Ta có: $\dfrac{U_{1}}{U_{2}}=\dfrac{n_{1}}{n_{2}}$
$⇒n_{2}=\dfrac{n_{1}.U_{2}}{U_{1}}$
Khi $U_{2}=10V:n_{2}=\dfrac{n_{1}.U_{2}}{U_{1}}=\dfrac{4000.10}{220}=\dfrac{2000}{11}$ vòng
Khi $U_{2}=12V:n_{2}=\dfrac{n_{1}.U_{2}}{U_{1}}=\dfrac{4000.12}{220}=\dfrac{2400}{11}$ vòng