A picture for reference:
Difficulty 1: I don't get why a salt bridge is needed. People say it's to maintain the overall neutrality. But in both cases where salt bridge is a tube of solution and a membrane, two half cells can be seen as one. So as long as no thing leaves either of the two half cells, the whole is always neutral.
Difficulty 2: as in the picture, cations go to the right cell, and anions go to the left. I can understand that anions go the left because the number of zinc ions increase as the reaction proceed. But in the right half cell, the electrons are being absorbed, its charge cancelled by that of the copper ion; I don't see any need for sodium ions to move to the right.
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