Electric and Magnetic Fields in Space
Key Terms
Action of Electric and Magnetic Fields on Matter
Accelerating electrons, part of every atom that makes up the dish, are the source of radio and television waves.
Therefore it is imperative to comprehend some of the properties of electrons.
Being that the mass of an electron is too small to measure, the only possible procedure is to find the charge-to-mass,
q/m, by balancing the forces of an electric and magnetic field on an electron. It was first measured with a cathoderay tube
in 1897, where all air is removed from the glass tube. An electric field pulls electrons out of the negatively-charged
cathode and accelerates them toward the positively-charged anode. Some of the electrons pass through a hole in the anode
and travel in a narrow toward a fluorescent screen. The screen glows at the point where electrons hit.Below is a picture
displaying a cathoderay
Electric and magnetic fields in the center of the tube exert forces on the electrons. Charged parallel plates produce a uniform electric field perpendicular to the beam. The electric field intensity, E, produces a force,qE, on the electrons that deflects them upward. Two coils produce a magnetic field at right angles to both the beam and the electric field. Remember that the force exerted by a magnetic field is perpendicular to the field and to the direction of motion of the particles.The magnitude of the force exerted by the magnitude field is equal to Bqv (B is magnetic field strength and v is the electron velocity).
The electric and magnetic fields are adjusted until the beam of electrons follow a straight, or undeflected, path. Then the forces due to the two fields are equal in magnitude and opposite direction. The equation used is Bqv=Eq.
If the electric fields is turned off, only the force de to the magnetic field remains. The magnetic force acts perpendicular to the direction of motion of the electrons, causing a centripetal acceleration. The electrons follow a circular path with radius r. Newton's second law gives: Bqv = m(v2/r).
When solving for q/m you get:
Mass of the Electron
The Mass Spectometer
The two dots on the screen which are two values of q/m, that were discovered when a British physicist J.J Thomson put into his tube, proved that atoms could have same chemical properties but have different masses; thus showing the existence of isotopes. With a mass spectrometer the masses of positive ions can be measured precisely. It creates an ion from an atom or molecule, then measures the mass/charge ratio.
Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio (m/z) of ions. It is most generally used to find the composition of a physical sample by generating a mass spectrum representing the masses of sample components. A mass spectrometer is a device used for mass spectrometry, and produces a mass spectrum of a sample to find its composition. This is normally achieved by ionizing the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. A typical mass spectrometer comprises three parts: an ion source, a mass analyzer, and a detector.(Ref:The Wikipedia Encyclopedia)
Regularly, ions pass through electric and magnetic deflecting fields to select ions with a specific velocity. However, those that go through undeflected move into a region with a uniform magnetic field. The equation r = mv/qB, from the equation Bqv = mv2/r, is used to find the circular path with a radius that they follow.
The velocity of the ion can be found from:
Substituting this expression for v in the previous equation gives the radius of the circular path:
r = (1/B)(2Vm/q)1/2.
From this equation, the charge-to-mass ratio of the ion is found to be:
Mass spectrometers can be used to seperate isotopes of atoms such as uranium, it is often used by chemists as a very sensitive tool to find small amounts of molecules in samples, even small as one molecule in 10 billion molecules. Many dangerous contaminants in the environment have been detected with this device.
Electric & Magnetic Fields in Space
Althouh you probably do not realize it, you rely on electromagnetic waves everyday. Signal broadcasr from television and radio stations, orbiting satellites, and even those emanating from distant galaxies are all electromagnetic waves. They are also used in common comsumer products such as microwave ovens, remote-control garage door openers, and cellular phones.
In 1860, a Scottish physicist James Maxwell postulated that the opposite of induction is also true; that a changing electric field produces a changing
magnetic field. He also suggested that charges are not necessary- a changing electric field alone would produce the magnetic field. He then predicted that both
accelerating charges and changing magnetic fields would produce electric and magnetic fields that move through space.
A combined electric and magnetic field that travels through space is an electromagnetic wave, or EM wave.
An antenna, which is a wire designed to transmit or receive electromagnetic waves, is connected to an alternating current (AC) source. The AC source produces.
a varying potential difference in the antenna that alternates at the frequency of the AC source, it propagates away from the antenna. If it were possible to see
invisible electromagnetic waves approaching, the changing fields would appear as the figure bellow. The electric field oscilates up and down, while the magnetic field
oscillates at right angles to the electric field. Both of the fields are at right angles to the wave direction. Note that an electromagnetic wave produced by an antenna is polarized; that is, its electric field is parallel to the antenna's conductor.
