# 17. Magnetic Forces

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## Learning Objectives

• To be able to describe the motion of a charge in a uniform magnetic field.
• To comprehend how a velocity selector works.

## Preparation

### Course Notes

Read through the course notes before watching the video.  The course note files may also contain links to associated animations or interactive simulations.

The Magnetic Field (PDF)

## Lecture Video

### Video Excerpts

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## Learning Activities

### Guided Activities

Read through the class slides. They explain all of the concepts from the module.

Slides (PDF)

### Self-Assessment

Do the Concept Questions first to make sure you understand the main concepts from this module. Then, when you are ready, try the Challenge Problems.

### Concept Questions

Concept Questions (PDF)

Solutions (PDF)

### Challenge Problems

Challenge Problems (PDF)

Solutions (PDF)

## Problem Solving Help

Watch the Problem Solving Help videos for insights on how to approach and solve problems related to the concepts in this module.

### Problem 1: Electron Gyrating in a Magnetic Field

An electron moves in a clock-wise direction in a semi-circle in a magnetic field. What is the direction and magnitude of the magnetic field that causes this motion? What is the time it takes for the electron to traverse the semi-circular path?

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» iTunes U (MP4 - 10MB)
» Internet Archive (MP4 - 10MB)

### Problem 2: Mass of an Isotope

An isotope of aluminum which is singly charged moves in the crossed electric and magnetic fields of the velocity selector of a mass spectrometer. The magnitude of the electric field is E and of the magnetic field B. What is the velocity that is selected by this arrangement? The ion then moves into a region with a magnetic field of the same magnitude, and with no electric field, and subsequently moves in a circle of radius R meters. What is the mass of this aluminum ion in terms of the quantities given?

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» iTunes U (MP4 - 14MB)
» Internet Archive (MP4 - 14MB)

### Problem 3: Force on a Wire in a Magnetic Field

A current carrying wire of length l carries current in the y-direction. It is immersed in a constant magnetic field B with a given orientation. What is the force per unit length on the wire? What is the total force on the wire?

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» iTunes U (MP4 - 9MB)
» Internet Archive (MP4 - 9MB)

### Problem 4: Magnetic Interaction of a Wire and a Rectangular Loop

A long straight wire carries a current I1. There is a rectangular loop with its long sides parallel to the wire, which carries a current I2. The long sides of the loop and the wire all lie in the same plane. The nearer side of the rectangular loop is a distance r1 away from the wire, and the more distance side is a distance r2 from the wire. What is the force that the rectangular loop feels as a result of the current in the wire?

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