Question 18.QE.9: Transformer for laptop Your laptop requires a 24-V emf to fu......
Transformer for laptop
Your laptop requires a 24-V emf to function. What should be the ratio of the primary coil turns to secondary coil turns if this transformer is to be plugged into a standard house AC outlet (effectively a 120-Vemf)?
Represent mathematically The ratio we are looking for is related to the coil emfs by Eq. (18.7):
\varepsilon_{\mathrm{s}}=\frac{N_{\mathrm{s}}}{N_{\mathrm{p}}} \varepsilon_{\mathrm{p}}
Solve and evaluate Solving for the ratio and inserting the appropriate values:
\frac{N_{\mathrm{p}}}{N_{\mathrm{s}}}=\frac{\varepsilon_{\mathrm{p}}}{\varepsilon_{\mathrm{s}}}=\frac{120 \mathrm{~V}}{24 \mathrm{~V}}=5
The primary coil needs to have five times the number of turns as the secondary coil. This is a step-down transformer, since the resulting secondary coil peak emf is lower than the primary coil peak emf.
Try it yourself: If the primary coil had 200 turns, how many turns should the secondary coil have to reduce the peak emf from 120 V to 6 V?
Answer: 10 turns.