One Chapter 8

INVESTIGATING NEW RAYS

• Dalton, in 1808 proposed that matter is made of atoms. All substances were either made of single atoms or combinations of atoms (molecules).
• He thought that atoms were indivisible.
• In the 19th century, experiments showed that atoms were divisible. As a result, new particles and forces were found.

CATHODE RAYS

• Geissler in the 19th century, Invented a new vacuum pump.
• He produced discharges of electricity in evacuated tubes of varying shape. He also produced difference colors of discharge by placing different gases in the tubes.

GAS DISCHARGE TUBE - A tube that allows an electric current to pass through a gas at low pressure.

ELECTRODES - Metal plates sealed in the ends of a gas discharge tube. (+ is the anode and - is the cathode.)

• When air is pumped out of the tube, the discharge across an induction coil stops, and the electrodes in the tube are connected by one or more violet streamers.

• At low enough pressure, a pink glow fills the entire tube. Continued decreases in pressure cause the pink glow to concentrate around the anode and a blue glow around the cathode. The space between the glows is dark (called Faraday's dark space).

• Continued reduction in pressure causes the dark space to expand, and the color at the electrodes to fade until the tube is dark, except for a faint green or violet glow around the anode. The sides of the tube fluoresce (usually green). The dark region is now called Crookes' dark space.

• Investigations centered on what was happening in the dark space. Early researchers decided that the glow in the gas originated at the cathode. For this reason, the discharge was called CATHODE RAYS, and the tube a CATHODE RAY TUBE (CRT).

MALTESE CROSS TUBE

• Plucker made an anode into a Maltese cross, and this produced a shadow in the glow at the end of the tube. This showed that the cathode rays traveled in straight lines.

PADDLE-WHEEL DISCHARGE TUBE

• Crookes reported that a paddle wheel placed in the path of the cathode rays turned. This proved that the cathode rays carried energy, and that they might be made of particles.
• This also indicates that the rays (particles) moved from the cathode to the anode.

CATHODE RAYS IN A MAGNETIC FIELD

• Crookes showed that the rays were deflected by a magnetic field.
• Crookes noted that charged particles in a magnetic field experience a force. Cathode rays behaved as if they were negatively charged particles.

CATHODE RAYS IN ELECTRIC FIELDS

• Arthur Schuster noticed that the particles were repelled from a negative plate and attracted to a positive plate.
• This is further proof that cathode rays are negatively charged particles.

CATHODE RAYS CARRY A NEGATIVE CHARGE

• Jean Perrin constructed an apparatus that had an anode made of a hollow aluminum cylinder that was open at both ends. At the end opposite to the cathode, was a cylinder that was closed at one end.
• The closed cylinder collected the cathode rays and was connected to an electroscope, which was used to determine the charge on the cathode rays.
• The electroscope showed that the cathode rays were negatively charged.

THOMSON'S TUBE

• The deflecting plates deflected the particles in one direction.
• Magnetic coils deflected the particles in the other direction.
• By adjusting the relative strength of the electric and magnetic fields, the particles went straight.
• From measurements and equations for deflecting particles by magnetic and electric fields, the charge to mass ratio was determined (1.76 X 10¹¹ C/kg).
• The charge to mass ratio (Q/M) was the same regardless of the potential difference used to accelerate particles.
• Q/M was the same for different cathode materials. This indicates that there must be a similarity between particles making up different cathode materials.
• Similar experiments with hydrogen ions, showed that the hydrogen lon's charge to mass ratio was 1836 times smaller than for cathode rays.
• If we assumed that equal charges were present on the hydrogen ions and cathode ray particles, then the mass of the cathode ray particles was 1/1836 of the mass for the hydrogen ion.
• The hydrogen atom is the smallest atom, yet these particles were smaller. This meant that the atom was not the smallest particle.
• Thomson concluded that cathode rays were light, fast moving, negative particles, that were a part of an atom.
• Thomson called the cathode ray particle, the ELECTRON.

PROPERTIES OF CATHODE RAYS

• They are produced by the negative electrode, or cathode, in an evacuated tube, and travel towards the anode.
• They travel in straight lines and cast sharp shadows.
• They have energy and can do work.
• They are deflected by electric and magnetic fields and have a negative charge.
• They are beams of tiny, negatively charged particles called electrons.

January 19, 2014