Final answer:
The input primary voltage Vp must be impressed on the primary coil for a transformer to operate properly, as it determines the output voltage Vs in accordance with the transformer equation.
Step-by-step explanation:
The voltage number that needs to be impressed on the primary coil for proper operation of a transformer is known as the input primary voltage Vp. According to Faraday's law of induction, the output voltage Vs from the secondary coil is almost entirely dependent on the input voltage Vp across the primary coil and the ratio of the number of loops in the primary and secondary coils. This relationship is commonly referred to as the transformer equation, which states that the ratio of the secondary to primary voltages equals the ratio of the number of loops in their respective coils. This principle is vital in understanding the transmission of electrical power through transformers.