Diode, Do I understand this correctly?

Alright heres the thing I am not getting. it says that when the cathode is more positive than the anode then no current will flow, which is waht I understand. What  I dont understand is where the 2 ends are the same. the second one down, as well as the third down, its equal. I dont understand how when they are equal, the + can only flow from the anode to the cathode and then the "-" can only flow from the cathode to anode. if they are equal then shouldnt the be able to flow in either direction across the diode?

also when it says "cathode is more positive than the anode then no current will flow", the last diode shows the anode is less positive than the cathode. 

any explaination is much appreciated

Where did you get the diagram from?.. link?..  Let me start with that I'm sick, feeling miserable, and at work; the brain isn't all it can be this morning. Having said that, numbering the pictures 1-4 with 1 at the top and 4 at the bottom, 1 & 4 are the same, and 2 & 3 are the same.

1&4 no current should flow, 2&3 current should flow as long as Vf is achieved.. for some reason the arrows are different colors; I don't know why, and that might play a part in what they are describing.. which is why I asked for a link..

hope this helps..

Jim

https://www.the12volt.com/diodes/diodes.asp is the link i got it from. \

When you say vf, do you mean aportential difference? thats just a wild guess but with the info given, id say its a good guess.

http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

http://www.play-hookey.com/ac_theory/ps_rectifiers.html

http://www.kpsec.freeuk.com/powersup.htm

The last one has a circuit like what you've posted.. I don't know if it would help you to imagine a resistor (or lightbulb) across the DC connections to show a completed circuit. All the bridge rectifier circuit does is direct the positive and negative of the AC input (which, being AC, constantly changes) to the proper DC output terminals.. it's not difficult to understand; read through the links and see if the light comes on in your head. I suspect once you see it, you have the 'Ah' moment and all will be clear.

If you can scrounge a copy of electronics workbench or one of the Radio Shack 'Learning Electronics' breadboards you can learn much more about this and other electronics..

http://www.radioshack.com/product/index.jsp?productid=2102913&cp=2032062.2032398.2032405&parentPage=family

Jim