transferring electricity over long distances?"


You can send waves over long distances, see how works the antennas, the lasers, the communication through satellites (don't forget that light is also e.m. field).


Q4: "What spectrum of light are the electromagnetic waves that are in a field created by electricity?"


This question is not clear - what you mean "e.m. waves that are in a field created by electricity"? A field around static electric charges contains no waves. Maybe after you read my answers above, you'll be able to express more clearly what you ask.


Q5: "Why is it not possible to induce current from the Earth's magnetic field? I thought Maxwell said that electric fields and magnetic fields were the same thing."


The law of induction of electromotive force is $\mathscr E = -\text d \Phi / dt$ where $\Phi$ is the magnetic flux. Thus, for generating an electromotive force $\mathscr E$ a variable magnetic field is needed in your coil. I am not sure whether the magnetic field of the Earth varies in time at all, and surely not as quickly and in the form as needed for producing current in your coil. Anyway, it's not known to me that we generate electricity from the Earth's magnetic field, see in Wikipedia how we generate current from magnetic flux.


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