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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Thermionic Emission Page 1

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1.

Study of a Vacuum Diode

 

Thermionic Emission:

  • The emission of electrons from the surface of a metal, when it is heated to a high temperature, is called thermionic emission.

    The electrons, so emitted, are known as thermions.
  • The minimum energy required to liberate an electron from the metal surface is called the work function of that metal. For example, caesium has the lowest work function at W0 = 1.75 eV, tungsten has a work function of 4.55 eV.

 

 

2.

Diode: Construction and Working

 

 


The two electrodes:

  1. Cathode: Cathode is a metal cylinder coated with oxides of BaO or SrO, which surround the filament in the indirectly heated type of diodes. In the directly heated type of diodes, filament itself serves as a cathode. The cathode emits electrons when heated.
  2. Anode (or plate): It is a cylinder enclosing the cathode. It collects electrons.

 

 

3.

Diode Characteristics

 

The circuit diagram

Circuit used for studying the characteristic
curves of a vacuum diode

 

  1. Keeping filament current If constant, change in plate current (Ip), is observed when plate voltage (Vp) is steadily increased. (Plate characteristics)
  2. Step (i) is repeated for different constant values of If. i.e.If1, If2, If3 etc.

    The graphs representing these observations are shown below. These curves are known as static characteristics of diodes.

Study of Characteristics (Conclusions)

A fixed number of electrons are emitted per second by the cathode, when a constant current If is passed through the filament.

For a small plate voltage,

  • Only a few electrons reach the plate; Ip is small.
  • The remaining electrons gather between the cathode and the plate and form a cloud known as space charge.

    Space Charge

    In a vacuum diode, the heated cathode emits electrons. These electrons are attracted by the plate when it is at a positive potential with respect to the cathode. If the plate is at zero potential or if its potential is slightly positive, the electrons are not attracted by the plate as soon as they are emitted. In other words, the rate at which the electrons are attracted by the plate is less than the rate at which they are emitted by the cathode. The excess number of electrons remain in the space between the cathode and the plate, forming a cloud of electrons. This electron cloud is called the space charge.

    This negatively charged electron cloud repels the electrons emitted by the cathode and prevents them from reaching the plate. Therefore, the plate current decreases still further.

    Remember
    1. The number of electrons emitted from the cathode depends upon the temperature of the cathode.
    2. Temperature of the cathode depends on the filament (heater) current.
    3. Higher the temperature, more is the number of electrons emitted.
    4. The potential of a plate with respect to cathode does not affect the rate of emission, but the number of electrons reaching the plate depends upon the potential.
    5. Plate current flows in the diode only when the plate is made positive w.r.t. cathode. No current can flow when the plate is negative w.r.t. cathode.
    6. Electron current flow within a diode takes place only from cathode to the plate. This is known as unidirectional conduction.

When plate voltage is increased,

  • The number of electrons reaching the plate increases.
  • Finally, a stage is reached where
    rate of emission of electrons by the cathode = rate of collection of electrons by the plate.
  • Now, the plate current reaches its maximum value known as the saturation current.

Diode is a "Valve": If plate of the diode is given a negative voltage with respect to the cathode, it repels electrons emitted by the cathode, hence the plate current is zero. Thus, current flow through a diode is unidirectional. Diode becomes conducting element only when plate is at positive potential. Hence, the diode is called a valve.

Conducting mode

Non-conducting mode

 

 

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