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The term electromagnetic wave describes the way electromagnetic radiation (EMR) moves through space. In many respects, an electromagnetic wave behaves similarly to ripples on water, or to sound traveling through a medium such as air. Shorter wavelength, or higher frequency, EMR carries more energy than longer wavelengths or lower frequencies. The relationship between light and electromagnetism was established by the work of the physicist James Clerk Maxwell in the 19th century. In quantum theory, electromagnetic radiation is produced when subatomic particles release energy.
The electromagnetic field of the earth is a strong force which keeps boats heading in the right direction and allows for effective sonar and compass activity.
We don't fully understand this force or how electromagnetic waves function, but we use them on a daily basis. Electromagnetic wave theory seems to indicate that there are intrinsic similarities between gravitational waves, light, and the space-time forces behind all of the universes main atomic forces. This editable Main Article is under development and not meant to be cited; by editing it you can help to improve it towards a future approved, citable version. In physics, an electromagnetic wave is a change, periodic in space and time, of an electric field E(r,t) and a magnetic field B(r,t). Because an electric as well as a magnetic field is involved, the term electromagnetic (EM) is used, a contamination of electric and magnetic.
Classically (non-quantum mechanically), EM radiation is produced by accelerating charges, for instance, by charges oscillating in a radio antenna.
Electromagnetic waves were predicted on theoretical grounds by James Clerk Maxwell in 1861 and first emitted and received in the laboratory by Heinrich Hertz a quarter century later. Assume that the snapshot in figure 1 is taken at time t, then at a certain point z we see an arrow of certain length representing E(z,t) and also a vector B(z,t).
Basically, the only property distinguishing different kinds of EM waves, is their wavelength, see figure 2.
As was pointed out above, the wavelengths of the various parts of the EM spectrum differ many orders of magnitude. It is known that EM waves can be linearly superimposed, which is due to the fact that they are solutions of a linear partial differential equation, the wave equation (see next section).
The electric field vectors in figure 1 are all in one plane, this is the plane of polarization, and a wave with one fixed polarization plane, is called linearly polarized.
The radiation of many lasers is monochromatic and linearly polarized (at least to a very good approximation). In this section it will be shown that the electromagnetic wave depicted in figure 1 is a solution of the Maxwell equations in the vacuum. Note that the displacement current (time derivative of E) is essential in this equation, if it were absent (zero), the field E would be a static, time-independent, electric field, and there would be no waves. Observe that E and B are related by the third and fourth Maxwell equation, which express the fact that a displacement current causes a magnetic field, and a changing magnetic field causes an electric field (Faraday's law of induction), respectively. The wave equation is without doubt the most widely studied differential equation in mathematical physics. We could now proceed in the very same way and solve the wave equation for B, but then we could easily overlook the relation between the two fields. Clearly, the cycle averaged intensity of a wave going through vacuum is constant, independent of the direction of propagation, z. The negative quantity is the loss of energy of the system per unit time and per unit volume (according to Joule's and Ohm's laws). As an example we give an order-of-magnitude-calculation of an electromagnetic energy density.
See Fourier expansion electromagnetic field for the expansion of the vector fields A (vector potential), E, and B together with the Fourier expressions for the EM energy and the EM momentum. See photons for an overview and see quantization of the electromagnetic field for a more detailed discussion. Complete the form below to receive an email with the authorization code needed to reset your password.
For more help in Hertz Experiment for Production and Detection Electromagnetic Wave please click the button below to submit your homework assignment. Enter your e-mail and subscribe to our newsletter for special discount offers on homework and assignment help. Electromagnetic radiation: same, same but differentWhat have radios, warm hugs and nuclear blasts got in common? Infrared images let us 'see' temperature; hotter things appear red and yellow, cooler things green and blue. Radio, microwaves, UV, visible light and gamma rays have completely different effects, but they're all exactly the same kind of thing: electromagnetic radiation (EMR).
The only difference between gamma-rays from nuclear bombs and the waves that let our TVs and microwaves do their thing is how much energy those waves have got. Electromagnetic radiation is just waves of moving energy, but when we tune into radio or TV stations, we don't talk about their energy — it's all about frequency.
Whether it ends up as microwaves, X-rays or sunlight, the energy that travels as electromagnetic radiation is always produced in the same way. But it's not just radio towers, X-ray machines and light bulbs that produce electromagnetic radiation; every bit of matter in the universe emits radiation, including you. Warm things like us give off body heat, and that heat energy travels as infrared radiation.
