Cathode Rays: Visualizing Electrons in a Discharge Tube

Cathode rays are streams of electrons emitted from the cathode (negative electrode) in a discharge tube when a high voltage is applied across a low-pressure gas.

They were crucial in discovering the electron and understanding the structure of the atom.

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1. Physical Setup

A typical cathode ray experiment uses a discharge tube:

• A glass tube with very low-pressure gas inside.
• Two metal electrodes sealed into the tube:
  • Cathode (−): negative electrode
  • Anode (+): positive electrode
• A high voltage source (several kV) connected between cathode and anode.

When the voltage is high and the gas pressure is low, electrons are pulled out of atoms near the cathode and accelerated towards the anode.

These fast-moving electrons form what we call cathode rays.

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2. What Are Cathode Rays, Exactly?

In modern terms:

Cathode rays are beams of electrons traveling in straight lines from the cathode to the anode in a vacuum or low-pressure gas tube.

Key points:

• They originate at the cathode.
• They move toward the anode.
• They are negatively charged (because electrons carry negative charge).
• They can be deflected by electric and magnetic fields.
• They can cause fluorescence (glow) when they hit certain materials (like ZnS coatings or glass zones).

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3. Evidence That Cathode Rays Are Electrons

Historically, several experiments showed cathode rays are made of tiny negatively charged particles (later called electrons):

(a) Straight-Line Travel and Shadow Formation

• Place a metal cross or object between cathode and screen.
• A sharp shadow forms on the fluorescent screen.
• This shows the rays travel almost in straight lines.

(b) Deflection by Electric Fields

Set up parallel plates inside the tube:

• Upper plate: positive
• Lower plate: negative

Observation:

• The cathode ray beam bends toward the positive plate, meaning the rays are negatively charged.

Mathematically, for a charge $q$ in an electric field $E$:

$$F = qE, \quad a = \frac{F}{m} = \frac{qE}{m}$$

Since the beam deflects consistently, it acts like many identical particles with the same $q/m$ ratio (charge-to-mass).

(c) Deflection by Magnetic Fields

• A magnetic field $\vec{B}$ applied perpendicular to the beam path causes a curved trajectory.
• The Lorentz force is $\vec{F} = q(\vec{v} \times \vec{B})$.
• From this, charge $q$ and mass $m$ of the particle can be studied.

(d) Thomson’s e/m Measurement

J. J. Thomson balanced electric and magnetic deflections and derived the specific charge:

$$\frac{e}{m} \approx 1.76 \times 10^{11} \; \text{C/kg}$$

This is the same for all cathode rays, irrespective of the gas or metal used. So the particles are universal – not atoms of a particular gas.

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4. Properties of Cathode Rays

1. Travel in straight lines (cast sharp shadows).
2. Negatively charged (deflected by E and B fields like negative charges).
3. Cause fluorescence: make certain materials glow.
4. Produce X-rays when striking heavy metals at high speed.
5. Have kinetic energy and can rotate a small paddle wheel (showing they have momentum).
6. Independent of gas and cathode material: always the same kind of particles (electrons).

So, cathode rays are not made of light; they are made of matter particles (electrons).

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5. Relation to the Electron

Thomson concluded:

• Cathode rays are constituents of all atoms.
• The particles have
  • Charge $e \approx 1.6 \times 10^{-19} \, \text{C}$
  • Mass $m \approx 9.11 \times 10^{-31} \, \text{kg}$

Thus, cathode rays revealed the electron, one of the fundamental building blocks of matter.

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6. What the Visualization Shows

The interactive visualization conceptually shows:

• A long horizontal discharge tube.
• On the left: a glowing cathode emitting a beam of electrons.
• The beam moves across the tube, causing a fluorescent screen to glow where it hits.
• You can:
  • Adjust the accelerating voltage to see the beam move faster.
  • Turn on an electric field and adjust its strength to see the beam deflect.
  • Turn on a magnetic field and adjust its strength to see curved paths.

This visually connects the idea “cathode rays = electrons” with trajectories controlled by classical forces.

Use the sliders and toggles to see how these invisible rays behave under different physical conditions.