Single correct · +4 / −1
In an ideal step-up transformer with Nₛ > Nₚ:
- (a) voltage increases, current increases
- (b) voltage increases, current decreases
- (c) voltage decreases, current decreases
- (d) voltage decreases, current increases
Answer and solution
Answer (b) voltage increases, current decreases
Solution
Physics sir · 4 steps
Voltage follows the turns, and since power stays the same, current has to go the opposite way.
- Voltage follows turns. The same changing flux links every turn of both coils, so each turn has the same emf. The voltage on a coil is therefore proportional to its number of turns.
Vₛ/Vₚ = Nₛ/Nₚ = Iₚ/Iₛ
- Power is conserved. An ideal transformer loses no energy, so Vₚ Iₚ = Vₛ Iₛ. If the voltage goes up by some factor, the current must go down by the same factor.
- Use the turns ratio. Nₛ/Nₚ > 1 gives Vₛ > Vₚ, so the voltage increases. Then Iₛ/Iₚ = Nₚ/Nₛ < 1, so Iₛ < Iₚ and the current decreases.
- So. In a step-up transformer the voltage increases and the current decreases, which is option (b).
The idea to keep. A transformer only trades voltage for current, it never creates power. Whatever happens to voltage, current does the opposite by the same turns ratio.
Answer from the NTA final answer key. Solution by PrepWiser.