repel

1) Normally, electrons repel each other according to Coulomb's law, and this repulsion is much greater that the gravitational attraction between them. However, in particular cases like supraconductivity, electrons can attract each other. 2) Coulomb's law: "If both charges have the same sign (like charges) then the product q1*q2 is positive and the direction of the force on q1 is given by r21; the charges repel each other. If the charges have opposite signs then the product q1*q2 is negative and the direction of the force on q1 is given by -r21; the charges attract each other." "charged elementary particles have mass that is far less than the Planck mass while their charge is about the Planck charge so that, again, gravitational forces can be ignored. For example, the electrostatic force between an electron and a proton, which constitute a hydrogen atom, is almost 40 orders of magnitude greater than the gravitational force between them." Source and further information: http://en.wikipedia.org/wiki/Coulomb's_law 3) "In condensed matter physics, a Cooper pair is the name given to electrons that are bound together at low temperatures in a certain manner first described in 1956 by Leon Cooper. Cooper showed that an arbitrarily small attraction between electrons in a metal can cause a paired state of electrons to have a lower energy than the Fermi energy, which implies that the pair is bound. In normal superconductors, this attraction is due to the electron - phonon interaction. The Cooper pair state is responsible for superconductivity, as described in the BCS theory developed by John Bardeen, John Schrieffer and Leon Cooper for which they shared the 1972 Nobel Prize. The reason for the pairing can be seen from a simplified explanation. An electron in a metal normally behaves as a free particle. The electron is repelled from other electrons due to their similar charge, but it also attracts the positive ions that make up the rigid lattice of the metal. This attraction can distort the positively charged ion lattice in such a way as to attract other electrons (the electron-phonon interaction). At long distances this attraction between electrons due to the displaced ions can overcome the electrons' repulsion due to their negative charge, and cause them to pair-up." Source and further information: http://en.wikipedia.org/wiki/Cooper_pair

Repel one another because they have similar charges.