Question

In 1886 Eugene Goldstein observed positively charged particles moving in the opposite direction to electrons in a cathode ray tube (illustrated below). From their mass, he concluded that these particles were formed from residual gas in the tube. For example, if the cathode ray tube contained helium, the canal rays consisted of \(\mathrm{He}^{+}\) ions. Describe a process that could lead to these ions.

Short answer

Electrons collide with helium atoms in the tube, ionizing them to form \(\mathrm{He}^{+}\).

Step-by-step solution

Understand the Context of Cathode Ray Tubes

The cathode ray tube is a vacuum tube containing a gas, in this instance, helium. When a high voltage is applied, it produces electrons that travel from the cathode (negative electrode) to the anode (positive electrode). Under these conditions, other particles, identified as positively charged, were observed by Eugene Goldstein. These are known as canal rays.

Analyze the Formation of Ions

In the cathode ray tube, the electrons emitted from the cathode can collide with helium atoms in the gas. These high-energy collisions can remove one or more electrons from helium atoms, leaving behind positively charged helium ions, \(\mathrm{He}^{+}\). This process is called ionization.

Identify the Ionization Process

The specific process that leads to ionization is when an energetic electron, traveling through the gas, strikes a helium atom and transfers enough energy to remove an electron from the helium atom. This removal results in the creation of a helium ion (\(\mathrm{He}^{+}\)) and a free electron: \[\mathrm{He} + e^- \rightarrow \mathrm{He}^+ + 2e^-\]where \(e^-\) is the electron that caused the ionization.

Conclusion of Ionization in the Tube

As the tube maintains a vacuum and a sufficient voltage, continuous collisions occur, sustaining the presence of these positively charged ions. The canal rays observed are mainly composed of these ions, moving in the direction opposite to the electron flow.

Key concepts

Cathode Ray Tube

A Cathode Ray Tube (CRT) is a type of vacuum tube that plays a crucial role in the discovery of atoms and ions. Inside a CRT, you'll typically find a gas, which in some experiments is helium. When a high voltage is applied across the electrodes, it creates a flow of electrons from the cathode (negative electrode) to the anode (positive electrode). CRTs were instrumental in scientific advancements of the late 19th and early 20th centuries.

These tubes demonstrate the behavior of electrons and positive ions in an electric field. Electrons move quickly towards the anode, while the canal rays, observed by Goldstein, move in the opposite direction. Goldstein's critical observation involved tracking these positively charged particles moving backward, providing insight into the nature of atomic and subatomic particles.

CRT technology also laid the foundation for the development of television displays and oscilloscopes. While modern flat screens have largely replaced them, understanding CRTs helps grasp classical physics experiments.

Helium Ions

In the context of cathode ray experiments, helium ions (\(\mathrm{He}^{+}\)) are produced when helium atoms lose an electron. When an energetic electron interacts with a helium atom, it can knock out one of the electron's orbitals. This results in the ionization of helium, converting it into a positively charged ion.

Helium is one of the noble gases, known for being chemically inert. However, under the influence of high-energy electrons, as in a CRT, it can become ionized. The resulting helium ions are a fundamental component of what Goldstein refers to as canal rays.

Helium ions are a typical product in many physics experiments involving ionization, as they have stable isotopes that make them easier to study. They provide valuable insights into electron-ion interactions and contribute to the foundational understanding of particle physics.

Energetic Electron Collision

When a high voltage is applied in a cathode ray tube, electrons gain significant kinetic energy as they accelerate towards the anode. As they zoom through the gas-filled space inside the tube, they collide with gas atoms. This action is what we refer to as energetic electron collisions.

These collisions are crucial because they provide the necessary energy to remove electrons from helium atoms, causing ionization. It is the kinetic energy of these speeding electrons that leads to the creation of positive ions when they collide with helium atoms.

In scientific terms, these collisions are described by considering the energy transfer mechanics involved. The energetic electrons transfer enough energy to the helium atoms to eject an electron, transforming them into ions. Understanding these collisions helps explain processes in various gaseous discharge devices, such as neon lights and certain types of lasers.

Positive Ion Formation

Positive ion formation is a process where neutral atoms lose electrons and become positively charged. In a cathode ray tube filled with helium, this occurs when high-energy electrons, generated by the applied voltage, strike helium atoms.

This interaction can be written using the equation:\[\mathrm{He} + e^- \rightarrow \mathrm{He}^+ + 2e^- \]Here, an energetic electron (\(e^-\)) hits a helium atom, removing one electron, resulting in a helium ion (\(\mathrm{He}^+\)) and releasing another electron.

Such ionization is foundational in understanding how charge transport occurs within gaseous mediums. Canal rays observed in cathode ray tubes are comprised mainly of such positive ions, and their study has been pivotal in advancing atomic theories of matter. This process is not only critical in scientific research but also in practical applications like mass spectrometry and ion propulsion.