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Maxwell modified Ampère’s law and used it with the three other basic laws of electromagnetism to predict the existence of electromagnetic waves and to derive their properties.
His theory predicted that EM waves of any frequency travel through vacuum at the same speed, a speed that closely matched measurements of the speed of light—strong evidence that light is an EM wave.
In honor of Maxwell’s achievements, the four basic laws of electromagnetism are collectively called Maxwell’s equations. They are:
1. Gauss’s law [Eq. (16-17)]: If an electric field line is not a closed loop, it can only start and stop on electric charges. Electric charges produce electric fields.
2. Gauss’s law for magnetism: Magnetic field lines are always closed loops since there are no magnetic charges ( monopoles ). The magnetic flux through a closed surface (or the net number of field lines leaving the surface) is zero.
3. Faraday’s law [Eq. (20-18)]: Changing magnetic fields are another source of electric fields.
4. The Ampère-Maxwell law says that changing electric fields can be a source of magnetic fields. Now, electric AND magnetic field lines form closed loops.
5. NET EFFECT: I need charges to create electric and magnetic fields, but THEN the electric and magnetic fields can propagate on their own, each sustaining the other. And the fields can carry energy and momentum through space.
The electric dipole antenna consists of two metal rods lined up as if they were a single long rod. The rods are fed from the center with an oscillating current.
For half of a cycle, the current flows upward; the top of the antenna acquires a positive charge and the bottom acquires an equal negative charge.
When the current reverses direction, these accumulated charges diminish and then reverse direction so that the top of the antenna becomes negatively charged and the bottom becomes positively charged.
When these charges reverse direction, they ACCELERATE, producing alternating electric and magnetic fields that can now propogate through space.
The result of feeding an alternating current to the antenna is an oscillating electric dipole.
This produces an electric (E) and magnetic (B) field perpendicular to each other, and BOTH perpendicular to the direction of the flow of energy(S): (A half-cycle later it looks like:)
Lookang many thanks to Fu-Kwun Hwang and author of Easy Java Simulation = Francisco Esquembre, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
By SuperManu - Self, based on Image:Onde electromagnetique.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2107870
lIn the time it takes the wave to oscillate once (the period, T) the wave MOVES a distance equal to the length of a wave (the wavelength :). So the speed of the wave is:
EXAMPLE: On the back of your microwave is a sticker that tells you the frequency of the “output” (the EM radiation inside when you turn it on). It should say 2450 MHz. The wavelength is then:
You can check this (google: “speed of light microwave cheese”)
!! At what frequency would an EM wave have a wavelength about as long as a human being? Where is this in the electromagnetic spectrum?
A supernova is an exploding star; a supernova is billions of times brighter than an ordinary star. Most supernovae occur in distant galaxies and cannot be observed with the naked eye. The last two supernovae visible to the naked eye occurred in 1604 and 1987.
Supernova SN1987a occurred 1.6 × 1021 m from Earth. When did the explosion occur?