Transformer Example

Here's a classic question on the workings of a transformer:
Jessie builds a transformer, connected to a 12 V a.c. power supply. She has 200 turns on the primary and 40 turns on the secondary. What is the output voltage?

From the information, we can say V_p = 12 V, N_p = 200 and N_s = 40. Putting in the numbers:

This can easily be solved to give V_s = 2.4 V. But since N_s is exactly 5 times smaller than N_p, it shouldn't be a surprise that V_s is also 5 times smaller than V_p!

Question: how many turns would she need on the secondary if she wanted a 24 V output?

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