Tag: Photoelectric Cell Class 12

Photoelectric Cell | Construction | Applications

Photoelectric Effect-

 

Before you go through this article, make sure that you have gone through the previous article on Photoelectric Effect.

 

We have discussed-

  • Photoelectric Effect is a phenomenon of emission of electrons from metal surfaces.
  • Photoelectric Effect occurs when a light of suitable frequency falls on the metal surface.

 

In this article, we will discuss about Photoelectric Cell.

 

Photoelectric Cell-

 

Photoelectric Cell is a device that converts light energy into electrical energy.

 

Also, it may also be defined as-

  • Photoelectric cell is an electron tube or a photo tube.
  • Electrons initiating an electric current in this electron tube originate by photoelectric emission.

 

Let us discuss the construction and working of a photoelectric cell.

 

Construction-

 

  • Photoelectric cell consists of an evacuated glass containing anode and cathode.
  • The cathode is semi cylindrical.
  • The cathode is coated with a photo sensitive alkali metal like rubidium, molybdenum etc.
  • This is to reduce the work function.

 

Working-

 

  • When light of suitable frequency is incident on the cathode, it emits the photoelectrons.
  • These photoelectrons are attracted by the anode which is positively charged.
  • This flow of electrons result in a flow of current.
  • The flowing current is proportional to the intensity of incident radiation.
  • This current may be further increased by filling some inert gas like argon into the glass.
  • The photoelectrons emitted by cathode ionize the gas by collision.
  • This collision results in increasing the current.
  • The positive ions move towards the cathode.
  • Negative ions and electrons move towards the anode.

 

 

Obviously, there is no conduction in the absence of light in the phototube.

 

Applications of Photoelectric Cell-

 

Photoelectric cell has its applications in the following-

  1. Burglar alarm
  2. Counting machines
  3. Street lights
  4. Home security system
  5. Fire alarm

 

1. Burglar Alarm-

 

  • A burglar alarm is fitted near the lock of almirah.
  • A thief comes and makes the light fall on the lock by match stick, torch etc.
  • The light is received by the photoelectric cell.
  • The flow of electrons start from cathode to anode.
  • This completes the circuit and alarm starts ringing.

 

 

2. Counting Machines-

 

  • Counting machines may be operated by using a photoelectric cell.
  • Whenever a currency note obstructs the incident light, flow of current stops for that duration.
  • This advances the machine by count one.

 

3. Street Lights-

 

  • Street lights may be operated by using a photoelectric cell.
  • The switch of the relay is attached with the photoelectric cell.
  • When the intensity of sunlight decreases below the required value, the switch is activated.
  • This switch on the street lights automatically.

 

4. Home Security System-

 

  • In a home security system, the light is produced by a source at one end of the circuit.
  • This light falls on a photoelectric cell sensor located at some distance away from it.
  • The electrons are emitted by the cathode.
  • When somebody walks between the light source and the sensor, the path of the beam is blocked.
  • The sensor registers a drop in light levels and sends a signal to the control box.
  • The control box is connected to either siren or a telephone autodialer.
  • The control box on receiving the signal responds accordingly.

 

 

5. Fire Alarm-

 

  • A photoelectric cell with a required circuit is installed in sensitive places.
  • When the fire breaks out, light falls on the photoelectric cell.
  • This causes the photoelectric emission which causes the current flow.
  • This makes the bell connected to the circuit start to ring.

 

To gain better understanding about Photoelectric Cell,

Watch this Video Lecture

 

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