Last updated on Wednesday, May 22nd, 2024
In e/m Thomson Method experiment, we determine the specific charge value e/m ratio by the Thomson method. Cathode ray tube is the main component of this experiment. As you know cathode ray tube has three components first is electron gun second is deflector plates for horizontal and vertical deflection and the third part is a fluorescent screen.
Objective : e/m (charge to mass) ratio of the electron is determined using the cathode ray tube in this experiment
e/m Thomson Method Experiment
The electron gun comprises a filament. Which on heating provide thermal energy to the cathode. As a result, electrons emitted from the cathode surface. Which further moves towards the anode, the potential difference between the cathode and anode decide the kinetic energy. The deflecting plates are connected with the positive terminal horizontally and vertically.
So providing the potential to these plates one can deflect the beam in any direction. In the (e/m ratio) Thomson method, there are two processes to determine the charge to the mass value of the electron (e/m ratio). In one case we use the two uniform circular coils while in the second one the two bar magnets.
e/m Thomson Method Experiment Viva Questions:
The two bar magnets are used in this experiment. So how to originate the formula a complete theory is explained.
1Q. What Is Cathode Ray Tube (CRT)?
A cathode ray tube (CRT) is a device used to display images on electronic devices such as computer monitors, televisions, and oscilloscopes. The CRT works by using an electron beam to produce an image on a phosphorescent screen.
Inside the CRT, there are several key components: a cathode (which emits electrons), an anode (which accelerates the electrons), a control grid (which controls the speed of the electrons), and a phosphorescent screen (which produces light when struck by the electrons).
When the CRT is turned on, a high voltage is applied between the cathode and anode, creating an electric field that accelerates electrons from the cathode towards the screen. The control grid modulates the speed of the electrons, allowing them to form a specific pattern on the screen, which creates an image.
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CRT technology was widely used in the past for various applications, such as in televisions and computer monitors, but it has largely been replaced by LCD and LED technology, which is more efficient and produces better image quality.
2Q. What is electron gun in CRT?
It is a device that generates a beam of high-energy electrons that are directed towards the screen of the CRT. The electron gun consists of a heated cathode, one or more control grids, and an anode.
When the cathode is heated, it releases a cloud of electrons through a process called thermionic emission. The control grids are used to focus and accelerate the electrons towards the screen of the CRT. The anode is used to maintain a high voltage between the cathode and the control grids, which accelerates the electrons towards the screen.
The shape and size of the electron beam can be controlled by adjusting the electric fields produced by the control grids. By manipulating the electric fields, the electron beam can be made to scan across the screen in a specific pattern, which allows it to create an image.
3Q. Why do we use filament in CRT?
The filament in a cathode ray tube (CRT) is used to heat the cathode, which is a key component of the electron gun. The cathode is responsible for emitting electrons through a process called thermionic emission.
The filament is typically made of tungsten and is heated by an electrical current passing through it. As the filament heats up, it emits a stream of electrons into the vacuum inside the CRT. These electrons are then accelerated towards the screen by a high voltage applied between the cathode and the anode.
4Q. How electrons emit from the cathode?
Electrons are emitted from the cathode in a process called thermionic emission. In thermionic emission, a material (in this case, the cathode) is heated to a high temperature, causing electrons to escape from the surface of the material.
The cathode in a cathode ray tube (CRT) is typically made of a material that has a low work function, which means that it requires relatively little energy to remove an electron from the surface of the material. When the filament in the CRT is heated, it emits a stream of electrons into the vacuum inside the CRT. These electrons are then attracted to the positively charged anode, which accelerates them towards the screen.
5Q. How do you accelerate electrons in CRT?
Electrons are accelerated in a cathode ray tube (CRT) by applying a high voltage between the cathode and the anode. The anode is typically a positively charged plate that is located near the screen of the CRT, while the cathode is located at the opposite end of the tube.
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6Q. What is the role of cathode and anode in CRT in reference to the emission of electrons?
The cathode and anode in a cathode ray tube (CRT) play crucial roles in the emission and acceleration of electrons that create the image on the screen.
The cathode is a negatively charged electrode located at one end of the tube. When heated, it emits a stream of electrons into the vacuum inside the tube through a process called thermionic emission. These emitted electrons constitute an electron beam that travels towards the screen.
The anode, on the other hand, is a positively charged electrode located near the screen. It accelerates the electron beam towards the screen by applying a high voltage between the cathode and the anode.
Deflecting Section of CRT
7Q. How deflecting plates affect the path of electron beams?
