Electromagnetism is a simple example of a gauge theory where the underlying potentials (the vector and scalar potentials) are defined only up to a gauge choice. The vector potential generates magnetic fields through its spatial variation and electric fields through its time dependence(1). Here, we report experiments in which we have produced a synthetic gauge field. The gauge field emerges only at low energy in a rubidium Bose-Einstein condensate: the neutral atoms behave as charged particles do in the presence of a homogeneous effective vector potential(2). We have generated a synthetic electric field through the time dependence of an effective vector potential, a physical consequence that emerges even though the vector potential is spatially uniform.