The relationship is given by Faraday 's law : The two equations for the EMF are , firstly , the work per unit charge done against the Lorentz force in moving a test charge around the ( possibly moving ) surface boundary ∂ Σ and , secondly , as the change of magnetic flux through the open surface Σ .
The relationship is given by Faraday 's law : The EMF is determined in this equation in two ways : first , as the work per unit charge done against the Lorentz force in moving a test charge around the ( possibly moving ) closed curve ∂ Σ ( `` t '' ) , and second , as the magnetic flux through the open surface Σ ( `` t '' ) .