Question

In a step-up transformer, the ratio of turns of primary to secondary coils is $1:10$ and primary voltage is $230V$. If the load current is $2A$, the current in the primary coil is:
(A). $20A$
(B). $10A$
(C). $2A$
(D). $1A$

Answer 1

Hint: There are two types of transformers; one is used to increase the voltage from one circuit to the other while the other is used to decrease the voltage. The ratio of the number of turns of secondary to primary coil is equal to the ratio of output voltage to input voltage and also equal to the ratio of input current to output current.
Formulas used:
$\dfrac{{{N}_{2}}}{{{N}_{1}}}=\dfrac{{{V}_{2}}}{{{V}_{1}}}=\dfrac{{{I}_{1}}}{{{I}_{2}}}$

Complete answer:
A transformer is a device which works on the principle of electromagnetic induction. It is used to increase or decrease the value of voltage between circuits. The voltage is increased by using a step-up transformer.
The ratio of primary to secondary coils is related to voltage and current as-
$\dfrac{{{N}_{2}}}{{{N}_{1}}}=\dfrac{{{V}_{2}}}{{{V}_{1}}}=\dfrac{{{I}_{1}}}{{{I}_{2}}}$
Here, ${{N}_{2}}$ is the number of turns in the secondary coil
${{N}_{1}}$ is the turns present in the primary coil
${{V}_{2}}$ is the output voltage
${{V}_{1}}$ is the input voltage
${{I}_{2}}$ is the current is the output current
${{I}_{1}}$ is the input current
The load or output current is given as $2A$ and the ratio of number of turns is $1:10$, therefore,
$\dfrac{{{N}_{2}}}{{{N}_{1}}}=10$. In the above equation, substituting given values to get,
$\begin{align}
  & \dfrac{{{N}_{2}}}{{{N}_{1}}}=\dfrac{{{I}_{1}}}{{{I}_{2}}} \\
 & \Rightarrow 10=\dfrac{{{I}_{1}}}{2} \\
 & \therefore {{I}_{1}}=20A \\
\end{align}$
The current in the input circuit is $20A$.
Therefore, the primary coil has a current of $20A$.

Hence, the correct option is (A).

Note:
The primary coil is the input circuit while the secondary coil is the output circuit. According to Ohm’s law, as the voltage increases, the current decreases that is why the voltage in the output circuit is more and the current has a small value. The electrical energy is transferred from one circuit to another through the transformer.