Purpose
Show how electrons were discovered and that they are charged.
Procedure
Refer to the instruction manual for the cathode ray tube (CRT).
Description
- Principle of CRT: Explains the functioning of the cathode ray tube.
- Electron Beam View: Provides a clear view of the electron beam from the gun.
- Power Requirements:
- Heater: 6.3VAC or DC Source needed.
- Accelerating Voltage: 150–300V Source required.
- Deflecting Plates: ±50VDC Source necessary.
- Power Supply:
- Model: S43951P (sold separately)
- Integrates heater, acceleration, and deflection voltages.
- Plate Dimensions: Measures 12 x 0.38 x 4.25 in. (30.5 x 0.97 x 10.8 cm).
- Weight: 12 oz. (350g).
Links
Experimental Setup
- Equipment: Partially evacuated glass cylinder of various shapes with two metal electrodes (one at each end).
- High Voltage Application: When a high voltage is applied between the electrodes, electrons travel in straight lines from the cathode to the anode.
Chemical Concepts Demonstrated
- Cathode rays
- Electrons
Demonstration
- Touch one end of a Crookes’ tube with a Tesla coil.
- Bring a horseshoe magnet near the tube.
- Alternative: Connect the tube to a vacuum pump.
Observations
- The tube emits a glow in a straight line until the magnet is introduced, bending the glow into a curved line.
Explanations
- Cathode Rays: Streams of negatively charged particles (electrons).
- Behavior of Cathode Rays:
- Contained in a glass tube that is partially evacuated.
- When connected to batteries, the cathode rays cause the opposite end of the tube to glow.
- Deflection by a magnetic field suggests that cathode rays are negatively charged.
- A solid object in the path of cathode rays casts a shadow on the wall opposite the cathode.
Historical Context
- Crookes Tube: An early electrical discharge tube invented by William Crookes and others (1869-1875) where cathode rays (electrons) were first discovered.
- Usage: Utilized by Crookes, Hittorf, Plücker, Goldstein, Hertz, Lenard, and others to study cathode ray properties, culminating in J.J. Thomson’s 1897 identification of cathode rays as negatively charged particles, later named electrons.
- Deflection by Magnetic Fields:
- Crookes placed a magnet across the neck of the tube, showing that the beam bends under a magnetic field, akin to electric currents in a generator, confirming that cathode rays obey Faraday’s law.