Step-up or Step-down?

Brief solution: Since the secondary coil has fewer turns than the primary coil, the voltage across the secondary coil is smaller than the voltage across the primary coil. Hence the transformer is a step-down transformer . $_\square$

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Detailed Solution: A one-phase transformer has two coils: a primary coil and a secondary coil as shown in the figure below.

The transformer core prevents the flux from leaking, so the flux in primary coil is equal to the flux in secondary coil. The flux in primary coil is proportional to $N_{P} \times I_{P} \times R_{P}$ . Similarly, the flux in secondary coil is proportional to $N_{S} \times I_{S} \times R_{S}$

$N_{P} \times I_{P} \times R_{P} = N_{S} \times I_{S} \times R_{S}$

The radius of both coils is the same. Since this is an ideal transformer, power in primary coil is equal to the power in secondary coil.

$V_{P} \times I_{P} = \text{ Power } = V_{S} \times I_{S}$

Thus, voltage is inversely proportional to the current.

$\frac{N_{P}}{V_{P}} =\frac{N_{S}}{V_{S}} \implies \frac{V_{S}}{V_{P}} = \frac{N_{S}}{N_{P}}$

Since the ratio $N_{S} < N_{P}$ , we can say that $V_{S} < V_{P}$ . Since the secondary voltage is less than primary voltage, the voltage is being stepped down, Therefore, this is a Step-down Transformer $_\square$

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Image Credit: Wikimedia Commons BillC

A step down transformer has more number of turns in the primary coil than the secondary coil