PHYS 2212 Module 9.3

Motional Emf

Learning Objectives

By the end of this section, you will be able to:

• Determine the magnitude of an induced emf in a wire moving at a constant speed through a magnetic field
• Discuss examples that use motional emf, such as a rail gun and a tethered satellite

Motional emf

Faraday’s law tells us that we can induce an emf if we change the magnetic flux in some way. Here, we’re going to look at another way to induce an emf with a different kind of change. Imaging we have a metal (conducting) rod that we are going to move through a magnetic field.

In this picture, the magnetic field is pointing out of the page (the dots) and the metal rod is going to move to the right with a speed v. To make the rod move, I’m going to literally grab it and pull it to the right. Nothing fancy here. 

As the bar moves to the right, all the atoms in the bar move to the right, because they are stuck in the bar. This means all the electrons move to the right, all the positive charges move to the right. Everything moves to the right at speed v. Using the RHR, figure out what will happen to the charges in the rod due to the magnetic force on them as they all move to the right through this B-field.