chem + phys q

[Jazz Man Tim]

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not sure where to put this...

In chemistry, galvanic cells. the Cathode where the reduction takes place is positivly charged..

yet in physics the cathode in cathode ray tubes is said to be a negative electrode
wtf?

can anyone explain why this is?

[xrtzx]

yeah, i know this too but my teacher explained it to me but i forgot, need help too thnx

[theChemCoach]

Excellent question!

• In a galvanic cell, the cathode is where reducation occurs. Reduction is the process of a species, lets say metallic cations, gain electrons - but where do they gain these electrons from? Well its not the solution (there are NO 'free' electrons to gain from the solution itself), these electrons are gained from the SURFACE of the metallic cathode.

Lets consider, what the surface of a cathode looks like;

• Think of the metal plates not as a lot of metal atoms packed together, but as a lot of metal ions floating in a sea of electrons. The boundary between the metal and a solution containing metal cations is pictured below.

• The metallic cations gain electrons from the surface of the metal, and form their respecitive solid deposits...

• As a consequence, the cathodic metal becomes POSITIVE, this instantaneously draw electrons from the ANODE. Hence the ANODE acts as the SOURCE of the electrons gained at the cathode.
  
  (You could now extend this response to a discussion of the physical processes at the anode, do you see how? note that the entire process is spontaneous and dynamic - i.e happening at the same time)
  
  • In CATHODE RAY TUBES: the cathode IS the source of electrons (just as the anode is in the galvanic cells);
  • Traditionally, metallic electrons are excited to the surface by heating (how it was orginally done), and then get emmitted and are attracted to the anode.
    • these days, electrons are being 'pumped' out of the anode and 'pumped' into the CATHODE by applying an externat potential difference (using a battery).
    so....
• electrons are ejected (creating the 'stream of electrons') from the cathode (excess surface electrons...hence negativ charge) to migrate to the anode (electron deficient)

• Think about this; cathode ray tubes (the ones in your lab and tv's) are more like an electrolytic cell than a galvanic cell.

Hope that all helps.

George

..

[nosadness]

why does the anode(tube) have a difficiency of electrons, i can see how that the cathode(tube) have an excess cause of the battery. But still the electrons goes to the anode(tube) so it is reduced, so shouldn't it be called a cathode(tube)

and what is a electrolytic cell?

[fush]

electrolytic cells require electricity to function while galvanic cells produce a current and electricity