What is the cause of Colour discharge in discharge tube?

What is the cause of Colour discharge in discharge tube?

It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.

Why discharge does not take place at very low pressure?

It is because when the gas is at very low pressure positive charged ions are not available to eject out the electron from the cathode hence the discharge current stops.No, vacuum cannot conduct electricity.

What happens to the gas inside the discharge tube when the pressure is decreased to 0.01 mm Hg?

At this pressure of about 0.01 mm of Hg: 0.01mm of hg At this pressure first striations disappear, the negative and cathode glow vanishes and the whole tube is filled with Crooke’s dark space. At this stage, luminous rays are seen to come out of the cathode. These rays are called cathode rays.

What happens in the discharge tube if a high voltage is applied at gas pressure of one atmosphere?

When high voltage is applied to the tube, the electric field accelerates the small number of electrically charged ions and free electrons always present in the gas, created by natural processes like photoionization and radioactivity.

What is the pressure in discharge tube?

—The increase in pressure of a gas in a discharge tube due to the discharge has been measured for pressures from about 0.1 to 20 mm of mercury for helium, hydrogen, oxygen, nitrogen, air, carbon monoxide and carbon dioxide.

Why low pressure is used in discharge tube?

You need the low pressure in the tube so that the passage of the beam of electrons is not impeded very much by collision with air molecules. You need a reasonably high accelerating voltage to give the electrons enough kinetic energy to make the fluorescent screen glow when the electrons hit the screen.

Why do gases conduct at low pressure?

At low pressure molecules of gas are far from each other due to which gas molecules have enough mean free path to accelerate. This means gas particles will acquire enough velocity and hence kinetic energy for the ionization of other molecules. And this is why gases conduct electricity at low pressure.

What happens discharge tube?

…is usually measured in an electric discharge tube in which a fast-moving electron generated by an electric current collides with a gaseous atom of the element, causing it to eject one of its electrons. (Chemists typically use joules, while physicists use electron volts.)

Why the tube was evacuated and high potential difference was created between the cathode and the anode?

To release electrons into the tube, they must first be detached from the atoms of the cathode. The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube.

Why a very high voltage is used in cathode ray tube?

Having a high voltage across the tube means that an electron can gain more kinetic energy between collisions with the air molecules and thus a smaller distance between gas molecules (not quite so low a pressure) can be used.

Why does the gas in a gas discharge tube need to be at low pressure in order for the gas to emit light when an electric current is passed through the tube?

1: Gas discharge tubes are enclosed glass tubes filled with a gas at low pressure, through which an electric current is passed. Electrons in the gaseous atoms first become excited, and then fall back to lower energy levels, emitting light of a distinctive color in the process.

What is perforated cathode?

Perforated cathode means cathode with holes. In production of anode rays, a perforated cathode ray tube is used which is pierced with holes through which anode rays passed and form red spot behind the cathode because of strike between rays and wall of discharge tube.

Why do you need low pressure in a discharge tube?

If the cathode ray experiment has a beam of electrons hitting a fluorescent screen which glows then: You need the low pressure in the tube so that the passage of the beam of electrons is not impeded very much by collision with air molecules.

When does the glow of a discharge tube end?

Once a glow discharge between electrodes has initiated; a low resistance current-path is formed, and passing a current sustains the discharge. Thus the tube resistance remains low, and the glow will continue, until the current falls below a value called the ‘extinction point’.

Why does a gas discharge tube light up?

Electrodes, incidentally, are not essential for gas breakdown. Any method that produces a strong-enough electric field, with or without electrodes, will also cause the gas (or regions therein) to glow; which is why discharge tubes light up when placed near resonant radio antennas and coils.

Why do you need a high voltage discharge tube?

You need a reasonably high accelerating voltage to give the electrons enough kinetic energy to make the fluorescent screen glow when the electrons hit the screen.