So electric fields are produced due to presence of charged particles and magnetic fields are produced due to motion of charged particles. In DC circuits, a current-carrying wire involves the motion of electrons, which are charged particles, so both magnetic and electric fields are produced. So why is it that only Alternating Current produces EM waves? Why do we need the changing magnetic fields? Similarly, is the motion of electrons around the atom’s nucleus what produces the continuous electromagnetic waves rather than only the transition between two energy levels that only does so?
[Physics] Why doesn’t Direct Current produce electromagnetic waves
electric-fieldselectromagnetic-radiationmagnetic fields
Related Solutions
For the magnetic field, the currents are one source of the magnetic, but this problem is more linked to the source of the current in the wire. For a conductor with finite conductivity, an electric field is needed in order to drive a current in the wire.
If we assume your wire is straight, this required field is uniform. One way to realize this field is by taking two oppositely charged particles and send them to infinity while increasing the magnitude of their charge to maintain the correct magnitude of electric field. In this limit, you will obtain a uniform electric field through all space.
Now, put your conductor in place along the axis between the voltage sources--a current will flow. In the DC case, this gives rise to the magnetic field outside of the wire. As for the electric field, a conductor is a material with electrons that can move easily in response to electric fields and their tendency is to shield out the electric field to obtain force balance. Because the electrons can't just escape the conductor, they can only shield the field inside the conductor and not outside the conductor. With this model, we see that the electric field is entirely set up by the source and placing the conductor in the field really just establishes a current. Note here that if you bend the wire or put it at an angle relative to the field, surface charges will form because you now have a field component normal to the surface.
For the limit of an ideal conductor, no electric field is needed to begin with to drive the current and so there isn't one outside the wire.
For the AC case, solving for the fields becomes wildly complicated very fast as now the electric field driving particle currents has both a voltage source and a time-varying magnetic source through the magnetic vector potential. The essential physics is the same, though, as the source will establish the fields (in zeroth order), and the addition of the conductor really just defines the path for particle currents to travel. In the next order, the current feeds back and produces electromagnetic fields in addition to the source(s) and will affect the current at other locations in the circuit.
I guess a short answer to your question is that there are always fields outside of the current-carrying wire and the electric field outside disappears only in the ideal conductor limit. Conductors generally do not require very strong fields to drive currents anyway so that the electric field outside is usually negligible, but don't neglect it for very large potentials in small circuits.
If them magnetic field is creating electric field and they combine to form EM waves, why does a compass show a magnetic field around the wire?
To produce EM waves, we do need the current to be time varying such that the magnetic field is time varying which induces a time varying electric field etc.
However, it isn't that case that the entire magnetic field produced by the time varying current is associated with EM radiation.
In the reactive near field close to the wire, there are time varying electric and magnetic fields that are not associated with EM waves (which transport energy away) but are, rather, associated with energy storage. From the linked article:
For example, current flowing in the antenna creates a purely magnetic component in the near-field, which then collapses as the antenna current begins to reverse, causing transfer of the field's magnetic energy back to electrons in the antenna as the changing magnetic field causes a self-inductive effect on the antenna that generated it. This returns energy to the antenna in a regenerative way, so that it is not lost.
This is all quite complicated in general but, for low frequency, e.g., 60Hz AC current along a wire in a circuit, the sinusoidally time varying magnetic field dominates.
Energy is alternately stored in this field (as the current magnitude increases) and returned (as the current magnitude decreases).
Best Answer
An EM wave is an oscillation of the EM fields- which requires them to vary in time. As you mentioned, DC produces a magnetic field, but a constant one.
For your second question, it is true that a classical orbiting-electron model would produce EM radiation, and it is one of the reasons that led to a quantum explanation: see the Bohr Model.