Question

A neutral iron bar is dragged to the left at speed v through a region with a magnetic field B points out of the page (Figure 20.122). Which diagram (1-5) best shows the state of the bar?

Short answer

The correct diagram is (3).

Step-by-step solution

Given Data

$\begin{array}{l}\text{speed}=v\\ \text{magnetic\hspace{0.33em}field}=B\end{array}$

Concept

The speed is perpendicular to the magnetic field.

Step 3(a): Find which diagram shows the state of the bar

When the bar moves through a uniform magnetic field, the particles with charge q > 0 inside the bar experience a magnetic force

$\overrightarrow{F}=q\left(\overrightarrow{v}\times \overrightarrow{B}\right)$

Here $\overrightarrow{B}$ is out of plane and $\overrightarrow{v}$ is directed towards the left.

$\overrightarrow{F}$ is directed upward.

This tends to push the +ve charges upward leaving –ve charges on the lower end.

Hence the correct diagram is (3).