The dynamical and equilibrium properties of charged polymers of random copolymerization (polyampholytes) are studied for both the single-chain and multichain cases with the use of molecular dynamics simulations. Single-chain polyampholyte has three temperature regimes under neutral conditions, which are characterized by an elongated Gaussian coil and a very dense globule for high and low temperatures, respectively, and by a transition between them at medium temperature, The gyration radius of single-chain polyampholyte shows a hysteresis against slow cyclic changes in temperature under the Coulomb force and short-range attraction force. The multichain polyampholyte takes a segregated globular phase at low temperatures and the wall-bound one-phase state with separated chains at high temperatures. The polyampholyte chains overlap significantly below critical temperature, at which glass transition takes place because of the Coulomb force. Added salt ions screen the electric field between the monomers and make the polyampholyte soluble when their density is comparable to that of the golyampholyte.