Question

A neutral copper bar oriented horizontally moves upward through a region where there is a magnetic field out of the page. Which diagram (1-5) in Figure 20.123 correctly shows the distribution of charge on the bar?

Short answer

The correct diagram is (3).

Step-by-step solution

Given Data

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

Concept

The speed is perpendicular to the magnetic field.

Find which diagram shows the distribution of charge on the bar

Magnetic force is given by Lorentz force law. This says the direction of the magnetic force is given by the cross product of velocity and magnitude field.

Therefore, $F=q\left(v\times B\right)$

Since the direction of velocity is in the upward direction and the direction of magnetic field is out of the page so by the right hand thumb rule the direction of force on the positive charge is towards right and that on the negative charge is towards left.

Hence the correct diagram is (3).