Electricity and Magnetism

Cathode Ray

1. What meant by a cathode ray?
   A cathode ray is a beam of fast-moving electrons.
   
2. Why it is called cathode ray?
   Because it emerges from the cathode
   
3. What is meant by thermionic emission?
   It is the break up of electrons off metals into the space surrounding them due to the gained energy from heat.
   
4. Explain the structure of a cathode ray tube?
   The tube is made of glass with all air removed from it.  Inside the glass there are two pieces of metal called electrodes.  One of these is called the anode and the other is called the cathode. 
    
   
5. Why the air is removed from the glass tube?
   It is removed so that the electrons are free to move without colliding with air molecules
    
6. Explain how do we generate cathode ray?
   We connect the filament of the cathode to a suitable power supply and we connect the anode to the positive terminal of a high voltage power supply as shown below.  When the filament heats the cathode, the electrons start to gain energy and escape from the cathode surface.  Electrons leaving the cathode are accelerated toward the anode due to the high potential and the positive charge on the anode.  The accelerated electrons form a fast-moving beam of electrons called "cathode ray".  
   
   
7. Explain why current does not flow in the circuit shown?
   In the circuit below, the anode has a negative charge and thus it will not attract the escaped electrons.  As a result no current will be detected by the ammeter shown in the circuit.
   
   
8. List the basic properties of cathode ray?
   1. Travel in straight lines
   2. A beam of negatively charged particles
   3. Can be deflected by electric field
   4. Can be deflected by magnetic field
   5. Produce fluorescence (glow) when they hit some materials.  This property is used to design screens for TVs, computers, oscilloscopes, ..etc.
      
9. Explain an experiment to show that cathode ray travels in straight lines?
   As shown in figure below, we put an object like the Maltese cross.  When the cathode ray accelerates toward the screen part of it will be blocked by the cross and the other part is reach the screen.  On the screen, we see an image exactly proportional to the cross, which indicate that electrons must have traveled in straight lines.
   
   
10. Explain an experiment to show that cathode ray can be deflected by electric field?
    The circuit below shows that when the cathode ray enters the electric filed set by the two plates it is deflected toward the positive terminal.  This happens because the positive charge on the top plate as shown attracts the electrons.
    
    
11. Explain an experiment to show that cathode ray can be deflected by magnetic field?
    
    
12. Explain an experiment to show that cathode ray is made of negatively charged particles?
    
13. Show the difference between the direction of flow of electron current and conventional current for the cathode ray?
    
14.