Thomson's Cathode Ray Tube Experiments

Summary

This script explains the basic workings of a cathode ray tube (CRT), the precursor to the modern television tube. It details how electrons, emitted from the negatively charged cathode, are attracted to the positively charged anode, producing visible light when they strike a coated surface. The script also covers how these electrons can be deflected by an electric and magnetic field, leading to the discovery of the electron's charge-to-mass ratio. In 1897, J.J. Thompson used this knowledge to calculate the ratio, a fundamental step in understanding the nature of the electron.

Takeaways

• 😀 A cathode ray tube (CRT) is a precursor to the modern television tube and is made of glass with most of the air evacuated.
• 😀 When two metal plates in the CRT are connected to a high voltage source, the negatively charged plate (cathode) emits an invisible ray.
• 😀 The cathode ray is attracted to the positively charged plate (anode), where it passes through a hole and continues toward the opposite end of the tube.
• 😀 When the cathode ray strikes a specially coated surface, it produces a bright fluorescence or light.
• 😀 An electric field applied across the CRT causes the cathode ray to be attracted by the positively charged plate, indicating the ray consists of negatively charged particles.
• 😀 The negatively charged particles in the cathode ray are electrons, which behave like tiny magnets when in motion.
• 😀 Electrons interact with external magnetic fields, causing them to be deflected by the magnetic field.
• 😀 Reversing the direction of the external magnetic field causes the electron beam to deflect in the opposite direction.
• 😀 In 1897, JJ Thompson determined the charge-to-mass ratio of an electron using both electric and magnetic fields to measure deflection angles.
• 😀 Using the deflection angles (Theta e for electric field and Theta B for magnetic field), Thompson calculated the charge-to-mass ratio of an electron as -1.76 * 10^8 coulombs per gram.

Q & A

What is a cathode ray tube (CRT)?

A cathode ray tube (CRT) is a glass tube, from which most of the air has been evacuated. It is the precursor to modern television tubes and produces light when an electron beam strikes a coated surface inside the tube.

What happens when the cathode is connected to a high voltage source?

When the cathode is connected to a high voltage source, it emits an invisible ray, known as the cathode ray, which is attracted to the positively charged anode.

What is the anode in a cathode ray tube?

The anode is the positively charged plate in a cathode ray tube. It attracts the negatively charged cathode ray and allows the electrons to pass through a hole and continue traveling to the other end of the tube.

What causes the fluorescence in a cathode ray tube?

The fluorescence or bright light is caused when the cathode ray strikes the specially coated surface inside the tube. The collision of the electron beam with the surface produces light.

What do the electrons in a cathode ray tube represent?

The electrons in a cathode ray tube are negatively charged particles that are emitted from the cathode and travel toward the anode, producing light when they strike the surface.

How do electrons behave when an external magnetic field is applied?

When an external magnetic field is applied, the moving electrons are deflected, behaving like tiny magnets. The direction of deflection depends on the direction of the magnetic field.

What happens when the direction of the external magnetic field is reversed?

When the direction of the external magnetic field is reversed, the beam of electrons is deflected in the opposite direction.

What was J.J. Thompson's contribution to the understanding of electrons?

In 1897, J.J. Thompson, an English physicist, determined the charge-to-mass ratio of an electron by adjusting the electric field and measuring the deflection angles of electrons in both electric and magnetic fields.

What is the charge-to-mass ratio of an electron according to J.J. Thompson?

J.J. Thompson determined the charge-to-mass ratio of an electron to be -1.76 × 10^8 coulombs per gram.

How did J.J. Thompson measure the charge-to-mass ratio of an electron?

Thompson adjusted the electric field so that the electrostatic deflection (Theta_e) was the same as the magnetic deflection (Theta_B), then used this balance to calculate the charge-to-mass ratio of the electron using a specific equation.