Are electromagnetic waves positively charged

to directory mode

The radiation of the Hertzian dipole

(Fig. 1) shows the emission of the Hertzian dipole in the near field. Watch the animation over several phases. Note that a dipole has a fixed emission frequency depending on its physical dimensions.

The red and blue arrows in the semi-transparent circle show the phase difference between the electric and magnetic fields at the location of this circle. Click and drag the mouse to move the circle within the animation. The phase differences in the near field and far field will be discussed later.

The kidney-shaped lines represent electric field lines. The dark red lines are directed the other way around than the light red lines. The circular lines in the, plane (shown in perspective) describe the field lines of the magnetic field. The black lines are directed the other way around than the blue lines.

For a more precise understanding of the dipole oscillation and the associated emission of electromagnetic waves, individual oscillation phases are considered below. The start time is. be the period of the dipole oscillation. The time sequence corresponds to that when comparing pendulum oscillation and oscillating circuit. You should be familiar with the process described there in the electrical oscillating circuit.

Time: 1/4 T

The dipole as a capacitor is fully charged, i.e. there is currently an excess of electrons at one end of the metal rod. The other end is positively charged accordingly. The voltage and the electric field between the ends are maximal. The field lines of the electric field point in arcs from one end to the other.

Time: 2/4 T

Driven by the electrical voltage, the electrons flow through the rod. At the moment 1/2 T the current through the rod is maximum. This current has a magnetic field, the field lines of which run in concentric circles around the rod. The magnetic field strength is maximum.

The electric field strength is zero. However, the electric field lines that were created a quarter period before do not disappear. They cut off and move away from the dipole as an electric vortex field at the speed of light. The cross-section of this vortex field has a characteristic kidney shape.

Time: 3/4 T

After three quarters of the period, the electrons have arrived at the other end of the rod. The electric field is now maximal again, but directed the other way around than at the time 1/4 T . The field lines now form arcs in the other direction.

The current in the dipole is zero and so is the magnetic field. However, the magnetic field lines that were created before do not disappear, but move away from the dipole as a magnetic field at the speed of light.

Time: 4/4 T

Again driven by the electrical voltage between the ends of the rod, the electrons now flow back in the opposite direction. They have a magnetic field, the field lines of which again form concentric circles around the axis of the current. Since the current flows in the other direction than half a period before, the magnetic field lines are now also directed the other way around.

The electric field is zero again at this point in time. The field lines that were present during the charge separation have pinched off again and are moving away from the dipole at the speed of light.

Now the process starts from the beginning.

Phase relationship of the electric and magnetic field

To a certain extent, the Hertzian dipole oscillates back and forth between an electric and a magnetic field. We have already got to know this behavior with the oscillating circuit. If the electrons are at the ends of the rod, the electric field strength is maximum and the magnetic field strength is zero. A quarter of a period later, the electrons flow with maximum current to the other end of the rod. Now the magnetic field surrounding this current is maximal and the electric field strength is zero.

So you can see that the oscillation of the electric field strength and the magnetic field strength are shifted by 90 ° against each other. However, this only applies in the so-called near field, i.e. in the immediate vicinity of the dipole. At a greater distance, i.e. in the far field of the dipole, electric and magnetic field strengths oscillate in phase.

Radiation pattern

And another difference between mechanical and electromagnetic waves becomes clear: A point-shaped sound source emits sound waves with spherical wave fronts, so-called spherical waves. The electromagnetic waves emitted by the dipole, on the other hand, are not spherical waves, but are spatially oriented. The reason is the spatial orientation of the transmitter itself. The intensity of the emitted waves is greatest in the equatorial plane of the dipole.