tech99 said:the dimensions of the experiment are too small for radiation to be significant
These two statements would seem to be at odds. The field propagates away from the wire at the speed of light c, so there will be a phase shift related to the frequency and the propagation distance, no?tech99 said:Or does the propagation time through free space introduce a phase shift?
Thanks for the comments so far. This was my initial assumption. However, the energy in the magnetic field is not being radiated, and does not permanently leave the wire. It is just stored, and then returns to the wire. So my idea is that equal energy flows away from and towards the wire, so there is no net outflow. So I don't think it is possible to detect a propagation delay, and it appears that the whole field at whatever distance from the wire cycles simultaneously. It seems similar to the case of the phase of a standing wave, where energy also flows both ways.berkeman said:These two statements would seem to be at odds. The field propagates away from the wire at the speed of light c, so there will be a phase shift related to the frequency and the propagation distance, no?
Thank you. Yes, agree. If the wire is driven, for instance, by a step function, the magnetic field will expand outwards at the speed of light, so there is a time delay. But if the wire is driven by a continuous sinusoid, then energy is flowing both ways equally (away from and towards the wire) so my idea is that no propagation delay can then be discerned.mfb said:Well, it depends on the scales of your setup, where d is the distance (scale) and f is the AC frequency:
##df \ll c##? -> no notable phase shift, radiative terms (you can never avoid that completely) are negligible.
##df \gg c##? -> large phase shift, radiative terms are dominant.
In between, it is complicated.
There is still a loss due to radiation, so that part of the signal does propagate away with the appropriate delay...tech99 said:But if the wire is driven by a continuous sinusoid, then energy is flowing both ways equally (away from and towards the wire) so my idea is that no propagation delay can then be discerned.
But at that distant point, there are two waves. One going outward and one coming inward. Both contain equal energy. So the phasor diagram will show the two rotating in opposite directions, and give constant phase.marcusl said:There is always a delay since information about the phase of the current travels from the wire to a distant point at the speed of light.
Where did the inward-moving wave come from?tech99 said:But at that distant point, there are two waves. One going outward and one coming inward
You should do the suggested sums yourself, based on c, and come to a conclusion. c is about 1m per 3 nanoseconds and the time period at 50Hz is 20ms. What sort of a fraction of a cycle does that represent for a distance of 10m?tech99 said:I think it will always be in phase
For an alternating current, when the current increases, the field moves outwards as it is built, and when the current decreases, the field moves inwards as it collapses.berkeman said:Where did the inward-moving wave come from?
But EM is a transverse wave, not a longitudinal wave. I guess I'm not understanding what you and @mfb are saying...tech99 said:For an alternating current, when the current increases, the field moves outwards as it is built, and when the current decreases, the field moves inwards as it collapses.
I understand your problem. What I am suggesting is that the energy flows outwards, then inwards, so there is no net flow, rather like a standing wave.berkeman said:But EM is a transverse wave, not a longitudinal wave. I guess I'm not understanding what you and @mfb are saying...
tech99 said:the magnetic field of a wire carrying an alternating current
Your OP was about the field generated by a wire, but now you are talking about coils separated by a distance? Now I'm really confused...tech99 said:So maybe if two coils of wire are separated by a distance, one driven and the other open circuit, we will not see a phase shift due to the distance.
A localized source generates only outgoing waves. An incoming wave can only be generated by an external source (think of a pair of communications antennas). The field of the source is computed by use of retarded potentials (see the wiki article) or the retarded Green function, both of which replace t by (t-r/c) as mfb pointed out.tech99 said:But at that distant point, there are two waves. One going outward and one coming inward. Both contain equal energy. So the phasor diagram will show the two rotating in opposite directions, and give constant phase.
Does not that apply only to radiated energy? Reactive energy is just stored so cannot be outflow only; net energy is not lost from the wire unless radiation takes place.marcusl said:A localized source generates only outgoing waves. An incoming wave can only be generated by an external source (think of a pair of communications antennas). The field of the source is computed by use of retarded potentials (see the wiki article) or the retarded Green function, both of which replace t by (t-r/c) as mfb pointed out.
Would it be better to say that the Energy of the Magnetic Field moves out and in? If the wire is carrying AC, the energy that exists in the fields around it will 'return' to the circuit and be dissipated in a resistive load, in the source resistance or stored in the Capacitances (or equivalent)in the system.tech99 said:For an alternating current, when the current increases, the field moves outwards as it is built, and when the current decreases, the field moves inwards as it collapses.
The phase of a magnetic field refers to the position of the field in its cycle of oscillation. It is measured in degrees or radians and indicates the direction and strength of the magnetic field at a specific point in time.
The phase of a magnetic field can be determined by measuring the time it takes for the field to complete one full cycle of oscillation, also known as the period. This can be done using instruments such as an oscilloscope or a magnetometer.
The phase of a magnetic field can be affected by various factors such as the strength of the magnetic field, the distance from the source of the field, and the material through which the field is passing. It can also be influenced by external magnetic fields and electromagnetic interference.
The phase of a magnetic field is important because it can provide valuable information about the behavior and properties of the field. It can also be used to determine the direction and magnitude of an electric current, as well as for navigation and communication purposes.
Yes, the phase of a magnetic field can be changed by altering the properties of the field or by introducing external factors. For example, the phase of an alternating current can be changed by adjusting the frequency or amplitude of the current. Additionally, the phase of a magnetic field can be shifted by the presence of other magnetic fields or by manipulating the material through which the field is passing.