If the coil turns are oriented perpendicular to the magnetic field lines, the flux is the product between the magnetic flux density B and the area A crossed. The area is constant, being equal to the cross-sectional area of ​​the transformer core, where the magnetic field varies over time based on the excitation of the primary. Since in an ideal transformer the same magnetic flux passes through both the primary and secondary coils, the instantaneous voltage across the primary winding is equal. Taking the ratio of the two equations for Vs and VP gives you the basic equation for increasing or decreasing voltage. Ideal Power Equation - The ideal transformer as circuit elements - If the secondary coils are connected to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally the transformer is perfectly efficient; all input energy is transformed from the primary circuit to the magnetic circuit and into the secondary circuit. If this condition is satisfied, the input electrical power must be equal to the output power.Pincoming = IpVp = Poutgoing = IsVsGiving the ideal transformer