Deflecting plates in a cathode ray tube (CRT) affect the path of the electron beam by applying an electric field that changes the direction of the electrons as they travel towards the screen.
In a CRT, the electron beam is normally directed towards the center of the screen, but deflecting plates can be used to move the beam horizontally or vertically. The deflecting plates consist of two charged plates that are placed perpendicular to the direction of the electron beam. When a voltage is applied to the plates, an electric field is generated that deflects the electrons as they pass between the plates.
By varying the voltage applied to the deflecting plates, it is possible to control the amount and direction of deflection. This allows the electron beam to be directed to different locations on the screen, creating an image.
8Q. Does electron beam deflects by the electrodes only?
The path of the electron beam in a CRT can be affected by a variety of components, including the deflecting plates, electrostatic lenses, magnetic fields, and the screen itself.
9Q. What is the formula for force on a charged particle in the presence of the electric field?
The formula for the force (F) on a charged particle in the presence of an electric field (E) is given by:
F = qE
where:
q is the charge of the particle
E is the strength of the electric field
10Q. What is the formula for force on charged particles in the presence of the Magnetic field?
The formula for the force (F) on a charged particle in the presence of a magnetic field (B) is given by:
F = q(v x B)
where:
q is the charge of the particle
v is the velocity of the particle
x represents the cross product operation between the velocity vector and the magnetic field vector
This formula is known as the Lorentz force law and describes the force that a charged particle experiences in a magnetic field
11Q. In the presence of a uniform magnetic field when the electron beam enters into the field perpendicularly, what type of path electron beam will follow?
When an electron beam enters a uniform magnetic field perpendicularly, the path of the electron beam will follow a circular path. This is because the magnetic field exerts a force on the moving electrons, which is perpendicular to both the velocity of the electrons and the direction of the magnetic field. The force acts as a centripetal force, causing the electrons to move in a circular path around the axis of the magnetic field.
The radius of the circular path is given by the formula:
r = (mv)/(qB)
where:
r is the radius of the circular path
m is the mass of the electron
v is the velocity of the electron
q is the charge of the electron
B is the strength of the magnetic field
As the electrons move in a circular path, they continue to emit radiation due to the acceleration caused by the magnetic field. This radiation is known as synchrotron radiation and is used in various applications such as particle accelerators and synchrotron light sources.
12Q. How centripetal force contribute in the question 11?
In the case of a charged particle moving in a uniform magnetic field, the force due to the magnetic field acts as a centripetal force that causes the particle to move in a circular path.
The centripetal force contributes to the circular motion of the charged particle by constantly changing the direction of its velocity, while the magnetic field provides the necessary force to keep it moving in a circle. The combination of the two forces results in the circular motion of the charged particle.
13Q. How to determine one the radius r in the centripetal force formula, what is a concept?
The radius (r) in the centripetal force formula is determined by the following formula:
r = mv / qB
where:
- m is the mass of the charged particle
- v is the velocity of the charged particle
- q is the charge of the charged particle
- B is the strength of the magnetic field
This formula is derived from the equation for the Lorentz force law, which describes the force experienced by a charged particle moving in a magnetic field. The force acts as a centripetal force, causing the particle to move in a circular path.
To determine the radius of the circular path, we can set the magnetic force equal to the centripetal force:
Fmag = Fcp
qvB = mv^2 / r
Solving for r, we get:
r = mv / qB
Therefore, the radius of the circular path is determined by the mass, velocity, charge, and strength of the magnetic field.
14Q.Does forward or reverse biasing can affect the path of the electron beam?
Yes, the path of an electron beam in a CRT can be affected by forward or reverse biasing of the electrodes.
In a CRT, the electrons are accelerated by a high voltage applied between the cathode and the anode. This voltage creates an electric field that accelerates the electrons towards the anode. The electrons then pass through a set of deflecting plates that can be selectively charged to deflect the beam in different directions.
If the anode or deflecting plates are forward-biased (i.e., connected with positive voltage), they will repel the positively charged ions that are produced when the electrons collide with the anode or the deflecting plates. This can cause the ions to deflect the electron beam, leading to distortion or blurring of the image on the screen.
The screen of Cathode Ray Tube
15Q. What type of material produces the light on the screen of CRT when electrons strike?
In a CRT, the screen is coated with a phosphor material that produces visible light when it is struck by electrons.
The phosphor coating on the screen of a CRT is usually made of a mixture of different phosphors to produce a full-color image. The phosphors are often arranged in small dots or stripes called “pixels” that can be selectively excited by the electron beam to create different colors and shades.
