Solenoids

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AP Physics 2 › Solenoids

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A transformer is plugged into a household outlet $\left ( 120V\right )$ that supplies of current. The transformer steps up the potential to having a secondary coil with turns. How many turns does the primary coil have?

$N_{1}=108$ turns

CORRECT

$N_{1}=7500$ turns

$N_{1}=65$ turns

$N_{1}=1080$ turns

Explanation

The relationship between the number of turns for the primary coil and secondary coil in a transformer ( $N_{1}$ and $N_{2}$ respectively) to the relative potentials is

$V_{2}=\frac{N_{2}}{N_{1}}V_{1}$

Solving for $N_{1}$ ,

$N_{1}=\frac{V_{1}}{V_{2}}N_{2}=108$

Suppose a $5\textup{cm}$ long solenoid has a current increase from zero to $2\textup{A}$ in $0.2\textup{s}$ . The induced EMF magnitude is $0.008\textup{V}$ . Find the inductance.

$0.0008 \textup{H}$

CORRECT

$0.004 \textup{H}$

$0.0002 \textup{H}$

$0.008 \textup{H}$

$0.00016 \textup{H}$

Explanation

Write the formula for inductance and substitute the givens. The length of the solenoid has no effect in this problem.

$L=\frac{\epsilon }{\frac{\Delta I}{\Delta t}}=\frac{\epsilon \Delta t}{\Delta I}=\frac{(0.008 \textup{V})(0.2 \textup{s})}{2 \textup{A}}=0.0008 \textup{H}$

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