If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it passes. The area is constant, equal to the cross-sectional area of ​​the transformer core, where the magnetic field varies with time depending on the excitation of the primary. Since the same magnetic flux passes through the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding is equal. Taking the ratio of the two equations for Vs and VP gives the basic equation for increasing and decreasing voltage. Ideal power equation - the ideal transformer as circuit elements - if the secondary coils are attached to a load that allows current to flow, electrical energy is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all incoming energy is transformed from the primary circuit to the magnetic circuit and to the secondary circuit. If this condition is met, the incoming electrical power must be equal to the outgoing power. Pincoming = IpVp = Poutgoing = IsVsGiving the ideal transformer