16Q. Does magnetic coil is already built in Cathode Ray Tube?
or
Does magnetic field source is inbuilt in CRT to produce the magnetic field or we use it from externally?
Yes, a magnetic coil is typically built into a cathode ray tube (CRT) as part of the electron beam deflection system.
17Q. Suppose you deflected the electron beam on screen by some distance, can you bring at original position by the external electric or magnetic field at the original position?
Yes, it is possible to bring the deflected electron beam back to its original position on the screen of a CRT using an external electric or magnetic field.
If the electron beam has been deflected by a set of charged deflecting plates, for example, it can be brought back to its original position by applying an electric field in the opposite direction. This can be done by adjusting the voltage applied to the deflecting plates, or by adding additional plates with the opposite charge to cancel out the deflection.
Similarly, if the electron beam has been deflected by a magnetic field, it can be brought back to its original position by applying a magnetic field in the opposite direction. This can be done by adjusting the current in the magnetic coil or by adding additional coils with the opposite polarity.
18Q. In reference to 17 th question which field is more suitable to handle this problem, electric or magnetic and why?
The choice of which field to use to correct the deflection of the electron beam in a CRT would depend on the cause of the deflection.
If the deflection is due to the presence of electric charges, then an electric field would be the most suitable to correct the deflection. For example, if the deflection is caused by a charged particle or a set of charged plates, then an electric field in the opposite direction can be used to cancel out the deflection.
19Q. What is the standard value of charge to mass ratio of the electron in JJ Thomson method?
The standard value of the charge to mass ratio of the electron determined using J.J. Thomson’s cathode ray tube experiment is approximately 1.76 x 10^11 C/kg.
20Q. What formula you have used to determine the charge to mass ratio of the electron and what is the origin of it?
The formula used by J.J. Thomson to determine the charge to mass ratio of the electron in his cathode ray tube experiment is:
e/m = B2 d2 / 2V r2
where e/m is the charge to mass ratio of the electron, B is the magnetic field strength, d is the distance between the plates that deflect the electron beam, V is the accelerating potential, and r is the radius of curvature of the electron beam.
e/m Thomson method experiment-Magnetic Field Determination
21Q. Are you using Helmholtz’s Coils or permanent bar magnets in this experiment to balance the electric force on the electron?
Yes!, bar magnet
22Q. If you are using Helmholtz’s Coils how you will determine the magnetic field, what is the formula used for it?
Helmholtz coils are a type of electromagnetic device that can produce a uniform magnetic field within a defined region of space. The magnetic field strength produced by Helmholtz coils can be calculated using the following formula:
B = (8μnI / 5)1/2
where B is the magnetic field strength in tesla (T), μ is the permeability of free space (4π x 10-7 T m/A), n is the number of turns per coil, and I is the current flowing through the coil in amperes (A).
To measure the magnetic field strength produced by the Helmholtz coils, a magnetometer or gaussmeter can be used. These devices measure the magnetic field strength at a specific point in space, and can be used to verify that the field is uniform and has the expected strength.
23Q. If you are using a bar magnet then how?
Using Gaussmeter
24Q. Does earth’s magnetic field depend on the latitude and altitude?
Yes, the strength and direction of the Earth’s magnetic field vary with both latitude and altitude.
At the equator, the Earth’s magnetic field is relatively weak, with a strength of around 30-60 microtesla (μT). As you move towards the poles, the field strength increases, reaching a maximum of around 60-70 μT near the magnetic poles. This variation in field strength with latitude is due to the fact that the Earth’s magnetic field is generated by the motion of molten iron in its core, which is influenced by the rotation of the Earth.
25Q. How do you will calculate the earth’s magnetic field at your place?
To calculate the Earth’s magnetic field at a specific location on Earth, you would need to take several measurements using a magnetometer or gaussmeter. These instruments measure the strength and direction of the local magnetic field, which can be influenced by a variety of factors, including the Earth’s core, the atmosphere, and nearby geological features.
26Q. Does the magnetic compass in the mobile phone can tell the approximate value of the earth’s magnetic field?
Yes, the magnetic compass in a mobile phone uses the Earth’s magnetic field to determine the phone’s orientation relative to magnetic north. The compass contains a small magnetometer that can detect changes in the Earth’s magnetic field and convert them into a digital reading of the phone’s orientation.
27Q. What are the unit and approximate value of the earth’s magnetic field?
The unit of the Earth’s magnetic field is tesla (T) or its subunit microtesla (μT). The approximate value of the Earth’s magnetic field at the surface ranges from about 25 to 65 microtesla (μT), depending on the location and the local geological features.
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