Name: electron speed selector in thomson’s tube
Uploaded: 2023-09-18T17:11:19.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Structure And Properties Of Matter

Atomic structure

Thomson's e/m Experiment

J. J. Thomson&#39;s experiment determined the ratio of charge to mass for electrons.
He used a setup with crossed electric and magnetic fields, such that only electrons with a specific speed, the ratio of the electric and magnetic fields, passed through it.
In an evacuated glass tube, electrons were accelerated from a hot cathode and transformed into a beam by the potential difference between the cathode and the two anodes.
The kinetic energy gained by the electrons equals the lost electric potential energy. So, he determined the electrons&#39; speed.
The velocity-selected electrons struck the fluorescent screen and made it glow.
Then, he solved the two equations to determine an electron&#39;s charge to mass ratio.
All quantities on the right-hand side are independent of the type of residual gas in the tube, the cathode material, or any other aspect of the experiment, implying that the electrons in the beam are a fundamental constituent of all matter.
The most precise calculated value of e/m is 1.759 x 1011 coulombs per kilogram.

thomson's e/m experiment

In a beam of charged particles created by a heated cathode, the particles move at different speeds. However, many applications need a beam with uniform particle speeds. An arrangement known as a velocity selector uses electric and magnetic fields to pick particles with a particular speed from the beam.
A particle with charge q, speed v, and mass m enters an area from the top, where the magnetic and electric fields are perpendicular both to the particle's motion and to one another. The magnetic field is into the plane of the paper, while the electric field E is to the left. If the charge q is positive, the magnetic force is towards the right with magnitude qvB, and the electric force is towards the left with magnitude qE. The two forces are equal in strength for the given field strengths E and B for a specific value of v, for which the total force is zero, given by
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J. J. Thomson (1856–1940) used this concept to determine the electron's charge-to-mass ratio in one of the classic physics experiments at the end of the 19th century. A voltage difference between the two anodes and cathodes in a highly evacuated glass container accelerates and shapes a beam of hot cathode electrons. The accelerating potential V determines the electrons' velocity v. The lost electric potential energy equals the acquired kinetic energy.
Only the electrons with a speed equal to E/B can pass straight through the crossed electric and magnetic fields and strike the fluorescent screen that glows at the point of impact.
The electron's charge-to-mass ratio is calculated to be
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Its value is determined as follows:
<img alt="Redox reaction mechanism, redox notation, chemical equations in equilibrium diagram." src="/files/ftp_upload/14197/14197_Equation_3.png" style="text-align: center; height: 34px;" data-altupdatedat="1761830439000" data-alt="Equation3">
The fact that Thomson only discovered one value for this number was an essential feature of his measurements. The cathode material, the remaining gas in the tube, or any other aspect of the experiment had no bearing on it. This independence demonstrated that the beam's constituent particles, now known as electrons, are a component of all matter. As a result, Thomson is credited with discovering the electron as the first elementary and truly fundamental particle.
The American scientist Robert Millikan could precisely measure the electron's charge 15 years after Thomson's investigations. This number and the charge-to-mass ratio made it possible to calculate the electron's mass. Currently, the most exact value that is obtainable is
<img alt="HPLC chromatogram, protein separation, chromatography result, retention time, analytical chemistry." src="/files/ftp_upload/14197/14197_Equation_4.png" style="text-align: center; height: 20px;" data-altupdatedat="1761830439000" data-alt="Equation4">

In a beam of charged particles created by a heated cathode, the particles move at different speeds. However, many applications need a beam with uniform ...

Education

JoVE Core

Physics

Unit 1

Magnetic Forces and Fields

KEY TERMS AND CONCEPTS

electron speed selector in thomson’s tube

A velocity selector uses electric and magnetic fields to choose charged particles with one speed from a beam. Many beams from a heated cathode contain particles moving at different speeds, so this setup helps produce a uniform beam. The electric and magnetic fields are perpendicular to the particle’s motion and to each other.

In this arrangement, a particle with charge q, speed v, and mass m enters from the top. The magnetic field points into the plane of the paper, and the electric field points to the left. For a positive charge, the magnetic force acts to the right with magnitude qvB, while the electric force acts to the left with magnitude qE. The two forces cancel when the speed has a specific value, given by E/B, so the total force becomes zero.

J. J. Thomson used this idea to measure the electron’s charge-to-mass ratio in a classic physics experiment from the end of the 19th century. In a highly evacuated glass container, a voltage difference between the anodes and cathodes accelerates hot cathode electrons and shapes them into a beam. The accelerating potential V sets the electrons’ speed, and the lost electric potential energy becomes kinetic energy. Only electrons moving at speed E/B pass straight through the crossed electric and magnetic fields and strike a fluorescent screen, which glows at the impact point.

Thomson calculated the electron’s charge-to-mass ratio from this beam. A key result was that he found only one value for this number. It did not depend on the cathode material, the remaining gas in the tube, or other details of the setup. That independence showed that the particles in the beam, now called electrons, are part of all matter. For that reason, Thomson is credited with discovering the electron as the first elementary and truly fundamental particle. About 15 years later, Robert Millikan measured the electron’s charge with high precision. Together with Thomson’s charge-to-mass ratio, this made it possible to calculate the electron’s mass.