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By clicking 'Send to a friend' you agree ABC Online is not responsible for the content contained in your email message. These original EMS pages will be available until May 31, 2011 at which point you'll automatically be redirected to the new pages. Different forms of EMR are distinguished by their wavelengths, which vary from many yards (meters) to a distance smaller than the diameter of an atomic nucleus. For example, if a light is shone onto a screen through a barrier with two narrow slits, a pattern of light and dark stripes is seen. This was given the name “ether,” sometimes spelled “aether,” and was thought to be an invisible material that filled space, but through which solid objects could pass unhindered. The wavelength is the distance between two identical points of the wave from cycle to cycle, for instance, the distance between one peak, or crest, and the next. This led to the study of electrodynamics, in which electromagnetic waves, such as light, are regarded as disturbances, or “ripples,” in an electromagnetic field, created by the movement of electrically charged particles.
According to this theory, the “wave” represents the probability of finding a particle, such as a photon, at a given location. For example, an electron in an atom can absorb energy, but it must eventually drop to a lower energy level and release the energy as EMR.
Radio, television, mobile phones and the Internet rely on the transmission of radio frequency EMR through air, space or fiber optic cables.
If we can learn to harness light and space-time in the future, we may find ways of transmitting information which are even more revolutionary than what is currently happening.
These forces are thought to be derived from one ultimate force, which is encapsulated in many theories concerning cosmology and the theory of everything. The first to see the applicability for communication purposes was the inventor of radiotelegraphy Guglielmo Marconi (around 1900). Note the enormous span in wavelengths, from one trillionth of a millimeter for gamma-rays (radioactive rays) up to the VLF (very low frequency) radio waves of about 100 kilometer. Furthermore, the sources of the radiations, the interactions with matter, and the detectors employed differ widely, too.
A linear superposition of waves is a solution of the same wave equation as the waves themselves.
So a time-dependent electric field that is not associated with a time-dependent magnetic field cannot exist, and conversely. In figure 1 the electric field depicts a particular solution, with special initial and boundary conditions. We assumed here that the direction of E defines the direction of the x-axis with unit vector ex along this axis. It follows that E and B are perpendicular (along the x-axis and y-axis, respectively) and are in phase.
The quantity represents the rate at which energy is produced per unit volume by ordinary Joulean (resistance) heating.
However, because all terms in the equations would be multiplied by the same volume, it is more convenient to consider densities and to divide out the volume.
One may look upon the quantity as the work (per unit time and unit volume) done by the Lorentz force q(v?B + E) on the moving particles constituting the current density J. This vector is perpendicular to the plane of E and B and by the right-hand rule, it points in the direction of propagation of the EM wave. The increase of field energy per unit time is the flow of radiation energy into the system. An induction coil is connected to two spherical electrodes with a narrow gap between them (the transmitter).
But if you could crank up the energy in radio waves a bit, you'd turn them into microwaves and you could zap your dinner with them.
In the case of EMR, it's how often you'd get hit by a wave of radiation if you stood in its way. So you're actually emitting higher energy radiation right now than your remote control or microwave oven. The in-line arrangement ensures the stone shows in the ultrasound image, and the disintegrate process are thus been given. The full range, in decreasing order of wavelength, goes from radio waves through microwaves, visible light, ultraviolet and X-rays to gamma rays and is known as the electromagnetic spectrum.
This is called an interference pattern: where the crests of the waves from one slit meet those from the other, they reinforce one another, forming a bright stripe, but where a crest meets a trough, they cancel out, leaving a dark stripe. Experiments designed to detect the ether by its effect on the speed of light in different directions all failed to find any evidence for it, and the idea was finally rejected.
EMR can also be defined in terms of its frequency, which is the number of crests that pass by in a given time interval. Low frequency radio waves mildly perturb atoms and molecules, while microwaves cause them to move about more vigorously: the material heats up. Unlike the non-existent ether, the electromagnetic field is simply the sphere of influence of a charged particle, and not a tangible, material thing.
Although it seems contradictory, EMR can behave as waves or as particles, depending on the type of experiment that is carried out. The wave-like nature of particles and the particle-like nature of waves have given rise to a great deal of scientific debate and some mind-boggling ideas, but no overall consensus about what it actually means. Depending on how it is observed, this radiation can appear as a particle or an electromagnetic wave. The lasers used to record and play DVDs and audio CDs use light waves to write to and read from the discs. Later applications are radio, television, radar, cellular phones, gps, and all kinds of wireless applications, from remote control of television sets to internet access in public places.


