Magnetism more important than electricity

[KingAntikrist]

Hi,

I had a question in my exam at my college that I did not quite know how to answer.

Why is the study of magnetism more important than of electricity in technology ? (I can't remember exactly the question)

 

[chrisbaird]

I think you have the question backwards. In my mind, electric effects are much more important to technology than magnetic effects. The reasons are:

1) Electric fields tend to be much stronger than magnetic fields
2) Most materials have a significant electric response (conductive, dielectric, etc) whereas very few materials have a significant magnetic response
3) Strong electrics fields are easier to create and control than strong magnetic fields

In the end, electric and magnetic effects are really facets of the same entity. So, it's important to understand both. We are surrounded by devices containing electric circuits, but really they are electromagnetic circuits because of induction, dynamos, etc.

 

[KingAntikrist]

Oh yea...thanks, you are right, I got the question backwards.

 

[RedX]

So, it's important to understand both. We are surrounded by devices containing electric circuits, but really they are electromagnetic circuits because of induction, dynamos, etc.

For electromagnetic waves, the electric and magnetic fields have the same strength in Gaussian units. In SI units, the electric field is stronger by a factor of the speed of light. But can you really compare things that have different units?

The voltages and currents of an AC generator propagate down wires at the speed \sqrt{\frac{1}{LC}} which is the speed of light, where L and C are the inductance and capacitance per unit length of the wires respectively. Their propagation is governed by a wave equation known as the telegrapher's equations.

So are the electric and magnetic fields created by an AC source electromagnetic waves? And if they are, don't the electric and magnetic fields have the same strength?

 

[chrisbaird]

In one sense, comparing electric field strengths and magnetic field strengths is like comparing apples and oranges because they have different units. But they both have the same end effects: exerting a force on a charge. So you can compare them in that sense. Antennas are designed to be coupled with the electric component of waves, not magnetic, because they get better response that way.

Yes, I would say AC sources propagate electromagnetic waves through circuits. Though we must be careful because people start picturing free, transverse, plane waves when we mention EM waves, which waves are only one special case.

 

[RedX]

Yes, I would say AC sources propagate electromagnetic waves through circuits. Though we must be careful because people start picturing free, transverse, plane waves when we mention EM waves, which waves are only one special case.

The waves from two parallel lines are transverse in both electric and magnetic fields. But are they true waves with the relation E=cB ?

From the derivation of the two wire transmission line, the magnetic field seems to be created from the changing current through the inductors, and not a displacement current: \frac{\partial E}{\partial t} which would be the hallmark of electromagnetic waves.

Another model for the transmission line is a ladder of inductors with capacitances on the rungs, and the displacement current is not taken into account with those lumped elements.

One would think the fields in a hollow waveguide would be more like electromagnetic waves, since they are solved with Maxwell's wave equations rather than a ladder of lumped elements. But does E=cB in this case?

 

[Born2bwire]

The waves from two parallel lines are transverse in both electric and magnetic fields. But are they true waves with the relation E=cB ?

From the derivation of the two wire transmission line, the magnetic field seems to be created from the changing current through the inductors, and not a displacement current: \frac{\partial E}{\partial t} which would be the hallmark of electromagnetic waves.

Another model for the transmission line is a ladder of inductors with capacitances on the rungs, and the displacement current is not taken into account with those lumped elements.

One would think the fields in a hollow waveguide would be more like electromagnetic waves, since they are solved with Maxwell's wave equations rather than a ladder of lumped elements. But does E=cB in this case?

In many waveguides (I'll ignore the ones that can support TEM along the direction of propagation like coaxial), the wave is bouncing around within the waveguide as opposed to traveling only in the direction of guided propagation. So if you have a hollow air filled waveguide, then yes, the waves inside would follow the same relations that they would in freespace (E is normal to B and both are normal to direction of propagation and are related by a constant.). It is just that the cumulative effect of the reflections off of the waveguide's walls is to direct the wave along a net direction of guided propagation.

 

[RedX]

In many waveguides (I'll ignore the ones that can support TEM along the direction of propagation like coaxial), the wave is bouncing around within the waveguide as opposed to traveling only in the direction of guided propagation. So if you have a hollow air filled waveguide, then yes, the waves inside would follow the same relations that they would in freespace (E is normal to B and both are normal to direction of propagation and are related by a constant.). It is just that the cumulative effect of the reflections off of the waveguide's walls is to direct the wave along a net direction of guided propagation.

If you have a coax cable, then there is current flowing on the outer surface, and a return current flowing in the inner conductor. The electric and magnetic fields in this coax cable are TEM.

If you have a hollow waveguide, then there is no return current. It seems like a hollow waveguide isn't a normal circuit at all. It seems like you're beaming E&M waves that are picked up by an antenna at the end of the tube.

How do you view a waveguide in a circuit? You have an AC generator that produces an alternating voltage, then you have an empty space where the beam goes through, and then a receiver that's connected to the load. Do you assign a resistance value for the empty space?

 

[Born2bwire]

If you have a coax cable, then there is current flowing on the outer surface, and a return current flowing in the inner conductor. The electric and magnetic fields in this coax cable are TEM.

If you have a hollow waveguide, then there is no return current. It seems like a hollow waveguide isn't a normal circuit at all. It seems like you're beaming E&M waves that are picked up by an antenna at the end of the tube.

How do you view a waveguide in a circuit? You have an AC generator that produces an alternating voltage, then you have an empty space where the beam goes through, and then a receiver that's connected to the load. Do you assign a resistance value for the empty space?

Yes, that is an apt descriptions of transmission lines. You are beaming an electromagnetic wave down the line, even in a coaxial cable.

It is difficult and confusing to pin down a causal relationship here. Even in a coaxial cable there are still electromagnetic waves. It is sufficient to note that if you have electromagnetic waves, you have currents. The causal relationship depends on what you suppose is your source. However, inside a transmission line there are no sources, strictly speaking. So the currents that exist in a waveguide (whether coaxial, rectangular, etc) are created by the electromagnetic waves. The waves induce the currents in the waveguides as they impinge on the walls and other interior objects. I would point out that you only have a truly physically returning current with a coaxial cable if you have DC currents. Otherwise, with AC currents the charges are oscillating in place. The electromagnetic waves that travel down the line carry the electric and magnetic fields that continually induce these currents on the walls and lines of the waveguide.

Looking at the terminations of the waveguide. Let's say I hook up my waveguide to an AC source. The AC source induces an AC voltage across the waveguide, which is nothing more than an AC electric field. A time-varying electric field means that there is a time-varying magnetic field and we see that the generator is directly inducing a wave across the opening to the waveguide. Basically what you can say is that the EM waves carry the potential difference that will excite the currents or voltages at the termination of your line.

I would also put in a word of warning about the use of TEM, TE and TM. When people use these in regards to waveguides and transmission lines, there is an implicit understanding that they are not talking about the orientation of the fields with their direction of propagation, but with the direction of guided propagation of the guide. If you were to solve for the modes of a hollow PEC waveguide you would find that the waves are still TEM in the sense of how they are actually traveling through the waveguide but will be TM or TE to the axial direction of the waveguide that we wish to propagate the waves along.