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Cathode Ray Experiment

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Last updated date: 27th Mar 2024
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What are Cathode Rays?

Cathode rays are a beam of negatively charged electrons traveling from the negative end of an electrode to the positive end within a vacuum, across a potential difference between the electrodes.


How Do the Cathode Rays Work?

The cathode is a negative electrode, Anode is the positive electrode. Since electrons are repelled by the negative electrode, the cathode is the source of cathode rays inside a vacuum environment. When a potential difference is applied, the electrons jump to an excited state and travel at high speeds to jump back-and-forth inside the vacuum glass chamber and when some cathode rays certain molecules of the cathode screen, they emit light energy. A wire is connected from anode to cathode to complete the electrical circuit.


Construction of a Cathode Ray Tube

Its Basic Components are: -


  • Electron Gun Assembly: - It is the source of the electron beams. The electron gun has a heater, cathode, pre-accelerating anode, focusing anode and accelerating anode.

  • Deflecting Plates: - They produce a uniform electrostatic field only in one direction, and accelerate particles in only one direction.

  • Screen: - The inner layer of the screen is coated with phosphorus, and produces fluorescence when cathode rays hit the screen by a process of phosphorus excitation.

  • Aquadag: - It is an aqueous solution of graphite used to collect the secondary emitted electrons which are required to keep the cathode ray in electrical equilibrium.


What is the Cathode Ray Tube Experiment?

In 1897, great physician J.J. Thompson, conducted his first cathode ray tube experiment to prove that rays emitted from an electron gun are inseparable from the latent charge. He built his cathode ray tube with a metal cylinder on the other end. The metal had two small diversions(slits), leading to an electrometer that could measure a small electric charge. From the first experiment, he discovered that the electrometers stopped measuring electric charge. From this, he deduced that the electric charge and the cathode rays must be combined and are the same entity.


Then he conducted a Second experiment, to prove the charge carried by the cathode rays was negative or positive. Now, he put a negatively charged metal plate on one side of the cathode rays to go past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the Cathode Ray Tube, he used a fluorescent coated tube that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent away from the negative plate and towards the positive plate. This proved that the cathode rays were negatively charged.


Then he performed the third experiment, to know the nature of the particles and reduce the mass of the particles as they had too small of a mass to be calculated directly. For the experiment, he used the cathode ray tube and with a high applied potential difference between the two electrodes, with the negatively charged cathode producing the cathode rays. He had already deduced that the particles were negatively charged. Firstly, he applied an electric field in the path between anode and cathode and measured the deflections from the straight path. Now he applied a magnetic field across the cathode ray tube by using an external magnetic field. The cathode ray is deflected by the magnetic field. Now he changed the direction of the external magnetic field and found that the beam of electrons is deflected in the opposite direction. From this experiment, he concluded that the electrostatic deflection is the same as the electromagnetic deflection for the cathode rays and he was able to calculate the charge to mass ratio of the electron.


After these three experiments, he deduced that inside the atom there consist of a subatomic particle, originally named ‘corpuscle’, then changed to ‘electron’ which is 1800 times lighter than the mass of hydrogen atom (Lightest atom).

 

Formula Used

The derivation of the formula used to calculate the charge to the mass ratio:


For Electric Field the force on a particle is

Force(F)=Charge(Q)*Electric field(E) ---<1>

For Magnetic Field the force on a particle moving with velocity is:


F=q*velocity(v)*Magnetic Field(B) ---<2>

From 1 and 2 we get,

V=E/B ----<3>

From the definition of Force,

Acceleration(a)= Force(f)/mass(m) ----<4>

Combining 1 and 4

a=q*E/m ----<5>

From Newton’s law Of motion, vertical displacement is:

Y= (1/2)*a*t*t ----<6>

From 5 and 6

q/m=(2*y*v*v)/x*x*E


Cathode Ray Tubes (CRT) 

The cathode ray tube (CRT) is a vacuum tube, in which electrons are discharged from the cathode and accelerated through a voltage, and thereby gains acceleration of some 600 km/s for every volt. These accelerated electrons collide into the gas inside the tube, thereby allowing it to glow. This enables us to see the path of the beam. Helmholtz coil, a device for producing a region of nearly uniform magnetic field, is also used to apply a quantifiable magnetic field by passing a current through them.


A magnetic field will cause a force to act on the electrons which are perpendicular to both the magnetic field and their direction of travel. Thus, a circular path will be followed by a charged particle in a magnetic field. The faster the speed of a charged particle in a magnetic field, the larger the circle traced out in a magnetic field. Contrarily, the larger the magnetic field needed for a given radius of curvature of the beam. The paths of the electrons are distorted by the magnet in CRT Tv when they are brought near the screen. The picture on the screen appears when the electrons accurately hit phosphors on the back of the screen. Because of this, different colors of light are emitted on the screen when the electrons are impacted. Hence, the electrons are forced to settle in the wrong place, thereby causing the distortion of the image and the psychedelic colors.


Postulates of J.J. Thomson’s Atomic Model

After the Cathode ray tube experiment, Thomson gave one of the first atomic models including the newly discovered particle. 


His model stated: -

  1. An atom resembles a sphere of positive charge with a negative charge present inside the sphere.

  2. The positive charge and the negative charge were equal in magnitude and thus the atom had no charge as a whole and is electrically neutral.

  3. His model resembles a plum pudding or watermelon. It assumed that positive and negative charge inside an atom is randomly spread across the whole sphere like the red part of the watermelon (positive charge) and the black seeds (negative charge).