Focus on a fixed point z, then in progressing time the two vectors E(z,t) and B(z,t) in the point z, grow to a maximum value, then shrink to zero, become negative, go to a minimum value, and grow again, passing through zero on their way to the same maximum value.
If we go at a fixed time a distance λ to the right or to the left we encounter the very same fields E and B.
So, it is not surprising that in the past different parts of the spectrum were discovered at different times and that electromagnetic radiation of different wavelengths is called by different names. Indeed, in special relativity E and c B can be transformed into one another by a Lorentz transformation of the electromagnetic field tensor, which shows their close relationship.
That is, E and B are simultaneously zero and attain simultaneously their maximum and minimum. The quantity is the energy density of the EM field and the time derivative is the rate of increase (power density). First one must assume what the shape is of the waves emitted by the antenna, are they spherical or cylindrical?
The coil provides short voltage surges to the sphere charging once positive, the other negative. Em waves are radiated at this frequency as a result of the oscillation (and hence acceleration) of free charges in the loop.
You wouldn't be stuck with the cricket or Kyle Sandilands, you could tune into anything from TV shows to sunshine and gamma-ray bursts. Keep increasing the energy and you'd be able to use those waves as a torch (visible light) before they started shining right through things (x-rays) and eventually, when they're at the other end of the energy spectrum, they'd be deadly gamma-rays. But don't get too cocky; we might make higher energy radiation than our appliances, but we don't pump out nearly enough of the stuff to do anything more useful than hug with it.
Light can also bend around an obstacle, like ocean breakers around a harbor wall: this is known as diffraction. It was apparent that light, and other forms of EMR, did not require any medium and could travel through empty space. X-rays and gamma rays pack much more of a punch: they can break chemical bonds and knock electrons from atoms, forming ions. This is what happens in a fluorescent tube: mercury atoms are brought into an energetically high state by collisions with electrons, and upon subsequent falling down to their lowest energy state they emit photons.
Electric component (red) in plane of drawing; magnetic component (blue) in orthogonal plane. If waves of different wavelengths are superimposed, then a non-monochromatic wave is obtained (the term multi-chromatic wave would be apt, but is never used).
The vector S, Poynting's vector, is the power flux, the amount of energy crossing unit area perpendicular to the vector, per unit time. A spark is generated between the spheres when the voltage between them reaches the breakdown voltage for air.
Hertz was able to detect these waves using a single loop of wire with its own spark gap (the receiver) this loop, placed several meters from the transmitter, has its own effective inductance, capacitance, and natural frequency of oscillation.
You'll get hit by way more visible light waves than radio waves each second because visible light is higher energy, and higher energy radiation has a higher frequency. Therefore, the frequency depends entirely on the wavelength: the shorter the wavelength, the higher the frequency. It also applies to subatomic particles, whole atoms and even quite large molecules, all of which can sometimes behave as waves. In science, our knowledge of the universe comes largely from analysis of light, radio waves and X-rays from distant stars and galaxies. The risk to patients from medical X-rays is considered negligible, but radiographers, who are exposed to them regularly, wear lead aprons — which X-rays cannot penetrate — to protect themselves. When we now plot E and B in the fixed point z as a function of time t, we see the same type (sine-type) function as in figure 1. As the air in the gap is ionized, it conducts, more readily and the discharge between the spheres becomes oscillatory.
Sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter.
And electrons vibrating in radio transmitters give off the low energy radiation we tune into. Ultraviolet light, present in sunlight, can cause sunburn and can also cause skin cancer if exposure is excessive.
The number of times per second that the vectors go through a full cycle is the frequency of the electromagnetic wave. From an electrical circuit viewpoint, this is equivalent to an LC circuit, where the inductance is that of the loop and the capacitance is due to the spherical electrodes. Thus, Hertz demonstrated that the oscillating current induced in the receiver was produced by em waves radiated by the transmitter. Radio waves have got the longest wavelengths, ranging from a few metres to waves that are kilometres long. When a steady state is reached (some time after the beginning of the transmission), the time derivative of vanishes. It's that range of wavelength and frequency that means we can fit lots of different radio and tv stations in.



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