WAEC Physics 2019 Paper

Concept:A $100\%$ efficient transformer has zero power loss, so the power supplied to the primary equals the power delivered by the secondary.Explanation:Input power is $E_p I_p$, while output power is $E_s I_s$.For $100\%$ efficiency, these two powers are equal:$$E_p I_p = E_s I_s$$In a transformer, the emf ratio equals the turns ratio:$$\frac{E_s}{E_p} = \frac{N_s}{N_p}$$Substituting $E_s = E_p \frac{N_s}{N_p}$ into the power equation gives:$$E_p I_p = E_p \frac{N_s}{N_p} I_s$$Cancelling $E_p$ on both sides and rearranging:$$N_p I_p = N_s I_s$$Therefore, the correct relation is $N_s I_s = N_p I_p$.Answer:C. $N_s I_s = N_p I_p$