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By the mid 1800s, physicists were well aware of the force experienced by moving charges in electric and magnetic fields. When the cathode ray tube was invented in 1855, they used their knowledge to investigate a strange new phenonemenon known as cathode rays. electricity for refrigeration heating and air conditioning 9th edition answers After some thirty years of debate about whether cathode rays were particles or waves, Thomson used a cathode ray tube and magnetic and electric fields to deduce that they were actually a stream of negatively charged particles now known as electrons. In this important experiment, he basically discovered the electron and measured the ratio of its mass to charge by balancing the electric and magnetic force on a cathode ray beam. This disovery had many important applications including the development of the cathode ray tube which was used in early televisions and computer screens.

Charged particles will always experience a force when placed in a magnetic field if they are moving in a direction that is NOT parallel to the field lines. The force will increase in proportion to the velocity, the magnetic field strength and the size of the charge. The force will be a maximum when the charge’s velocity vector is perpendicular to the magnetic field lines.

Note that Crookes also believed that cathode rays could be deflected by an electric field but never succeeded in demonstrating this experimentally. electricity trading strategies It was Thompson in 1897 that showed definitively that cathode rays were particles when he demonstrated deflection in an electric field. When Crooke’s did the experiment, the technology was not sufficiently developed to show deflection, as his tubes could not be evacuated to a low enough pressure. At these higher pressures, electrons were ionising the gas in the tube and the positive and negative ions were forming layers at the sides of the tube and shielding the electrons from the effects of the external electric field.

Some of the properties above suggested to physicists that cathode rays were a wave similar to light. For instance, they produced energy, they travelled in straight lines, they produced similar chemical reactions to those produced by light and they were not deflected by electric fields. Yet cathode rays were deflected by a magnetic field as if they were negatively charged particles. This apparently inconsistent behaviour of cathode rays led to much controversy over whether the rays were a stream of negatively charged particles or a form of EM wave like light.

By the end of the 19th Century, the argument in favour of cathode rays being charged particles had become much stronger. By then, it had been shown by Eugen Goldstein that the rays could be deflected by electric fields and by Jean Perrin that the charge on the rays was negative. The final piece of evidence was provided by Thompson in a brilliant experiment conducted in 1897.

Karl Braun invented a cathode ray tube with a fluorescent screen making up one end of the tube in 1897. Shortly after this, Thomson used this technology to perform a series of experiments to determine the true nature of cathode rays. Having established that cathode rays were negatively charged particles, Thomson analysed his data to determine exactly what these particles were. At this point it is important that you understand that no subatomic particles were known to exist.

To investigate the particles making up the cathode ray, Thomson made measurements of the charge to mass ratio of cathode rays. static electricity zap Thomson’s method used both electrostatic and magnetic deflection of the cathode rays to determine the charge to mass ratio for the particles that made up the cathode rays. The apparatus shown in the following diagrams could be set up so that it created a magnetic field that was perpendicular to both the electric field and the trajectory of the cathode rays and effectively cancel these two forces.

Thomson assumed the cathode ray to be made up of particles with a negative charge. He then considered the cathode ray beam to be made up of particles of mass m and charge e, travelling at velocity v. gasbuddy map He then considered what happened when the beam passed through an electric field in the region between the charged plates and through a magnetic field.

To work out the velocity, Thomson created an experiment where a beam of cathode rays simultaneously passed through an electric and magnetic field. He adjusted B so that the beam was undeflected as shown in the diagram on the right, creating a situation where B and E were equal and opposite. Since both E and B were known or calculable quantities, the velocity of the beam could be determined using the equations on the right.

A cathode ray oscilliscope contains a cathode ray tube as its main component. The important parts of a cathode ray tube include the electron gun, the deflecting plates and the fluorescent screen. In the electron gun the heating filament heats the cathode, releasing electrons by thermionic emission. A number of electrodes are used to control the brightness of the beam, to focus the beam and accelerate the electrons along the tube. Electrons are negatively charged particles and the positively charged anode develops a strong electric field that exerts a force on the electrons, accelerating them along the tube.

Regulating the number of electrons striking the screen controls the brightness of the display. electricity through wood This is achieved using a grid situated between the anode and the cathode that is at a more negative voltage than the cathode. The grid repels electrons, reducing the number reaching the anode and decreasing the number hitting the screen. Increasing the negative voltage at the grid with respect to the cathode will, therefore, decrease the brightness.

The electron gun contains two anodes and the beam is focused by changing the positive voltages on the anodes. Two sets of parallel deflecting plates are charged to produce an electric field that can deflect the beam of electrons separately, up or down or left or right. electricity lessons grade 6 These fields are used to move the beam so that the electrons can be directed to all points on the fluorescent screen. The glass screen is coated with layers of fluorescent material. It emits light when high-energy electrons strike it.

A cathode ray oscilloscope (CRO) this system to create a waveform on a screen. A CRO uses a cathode ray to display a variety of electrical signals. The horizontal direction is usually provided by a time base (or sweep generator) which allows the voltage (on the vertical axis) to be plotted as a function of time (on the horizontal axis). This enables complex waveforms or very short pulses to be displayed and measured. The voltage input is amplified and the signal directed towards the vertical deflection plates. no electricity jokes The horizontal time sweep signal is referred to the horizontal deflection plates provide a voltage that drives the beam across the screen at a constant rate. Both sets of plates work together to produce a waveform on the fluorescent screen.

Older style televisions or computer screens work in basically the same way as a cathode ray oscilliscope but with some notable differences. An electron gun still produces a cathode ray beam but in a television the beam is deflected by electromagnets or coils of wire which produce a magnetic force rather than an electrical one. Colour televisions also contain three electron guns which each focus their electron beams on one of three little dots on the back of the screen, one for red, one for green, and one for blue. They chose these colours because together, red, green, and blue create white light, and, in different combinations, can create any colour.

Standard televisions use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second but only half of the lines are painted per frame. The beam paints every other line (odd numbered lines) as it moves down the screen. gas 1940 hopper Then, the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times per second. Most computer monitors use progressive scanning because it significantly reduces flicker. Because the electron beam is painting all 525 lines 30 times per second, it paints a total of 15,750 lines per second. Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on).

When a television station wants to broadcast a signal to your television, or when your DVD wants to display the movie on a disc on your television, the signal needs to mesh with the electronics controlling the beam so that the television can accurately paint the picture that the television station or DVD sends. The television station or VCR therefore sends a well-known signal to the television that contains three different parts: