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

Das Experiment von J. J. Thomson bestimmte das Verhältnis von Ladung zu Masse für Elektronen.
Er verwendete einen Aufbau mit gekreuzten elektrischen und magnetischen Feldern, so dass nur Elektronen mit einer bestimmten Geschwindigkeit, dem Verhältnis von elektrischem und magnetischem Feld, hindurchgingen.
In einer evakuierten Glasröhre wurden Elektronen von einer heißen Kathode beschleunigt und durch die Potentialdifferenz zwischen der Kathode und den beiden Anoden in einen Strahl umgewandelt.
Die von den Elektronen gewonnene kinetische Energie entspricht der verlorenen elektrischen potentiellen Energie. Also bestimmte er die Geschwindigkeit der Elektronen.
Die geschwindigkeitsselektierten Elektronen trafen auf den fluoreszierenden Schirm und brachten ihn zum Leuchten.
Dann löste er die beiden Gleichungen, um das Ladungs-zu-Masse-Verhältnis eines Elektrons zu bestimmen.
Alle Größen auf der rechten Seite sind unabhängig von der Art des Restgases in der Röhre, dem Kathodenmaterial oder einem anderen Aspekt des Experiments, was bedeutet, dass die Elektronen im Strahl ein grundlegender Bestandteil aller Materie sind.
Der genaueste berechnete Wert von e/m ist 1,759 x 1011 Coulomb pro Kilogramm.

<ol>
	<li>Young, H.D. und Freedman, R.A. (2012). Universitätsphysik mit moderner Physik. San Francisco, Kalifornien: Pearson; Abschnitt 27.5; Seiten 896–897.</li>
</ol>

thomson's e/m experiment

In einem Strahl geladener Teilchen, der durch eine erhitzte Kathode erzeugt wird, bewegen sich die Teilchen mit unterschiedlichen Geschwindigkeiten. Viele Anwendungen erfordern jedoch einen Strahl mit gleichmäßigen Teilchengeschwindigkeiten. Eine Anordnung, die als Geschwindigkeitsfilter bekannt ist, verwendet elektrische und magnetische Felder, um Teilchen mit einer bestimmten Geschwindigkeit aus dem Strahl auszuwählen.

Ein Teilchen mit Ladung q, Geschwindigkeit v und Masse m tritt von oben in einen Bereich ein, in dem die magnetischen und elektrischen Felder sowohl zur Bewegung des Teilchens als auch zueinander senkrecht stehen. Das magnetische Feld zeigt in die Ebene des Papiers, während das elektrische Feld E nach links zeigt. Wenn die Ladung q positiv ist, wirkt die magnetische Kraft mit dem Betrag qvB nach rechts, und die elektrische Kraft wirkt mit dem Betrag qE nach links. Die beiden Kräfte sind bei den gegebenen Feldstärken E und B gleich stark und heben sich für einen bestimmten Wert von v auf, für den die Gesamtkraft null ist, was durch die folgende Gleichung dargestellt wird:

<img alt="Equation1" src="/files/ftp_upload/14197/14197_Equation_1.png" style="text-align: center; height: 40px;" />

J. J. Thomson (1856-1940) nutzte dieses Konzept, um das Verhältnis von Ladung zu Masse des Elektrons in einem der klassischen Physikexperimente am Ende des 19. Jahrhunderts zu bestimmen. Ein Spannungsunterschied zwischen den beiden Anoden und Kathoden in einem stark evakuierten Glasbehälter beschleunigt und formt einen Strahl heißer Kathodenelektronen. Die Beschleunigungsspannung V bestimmt die Geschwindigkeit v der Elektronen. Die verlorene elektrische potenzielle Energie entspricht der gewonnenen kinetischen Energie.

Nur die Elektronen mit einer Geschwindigkeit von E/B können direkt durch die gekreuzten elektrischen und magnetischen Felder hindurchgehen und die fluoreszierende Leinwand, die am Aufschlagpunkt leuchtet, treffen.

Das Verhältnis von Ladung zu Masse des Elektrons wird berechnet als:

<img alt="Equation2" src="/files/ftp_upload/14197/14197_Equation_2.png" style="text-align: center; height: 42px;" />

Sein Wert wird wie folgt bestimmt:

<img alt="Equation3" src="/files/ftp_upload/14197/14197_Equation_3.png" style="text-align: center; height: 34px;" />

Die Tatsache, dass Thomson nur einen Wert für diese Zahl entdeckte, war ein wesentliches Merkmal seiner Messungen. Das Kathodenmaterial, das verbleibende Gas in der Röhre oder andere Aspekte des Experiments hatten keinen Einfluss auf die Zahl. Diese Unabhängigkeit bewies, dass die Teilchen, aus denen die Strahlen bestehen und die heute als Elektronen bezeichnet werden, ein Bestandteil aller Materie sind. Infolgedessen wird Thomson die Entdeckung des Elektrons als erstes elementares und wirklich fundamentales Teilchen zugeschrieben.

Der amerikanische Wissenschaftler Robert Millikan konnte die Ladung des Elektrons 15 Jahre nach den Untersuchungen von Thomson genau messen. Durch diese Zahl und das Verhältnis von Ladung zu Masse war es möglich, die Masse des Elektrons zu berechnen. Derzeit ist der genaueste Wert, der sich ermitteln lässt

<img alt="Equation4" src="/files/ftp_upload/14197/14197_Equation_4.png" style="text-align: center; height: 20px;" />

In einem Strahl geladener Teilchen, der durch eine beheizte Kathode erzeugt wird, bewegen sich die Teilchen mit unterschiedlichen Geschwindigkeiten. Viele Anwendungen benötigen jedoch einen Balken mit gleichmäßigem ...

Education

Lehre

JoVE Core

Physics

Physik

Unit 1

Einheit 1

Magnetic Forces and Fields

Magnetische Kräfte und Felder

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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

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magnetic flux

Magnetic Flux Through Surfaces

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motion of a charged particle in a magnetic field

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

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magnetic force

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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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torque on a current loop in a magnetic field

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Current Loop Torque in Electric Motors

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the hall effect

Hall Voltage and Charge Carriers

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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.

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

The Bohr Model

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The Quantum-Mechanical Model of an Atom

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Subatomic Particles

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Quantum Numbers

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Atomic Orbitals

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Atomic structure
• Evolution of atomic models/theory
• Electrons
• Electron configurations

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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atomic-radii-and-effective-nuclear-charge

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This process can be either ...

Alkali Metals

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Halogens

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Noble Gases


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Periodic Table
• Properties
• Trends

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Periodic Table

Types of Chemical Bonds

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Lewis Symbols and the Octet Rule

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Ionic Bonding and Electron Transfer

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The Born-Haber Cycle

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trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

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Covalent Bonding and Lewis Structures

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lewis-structures-of-molecular-compounds-and-polyatomic-ions

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	...

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VSEPR Theory and the Basic Shapes

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VSEPR Theory and the Effect of Lone Pairs

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Valence Bond Theory

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Hybridization of Atomic Orbitals II

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Molecular Orbital Theory II

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Chemical bonding
• lonic
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Chemical bonding

Chemical Formulas

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ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

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molecular-compounds-formulas-and-nomenclature

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Molecular Models

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Representing compounds
• Formula writing
• Nomenclature
• Models and shapes (Lewis structures, ball and stick, molecular geometries)

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Representing compounds

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Measurement: Derived Units

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Uncertainty in Measurement: Accuracy and Precision

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Uncertainty in Measurement: Reading Instruments

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Dimensional Analysis

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Elements: Chemical Symbols and Isotopes

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Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Quantifying matter

Content elaboration: In earlier grades, properties of materials were quantified with measurements that were always associated with some error. In this course, scientific protocols for quantifying the properties of matter accurately and precisely are studied. Using the International System of Units (SI), significant digits or figures, scientific notation, error analysis and dimensional analysis are vital to scientific communication. There are three domains of magnitude in size and time: the macroscopic (human) domain, the cosmic domain and the submicroscopic (atomic and subatomic) domain. Measurements in the cosmic domain and submicroscopic domains require complex instruments and/or procedures. Matter can be quantified in a way that macroscopic properties such as mass can reflect the number of particles present. Elemental samples are a mixture of several isotopes with different masses. The atomic mass of an element is calculated given the mass and relative abundance of each isotope of the element as it exists in nature. Because the mass of an atom is very small, the mole is used to translate between the atomic and macroscopic levels. A mole is equal to the number of atoms in exactly 12 grams of the isotope carbon-12. The mass of one mole of a substance is equal to its molar mass in grams. The molar mass for a substance can be used in conjunction with Avogadro's number and the density of a substance to convert between mass, moles, volume and number of particles of a sample.

Quantifying matter

Electron Speed Selector in Thomson’s Tube

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