 

Practical Uses of Cathode Ray Tube Experiment

In ancient times, the cathode ray tubes were used in the beam where the electron was considered with no inertia but have higher frequencies and can be made visible for a short time.


Many scientists were trying to get the secrets of cathode rays, while others were in search of the practical uses or applications of cathode ray tube experiments. And the first search was ended and released in 1897 which was introduced as Karl Ferdinand Braun’s oscilloscope. It was used for producing luminescence on a chemical affected screen in which cathode rays were allowed to pass through the narrow aperture by focusing into the beams that looked like a dot. This dot was passed for scanning across the screen which was represented visually by the electrical pulse generator. 


Then during the first two to three decades of the twentieth century, inventors continued to search the uses of cathode ray tube technology. Then inspired by Braun's oscilloscope, A. A. Campbell advised that a cathode ray tube would be used for projecting video images on the screen. But, this technology of the time did not get matched with the vision of Campbell-Swinton. It was only until 1922, when Philo T. Farnsworth developed a magnet to get focused on the stream of electrons on the screen, for producing the image. Thus, the first kind of it, Farnsworth, was quickly backed up by Zworykin’s kinescope, known as the ancestor of modern TV sets.


Nowadays, most image viewer devices are made with the help of cathode ray tube technology including the guns of electrons which are used in huge areas of science as well as medical applications. One such use for cathode-ray tube research is the microscope invented by Ernst Ruska in 1928. The microscope based on electrons uses the stream of electrons to magnify the image as the electrons have a small wavelength which is used for magnifying the objects which are very small to get resolved by visible light. Just like Plucker and Crookes work, Ernst Ruska used a strong field of magnetic lines for getting it focused on the stream of electrons into an image.


Solved Example:  

Question: The charge of an electron e=1.602∗10−19 and its is mass m=9.11∗10−31. The stopping potential of an electron traveling in a cathode ray tube is V=5V. Find the velocity of an electron traveling (where charge of an electron e=1.602∗10−19 and mass m=9.11810−31).


Answer: Here we need to find the velocity of traveling electrons using the given stopping potential.


We know that eV=12mv2, the charge(e) and mass(m) of the electron is also given as,

e=1.602∗10−19 and m=9.11∗10−31

By substituting the values of e, m, V.(1.602∗10−19)(5)

=12(9.11∗10−31)(v2)v2

=(1.602∗10−19)(5)(2)9.11∗10−31v

=1.33∗106m/s             

FAQs on Cathode Ray Experiment

1. What is the procedure of the Cathode Ray Experiment?

The apparatus of Cathode Ray Experiment is arranged in such a way that the terminals have high voltage with the internal pressure, which is reduced by removing the air inside the CRT. Because of the high voltage in the terminal,  the partial air inside it is ionized and hence gas becomes the conductor. The electric current propagates as a closed-loop circuit. In order to recognize and measure the ray produced, a dipole is set up. The cathode rays will begin deflecting and repel from the dipole and move towards the anode because of the dipole. The phosphorescent substance is arranged in such a way that the rays strike the substance. And hence, it causes small sparks of light, which detects the stream of rays.

2. What are Cathode ray tubes?

Cathode ray tubes (CRT) are a presentation screen that produces pictures as a video signal. Cathode ray tubes (CRT) is a type of vacuum tube that displays pictures when electron beams from an electron gun hit a luminous surface. The CRT produces electron beams, accelerates them at high speed, and thereby deflecting them to take pictures on a phosphor screen. Electronic presentation gadgets being the most established and least expensive electronic presentation innovation, were initially made with CRTs. CRTs work at any aspect ratio, at any resolution, and geometry without the need to resize the picture. CRTs work on the principle of an optical and electromagnetic phenomenon, called cathodoluminescence.

3. What are the applications of Cathode ray tubes?

The following are the applications of Cathode ray tubes.

  • The main components of a cathode ray tube (CRT) includes A Vacuum tube holding an electron cannon and a screen lined with phosphors.

  • The technology of Cathode ray tubes is used by Televisions and computer monitors. Three electron cannons correlate to corresponding types of phosphors in color CRTs, one for each main color viz red, green, and blue.

  • Ancient computer terminals and black and white televisions are examples of monochromatic CRTs.

  • cathode ray tube (CRT) is also used in oscilloscopes, which are machines that display and analyze the waveform of electronic signals.

  • A cathode ray amusement device was the very first video game to be produced, which were used in old military radar screens.

4. What are the basic principles of the CRT?

There are three basic principles of the CRT as the following:

  • Electrons are released into a vacuum tube from very hot metal plates.

  • The released electrons are accelerated and their direction of movement is controlled by using either a magnetic field from a coil that is carrying an electric current or by a voltage between metal plates.

  • A high-velocity beam of electrons hits some materials such as zinc sulfide. The spot is created on the fluorescent screen, and it causes material, called a phosphor, to glow, giving a spot of light as wide as the beam.

5.  How to understand the concept of the Cathode Ray Experiment easily?

Students can understand the concept of the Cathode Ray Experiment easily with the help of a detailed explanation of the Cathode Ray Experiment provided on Vedantu. Vedantu has provided here a thorough explanation of the Cathode Ray Experiment along with Cathode Rays, How Do the Cathode Rays Work, Construction of a Cathode Ray Tube, Postulates of J.J. Thomson’s Atomic Model, and Practical Uses of Cathode Ray Tube Experiment along with examples. Students can learn the concepts of all the important topics of Science subject on Vedantu.