The every day use of the magnetic part in EM radiation

[Wille]

As I understand it, it is the magnetic part of electromagnetic radiation which creates a current (AC) in a radio receiver (i.e in an antenna) which can be translated into sound/graphics and so on.
Is this correct? I mean in the sense that it is the magnetic field in the incoming radiation which is useful here. Or is it the electric field?

Same question for the microwave oven. Is it the magnetic field in the microwaves which oscillate the water molecules (being dipoles) in the food? Or is it the electric field in the radiation?

Further, in an induction stove the AC current in the coil under the plate creates an oscillating magnetic field which in turn creates currents in the pot on the stove (and also twist” dipoles if the material is magnetic (which it should be to get enough heat)). Is this magnetic field, the same magnetic field which is a part of the electromagnetic radiation coming from the AC current in the coil under the plate? Or are these two, two separate magnetic fields?

And finally, if the magnetic field creates AC in the radio receiver mentioned earlier, shouldn’t that AC in turn create electromagnetic radiation with the same frequency as the incoming signal, i.e become a sender?

Thanks

 

[zoki85]

Summary:: As I understand it, it is the magnetic part of the electromagnetic radiation that is used in for instance radio receivers and microwave ovens, is that correct? When is the electric field of direct use?

Not entirely. Receiving loop antennae use the magnetic part while line antenna use the E-field part

 

[tech99]

Summary:: As I understand it, it is the magnetic part of the electromagnetic radiation that is used in for instance radio receivers and microwave ovens, is that correct? When is the electric field of direct use?

As I understand it, it is the magnetic part of electromagnetic radiation which creates a current (AC) in a radio receiver (i.e in an antenna) which can be translated into sound/graphics and so on.
Is this correct? I mean in the sense that it is the magnetic field in the incoming radiation which is useful here. Or is it the electric field?

Same question for the microwave oven. Is it the magnetic field in the microwaves which oscillate the water molecules (being dipoles) in the food? Or is it the electric field in the radiation?

Further, in an induction stove the AC current in the coil under the plate creates an oscillating magnetic field which in turn creates currents in the pot on the stove (and also twist” dipoles if the material is magnetic (which it should be to get enough heat)). Is this magnetic field, the same magnetic field which is a part of the electromagnetic radiation coming from the AC current in the coil under the plate? Or are these two, two separate magnetic fields?

And finally, if the magnetic field creates AC in the radio receiver mentioned earlier, shouldn’t that AC in turn create electromagnetic radiation with the same frequency as the incoming signal, i.e become a sender?

Thanks

For the case of a car radio antenna, a voltage is induced on it by a passing EM wave. It acts as a sensor for the electric field but extracts very little energy from it. This is because very little current can flow due to the high capacitive reactance of the rod.
For a loop antenna in a small radio, or a ferrite rod antenna, it couples to the B field and acts as a sensor in a similar way.
Efficient receiving antennas would be fairly large relative to the wavelength and also resonant. This allows a large current to flow, so we have both a voltage and a current, which means energy is being extracted from a passing wave. The current flowing in the antenna means that the electrons are being accelerated, and so it radiates. In a typical case, only half the incident power is extracted and half re-radiated. The re-radiated energy is of such phase as to partially cancel the incoming wave in the vicinity of the antenna, so we see a localised partial shadow behind the antenna.

 

[anorlunda]

I mean in the sense that it is the magnetic field in the incoming radiation which is useful here. Or is it the electric field?

Wrong. It is the electromagnetic energy. Electric and magnetic are different ways to look at the same thing. They are not independent quantities.

Edit: Consider a man on a train with a charged particle. He sees a static charge and an electric field. But a second man standing on the train platform sees a moving charge and a magnetic field. Both men are right. They have two descriptions of the same thing.

 

[Wille]

Wrong. It is the electromagnetic energy. Electric and magnetic are different ways to look at the same thing. They are not independent quantities.

Edit: Consider a man on a train with a charged particle. He sees a static charge and an electric field. But a second man standing on the train platform sees a moving charge and a magnetic field. Both men are right. They have two descriptions of the same thing.

Ok thanks for your answer. What about the microwave oven, is it the same way of thinking? One cannot say that it is either the electric field or the magnetic field of the electromagnetic radiation that oscillates the water molecule? Or is something different in that case?

And also the part about the induction cooktop, is the magnetic field created by the current in the copper coil the very same magnetic field that is a part of the electromagnetic radiation created by the AC in the coil?

 

[anorlunda]

It applies everywhere, but since it depends on motion speed is a factor.

Light and radio, and all EM radiation always moves at the speed of light. In a microwave oven, we have radiation.

In an electrical circuit, the charges move very slowly. And in that case we can have capacitors using primarily electric fields, and inductors using primarily magnetic fields.

We can also use Maxwell's equations. You may not have studied Maxwell's equations yet, but they treat the electric and magnetic fields separately.

It sounds confusing, yes they are the same thing, no they are separate. It depends on the context. It is something you need to work at studying in several courses. Questions on the Internet are not enough to learn this topic.

 

[vanhees71]

I'd rather express it differently: There's one fundamental entity in Nature called the electromagnetic field. Its split into electric and magnetic components depends on the reference frame of the observer.

E.g., a charge distribution which is entirely at rest for an (inertial) observer leads to an electromagnetic field which has only electric components.

For an observer, who's moving with constant velocity to the other observer there are both electric and magnetic components.

A time-dependent (wave) field has always both electric and magnetic components.