A modern and more general definition uses the Lagrangian L (or equivalently the Hamiltonian H ) of a system. Usually, L (or H ) of a system describing an interaction can be separated into a kinetic part T and an interaction part V : L = T − − V (or H = T + V ). In field theory, V always contains 3 fields terms or more, expressing for example that an initial electron (field 1) interacted with a photon (field 2) producing the final state of the electron (field 3). In contrast, the kinetic part T always contains only two fields, expressing the free propagation of an initial particle (field 1) into a later state (field 2). The coupling constant determines the magnitude of the T part with respect to the V part (or between two sectors of the interaction part if several fields that couple differently are present). For example, the electric charge of a particle is a coupling constant that characterizes an interaction with two charge-carrying fields and one photon field (hence the common Feynman diagram with two arrows and one wavy line). Since photons mediate the electromagnetic force, this coupling determines how strongly electrons feel such a force, and has its value fixed by experiment. By looking at the QED Lagrangian, one sees that indeed, the coupling sets the proportionality between the kinetic term T = ψ ψ ¯ ¯ ( i ℏ ℏ c γ γ σ σ ∂ ∂ σ σ − − m c 2 ) ψ ψ − − 1 4 μ μ 0 F μ μ ν ν F μ μ ν ν and the interaction term V = − − e ψ ψ ¯ ¯ ( ℏ ℏ c γ γ σ σ A σ σ ) ψ ψ .