Electron Speed Selector in Thomson’s Tube

magnetism

Magnetic Poles and Everyday Uses

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magnetic fields

Magnetic Force on Moving Charges

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magnetic field lines

Reading Magnetic Field Lines

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magnetic flux

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motion of a charged particle in a magnetic field

motion-of-a-charged-particle-in-a-magnetic-field

Charged Particle Paths in Magnetic Fields

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magnetic force

Magnetic Fields and Moving Charges

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magnetic force on a current-carrying conductor

magnetic-force-on-a-current-carrying-conductor

Current and Magnetic Field Around a Wire

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magnetic force on current-carrying wires: example

magnetic-force-on-current-carrying-wires-example

Balancing Weight with Magnetic Force

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force on a current loop in a magnetic field

force-on-a-current-loop-in-a-magnetic-field

Current Loops and Torque in Motors

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torque on a current loop in a magnetic field

torque-on-a-current-loop-in-a-magnetic-field

Current Loop Torque in Electric Motors

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the hall effect

Hall Voltage and Charge Carriers

Hall voltage and charge carriers are explained through Edwin H. Hall’s 1879 experiment. His setup helped identify the polarity of the main charge carriers in a conducting material. It also showed, for the first time, that the charge carriers in most metals are negative. A metal slab is placed in a constant magnetic field. The electrons move from right to left, so the magnetic force pushes them toward the top edge of the slab. This leaves excess negative charge at the top and excess positive...

A velocity selector uses electric and magnetic fields to choose charged particles with one speed from a beam. Many beams from a heated cathode contain particles moving at different speeds, so this setup helps produce a uniform beam. The electric and magnetic fields are perpendicular to the particle’s motion and to each other. In this arrangement, a particle with charge q, speed v, and mass m enters from the top. The magnetic field points into the plane of the paper, and the electric field...

key terms and concepts

Explore how crossed electric and magnetic fields select electron speed, how Thomson measured the electron charge-to-mass ratio, and why the result proved electrons are universal.

Electron Speed Selector in Thomson’s Tube | JoVE K12

physics

JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

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Content elaboration: Physical Science included properties and locations of protons, neutrons and electrons, atomic number, mass number, cations and anions, isotopes and the strong nuclear force which holds the nucleus together. In this course, the historical development of the atomic model and the positions of electrons are explored in greater detail. Atomic models are constructed to explain experimental evidence and make predictions. The changes in the atomic model over time exemplify how scientific knowledge changes as new evidence emerges and how technological advancements like electricity extend the boundaries of scientific knowledge. Thompson's study of electrical discharges in cathode-ray tubes led to the discovery of the electron and the development of the plum pudding model of the atom. Rutherford's experiment, in which he bombarded gold foil with a-particles, led to the discovery that most of the atom consists of empty space with a relatively small, positively charged nucleus. Bohr used data from atomic spectra to propose a planetary model of the atom in which electrons orbit the nucleus, like planets around the sun. Later, Schrödinger used the idea that electrons travel in waves to develop a model in which electrons travel randomly in regions of space called orbitals (quantum mechanical model). Based on the quantum mechanical model, it is not possible to predict exactly where electrons are located but there is a region of space surrounding the nucleus in which there is a high probability of finding an electron (electron cloud or orbital). Data from atomic spectra (emission and absorption) gives evidence that electrons can only exist at certain discrete energy levels and not at energies between these levels. Atoms are usually in the ground state where the electrons occupy orbitals with the lowest available energy. However, the atom can become excited when the electrons absorb a photon with the precise amount of energy (indicated by the frequency of the photon) to move to an orbital with higher energy. Any photon without this precise amount of energy will be ignored by the electron. The atom exists in the excited state for a very short amount of time. When an electron drops back down to the lower energy level, it emits a photon that has energy equal to the energy difference between the levels. The amount of energy is indicated by the frequency of the light that is given off and can be measured. Each element has a unique emission and absorption spectrum due to its unique electron configuration and specific electron energy jumps that are possible for that element. Being aware of the quantum mechanical model as the currently accepted model for the atom is important for science literacy as it explains and predicts subatomic interactions, but details should be reserved for more advanced study. Electron energy levels consist of sublevels (s, p, d and f), each with a characteristic number and shape of orbitals. Orbital diagrams and electron configuration can be constructed to show the location of the electrons in an atom using established rules. Valence electrons are responsible for most of the chemical properties of elements. In this course, electron configuration (extended and noble gas notation) and orbital diagrams can be shown for any element in the first three periods. Although the quantum mechanical model of the atom explains the most experimental evidence, other models can still be helpful. Thinking of atoms as indivisible spheres is useful in explaining many physical properties of substances, such as the state (solid, liquid or gas) of a substance at room temperature. Bohr's planetary model is useful to explain and predict periodic trends in the properties of elements.

The Periodic Table

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Periodic Classification of the Elements

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Periodic Table
• Properties
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Quantifying matter

Electron Speed Selector in Thomson’s Tube

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