Problem 107 - Chemistry: An Atoms-Focused Approach

Question

In April \(1897,\) J.J. Thomson presented the results of his experiment with cathode-ray tubes in which he proposed that the rays were actually beams of negatively charged particles, which he called "corpuscles." a. What is the name we use for the particles today? b. Why did the beam deflect when passed between electrically charged plates? c. If the polarity of the plates were switched, how would the position of the light spot on the phosphorescent screen change?

Step-by-Step Solution

Answer

Answer: The particles J.J. Thomson called "corpuscles" are today known as electrons. When the polarity of electrically charged plates is reversed, the electrons' deflection direction changes due to the altered electric field, causing the light spot on the phosphorescent screen to change its position.

Step 1: a. Identifying the modern name for the particles

The particles which J.J. Thomson called "corpuscles" are what we today know as electrons. They are negatively charged particles and are essential components of atoms.

Step 2: b. Understanding beam deflection

When the beam of negatively charged particles (electrons) passes between electrically charged plates, it gets deflected due to the forces acting on it by the electric field created between the plates. The electric field causes an attractive or repulsive force on the charged particles (depending on the polarity of the plates), leading to a change in the direction of their motion and the resultant beam deflection.

Step 3: c. Predicting the change in the light spot position

If the polarity of the plates is switched, the electric field between the plates will change directions. The negatively charged electrons, initially attracted to one plate, will now be attracted to the other plate in the reversed field. This will cause the light spot on the phosphorescent screen to change its position, as the electrons' deflection direction will be opposite to what it was before.

Key Concepts

J.J. Thomson

In 1897, J.J. Thomson made a groundbreaking discovery while experimenting with cathode-ray tubes. These tubes were glass containers that housed gases at very low pressures. Thomson's work led to the discovery of what we know today as electrons.
Thomson referred to these particles as "corpuscles," believing them to be the fundamental building blocks of atoms. His experiments showed that these corpuscles were negatively charged. This was a revolutionary idea at the time, as it challenged the prevailing notion of atoms being indivisible.
Thomson's experiments not only changed our understanding of atoms but also paved the way for future research in atomic physics. His identification of these negatively charged particles helped establish the electron's role in atomic structure.

Electrons

Electrons are subatomic particles with a negative charge, crucial to the structure of atoms. They are found orbiting the nucleus, which contains positively charged protons and neutral neutrons.
Electrons have a very small mass compared to protons and neutrons, but they are essential for chemical reactions and electricity. Their negative charge balances out the positive charge of protons in an atom, making the atom electrically neutral.
Without electrons, atoms wouldn't bind together to form molecules. Electrons also allow for the flow of electricity; when they move through a conductor, they create an electric current.

Electric fields

An electric field is a region around charged particles where forces are exerted on other charges. It can either attract or repel other charged particles depending on their charges. In Thomson's experiments, electric fields played a crucial role in demonstrating the existence of electrons.
When electrons pass through an electric field created by charged plates, they experience forces causing them to deflect from their original path. This deflection confirms the presence and charge of the particles.
The direction and magnitude of the force on a charged particle in an electric field depend on both the charge of the particle and the strength and direction of the field itself.

Beam deflection

Beam deflection is a key outcome in J.J. Thomson's cathode-ray tube experiments. It refers to the change in the trajectory of the electron beam when it passes through an electric or magnetic field.
The direction of deflection for a negative electron beam depends on the polarity of the charged plates generating the field. If the electric field applies a force to the electrons, this force will either attract them towards a positive plate or repel them from a negative one.
This concept illustrates how electron beams can be manipulated by electric or magnetic fields, which is vital for technologies like old television sets and oscilloscopes, where controlling the beam's path is essential for image and signal display.