Analytical mean-field theories and lattice model simulations have been used to study the charge-induced conformational changes of single polymer molecules. The compact-to-extended transition induced by charge is found to be first-order (i.e., two-state transition with a transition state) in the presence of strong short-range interactions at low temperatures. Short-range interactions decay much faster than electrostatic energy so expansion below a minimal value cannot produce electrostatic compensation for short-range energy loss. This is the origin of a free energy barrier (transition state) between the compact and the extended states. If the short-range interactions are weak in comparison with attractive and repulsive Coulomb interactions, the transition is expected to be second-order (one-state transition without a transition state). The prediction is compared to the computer simulation of the exhaustive enumeration of all 12-mer cubic lattice polymer conformations using different potentials, and qualitative agreement is found. Implications for protein folding and unfolding are discussed.