# A step-up transformer has 300 turns on its primary coil and 90,000 turns on its secondary coil. If the current flowing through the secondary is .5 amps, what is the current flowing in the primary?

Nov 22, 2015

$150 \text{A}$

#### Explanation:

${V}_{s} / {V}_{p} = {N}_{s} / {N}_{p} \text{ } \textcolor{red}{\left(1\right)}$

$N$ refers to the number of turns

$V$ is voltage

$s$ and $p$ refer to the secondary and primary coil.

From the conservation of energy we get:

${V}_{p} {I}_{p} = {V}_{s} {I}_{s} \text{ } \textcolor{red}{\left(2\right)}$

From $\textcolor{red}{\left(1\right)}$:

${V}_{s} / {V}_{p} = \frac{900 \cancel{00}}{3 \cancel{00}} = 300$

$\therefore {V}_{s} = 300 {V}_{p}$

Substituting for ${V}_{s}$ into $\textcolor{red}{\left(2\right)} \Rightarrow$

$\cancel{{V}_{p}} {I}_{p} = 300 \cancel{{V}_{p}} \times 0.5$

$\therefore {I}_{p} = 150 \text{A}$

Seems a big current.