Physics 208-02 – Introductory Physics II

Solution Set 9

Nazareth College

Department of Chemistry & Biochemistry

Robert F Szalapski, PhD – Adjunct Lecturer

www.CallMeDrRob.com

Spring 2012

## § Chapter 20, Problem 9

Alpha particles of charge and mass are omitted from a radioactive source at a speed of . What magnetic field strength would be required to bend them in a circular path of radius ?

Assuming that the magnetic field is oriented perpendicular to the velocity vector of an alpha particle (so that ), the magnetic force provides the centripetal acceleration.

## § Chapter 20, Problem 10

Determine the magnitude and direction of the force on an electron traveling horizontally to the east in a vertically upward magnetic field of strength .

The magnitude is given by

To find the direction, point the thumb of your right hand to the east and your fingers up, and the palm of your hand will be directed south. Because an electron has a negative charge, the direction of the force will be reversed. The direction is north.

## § Chapter 20, Problem 14

A (kinetic energy) proton enters a magnetic field in a plane perpendicular to the field (so that ). What is the radius of its path?

With the magnetic force on the proton supplying the centripetal force,

We will first need to find the speed, .

We may now solve for .

## § Chapter 20, Problem 16

What is the velocity of a beam of electrons that go undeflected when passing through perpendicular electric and magnetic fields of magnitude and ? What is the radius of the electron orbit if the electric field is turned off?

This arrangement is called a velocity selector. For it to work the electric field, the magnetic field and the velocity of the electron must all be mutually perpendicular. Assuming that it is set up correctly so that the electric force and the magnetic force oppose one another

If the electric field is switched off it will move in a circle with the centripetal force supplied by the magnetic force.