The most common method of generating waves of higher frequencies is to use a coil and capacitor connected in a series circuit. If the capacitor is charged by a battery,
the potential differenc across the capacitor creates an electric field.The storede electrons flow throught the coil and create a magnetic field when the battery is removed; after it is discharged, the coil collapses. A back-EMF develops that recharges the capacitor, but in the opposite direction this time.
Once again it discharges and the cycle continues. The number of oscillations each second is called the frequency, which depends on the size of the capacitor & the coil. The antenna, connected across the capacitor, extends the fields of the capacitor into space.
A good model to help you comprehend a coil and capacitor circuit is a pendululm analogy. The electrons in a coil and capacitor are represented by the pendulum bob, which moves fastest when its displacement vertical is zero. When the bob is at its greatest angle, its velocity is zero & its displacement from
the vertical is largest. The model can be used when using energy. When displacement is greatest, potential energy of the pendulum is largest. When the velocity is greatest, the kinetic energy is largest. The sum of both potential & Kinetic energy results in constant total energy. The energy stored in a magnetic field
is greatest when the current is largest. However, the all energy is in the electric field & the electric field is largest when the current is zero. The sum of both magnetic field energy & electric field energy are constant. The electromagnetic waves carry this energy through space in the form of electric & magnetic fields through magnetic
radiation.
Just as the pendulum stops swinging when left alone, because of resistence in the curcuit, the oscillations in a coil & capacitor will die out as well if no energy is added. Gentle pushes, however will keep a pendulum moving moving.The largest amplitude swing occurs when the frequency of pushing is the same as the frequency of swinging.
Voltage pulses applied to the coil-capacitor circuit at the right frequency keep th oscillating going: one way of doing this is to add a second coil to form a transformer. The AC induced in the secondary coil is increased by an amplifier and added back to the coil and capacitor. This type of circuit can produce frequencies up to
approximatetly 100 MHz
Electromagnetic waves are caused by the acceleration of electrons in an antenna. When the electric field strikes another antenna, they accelerate the electrons in it. When the antenna is turned to the direction of the polarization of the wave, the acceleration is largest; or when it is parallel to the direction of the electric fields in the wave.
While a simple wire antenna can detect electromagnetic waves, several wires can be used to increase the detected EMF. Electric fields generated in the individual wires from constructive interferance patterns that increase the strength of the signal. Giant parabolic dishes focus waves with wavelengths of 2-6cm on the antennas held by the tripod
above the dish.
Radio and television waves are used to transmit information across space. Only if the information broadcasted is comprehended, then it is possible to select waves of a particular station. For this process to be a succes, the coil and capacitor circuit is to be connected to the antenna. The capitance is adjusted until the oscillation frequency of the circuit
equals the frequency of the desired wave; this would be the only frequency to cause significant oscillations of the electrons in the circuit. The information is then amplified and ultimately drives a loudspeaker. This combination is known as a receiver.
Roentgen could not determine how the radiation was carried through space or why it had such penetrating power. For this reason he called the radiation "X-rays." He took
the name from the mathematician's use fo "X" to mean the unknown quantity in a problem. The formal name given the radiation is Roentgen rays, in honor of the discoverer.
We still use the name X ray, even though scientists have known the answers to Roentgen's problems for many years.
It is now known that X rays are high frequency electromagnetic waves. They are produced when electrons are accelerated to high speeds by means of potential differences of
20 000 volts or more. When the electrons crash into matter, their kinetic energies are converted into X-rays.
Electrons are accelerated to these speeds in cathode-ray tubes, such as the picture tube in a television. When the electrons hit the faceplate, they cause the colored
phosphors to glow. The sudden stopping of the electrons can also produce X rays. The faceplate glass contains lead to stop the X rays and protect viewers.
Electromagnetic Waves
In 1821, while performing a demonstration for his students, Oersted noticed that an electric current caused the needle in a nearby compass to deflect.
Oersted realized that his observation displayed a fundamental connection between electricity and magnetism. He concluded that an electric current in a
conductor produces a magnetic field, and that a changing electric current produces a changing magnetic field.
Production of Electromagnetic Waves
An AC source connected to an antenna can transmit electromagnetic waves. The wave frequency is equal to the frequency of the rotating AC generator and is limited to about
1 kHz. The range of frequencies and wavelengths that make up all forms of electromagnetic radiation, electromagnetic spectrum.
Reception of Electromagnetic Waves
X-Ray
In 1895 Wilhelm Roentgen of Germany discovered a kind of radiation that could pass through solid matter. Furthermore it could make a photographic image of the interior
of the solid. The value of this discovery was recognized at once. As soon as manufacturers could produce the needed apparatus, doctors began using the radiation in many
ways. It helped in setting broken bones and locating embedded pieces of metal such as bullets. With experience, new uses were added constantly.
References
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