# The fields and waves associated with a transformer

Induction in a transformer happens via a fluctuating magnetic field in coil A inducing a voltage and hence current in coil B.

1. An electric field is associated with a fluctuating magnetic field. Is the electric field at right angles to the magnetic flux lines?
2. Is this magnetic field actually an electromagnetic wave? What part, if any, do electromagnetic waves/fields have in the operation of a transformer? Do electromagnetic waves travel down the wire?
• Magnetic field is classical. See the accepted answer here
– jonk
Sep 5 '18 at 6:16
• With no-load on a transformer, the core energy is very small. With a massive secondary load, the core energy becomes massive and the external magnetic field is much greater. Sep 5 '18 at 12:09
• @analogsystemsrf I think you need to go back to school on the basis of what you said. Flux in the core is largely independent of secondary load current; it is determined by the un-loaded inductance of the primary and the current it takes from the voltage supply. Secondary load currents flow in primary and secondary and create equal and opposite ampere-turns hence they cancel. Sep 5 '18 at 18:22

An electric field is associated with a fluctuating magnetic field. Is the electric field at right angles to the magnetic flux lines?

The simple answer is yes as per this diagram from here: -

For a transformer, the changing flux is contained within the core and the windings that surround the core limbs "collect" the field that is at right angles to the magnetic flux lines. Two turns of a winding collect twice the voltage etc..

Is this magnetic field actually an electromagnetic wave? What part, if any, do electromagnetic waves/fields have in the operation of a transformer?

No. It is a magnetic field (and an associated electric field) and no part of understanding the operation of a transformer is relied upon by using electromagnetic wave theory. Electric and magnetic fields are not forming an EM wave because they are temporally shifted; it is the rate of change of magnetic flux that delivers voltage; BUT in an EM wave, electric and magnetic fields rise and fall with time syncronicity: -

This doesn't happen with a transformer because the induced voltage is $N\dfrac{d\Phi}{dt}$.

Do electromagnetic waves travel down the wire?

If you dig deep enough you will find that all currents and voltages can be thought of as EM waves and, as such, all wires can be thought of as transmission lines. Do you need this to understand "regular" transformers?

No!

Do you need this to understand RF transformers? Sometimes when in the upper VHF region and beyond and certainly as you approach the microwave range it is important to understand what effects can happen.

• That's a good answer. Sep 6 '18 at 5:45
• @user1897830 thank you but you are allowed to upvote answers that are useful to you and, in the case that one answer to a question is seen by you as "stand-out" then you can mark that answer as accepted. You've asked 23 questions now and not formally accepted any answers and there are a few good answers that you should consider formally accepting. This is how you accept an answer and overall it is in your interest to do this to pay-back the time spent by people making answers. Sep 6 '18 at 7:12
1. It's hard to answer. Each point in the space (or near the transformer) has its own electric field (E) direction and magnetic field (B) direction, so it's hard to say in general whether E and B is perpendicular.

2. No, the EM field itself is not an EM wave. An operating transformer do emit EM wave. However, due to most transformer works at extremely low frequency (50Hz/60Hz), the emitted power is extremely small. The low frequency EM wave does not travel in metal conductors.

Read Nikola Tesla's patent on the AC transformer from 1890.
ok too much "legalese" from the Attorneys.

Basically the Vac, and f with primary inductance creates the mutual flux in the core that couples the secondary turns and produces the voltage ratio. This generates a small magnetic current with no load but is necessary to couple the primary and secondary in the shared core.

E(Electric) fields from voltage to H(magnetic)fields from current are both at right angles to each other and the direction of the wire.

The Alternating voltage induces alternating current sinewaves are not the same as travelling waves on a long transmission line. The load also draws a current wave from the secondary voltage. If is a linear load, it is also a sinewave current.

Don't confuse the two distinct uses here of the term "wave".

Since the wavelength for 50Hz is far greater than the length of wire used there are no travelling-wave effects from the transformer itself.

Also read Maxwell's Treatise on Magnetism and Electricity at archive.org (search and if you fail re-search it again that's what your education is all about.

Learning how to learn.

For the 11 to 19 yr old

Is current exciting?

Although this video is about antennas, I think the same principles could be applied to a transformer regarding the magnetic field, electric field and electromagnetic waves. Electromagnetic Wave Propagation