Physics · Alternating Current · NEET
No. This is the most common mix-up. In an ideal transformer power is conserved: Vp x Ip = Vs x Is. So if voltage is stepped up, current must go DOWN by the same factor. Example (NCERT): 220 V at 10 A steps up to 440 V at 5.0 A. Voltage doubled, current halved. A step-up transformer raises voltage but lowers current.
The rule is: the side with MORE turns has the HIGHER voltage. Step-up: secondary has more turns (Ns > Np), so output voltage is higher. Step-down: secondary has fewer turns (Ns < Np), so output voltage is lower. Just remember 'more turns = more volts' and you never have to memorise which is which.
Use power balance for an ideal transformer: input power = output power, so Vp x Ip = P (the load power). Rearrange: Ip = P / Vp. Example: a 44 W lamp on a 220 V mains gives Ip = 44/220 = 0.2 A. You do NOT need the turns ratio when the load power is given directly.
Yes. Primary = the coil connected to the AC source (input). Secondary = the coil connected to the load/device (output). 'Step-up' and 'step-down' are defined by comparing secondary to primary, so always read the question to see which side is the source.
No. A transformer needs a CHANGING magnetic flux to induce an emf in the secondary (mutual induction). Steady DC gives constant flux, so no emf is induced and no output appears. Transformers work only with AC — this is a key reason AC is used for power transmission.
In an ideal transformer, the turns ratio is Np/Ns = 1/2. The ratio Vs : Vp is equal to (symbols carry usual meaning):
A 12 V, 60 W lamp is connected to the secondary of a step-down transformer whose primary is connected to AC mains of 220 V. Assuming the transformer to be ideal, the current in the primary winding is:
A step-down transformer connected to an AC mains supply of 220 V is made to operate an 11 V, 44 W lamp. Ignoring power losses, the current in the primary circuit is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For an ideal transformer: Vs/Vp = Ns/Np = Ip/Is. The voltage ratio equals the turns ratio, and the current ratio is the inverse of the turns ratio.
If Ns > Np then Vs > Vp — voltage is raised. The secondary (output) side has more turns and higher voltage but lower current. That is a step-up transformer.
Because power is conserved: Vp x Ip = Vs x Is (ideal). If Vs is larger, Is must be smaller so the product (power) stays the same. Energy is never created.
Step-up transformers raise voltage at the power station so current is small. Small current means less I squared R heating loss in long transmission lines, so more power reaches homes.
No. A transformer only changes voltage and current. The output AC has exactly the same frequency (50 Hz in India) as the input.
