Magnetic Fields Tutorial 1 - Magnetic Fields



Force On A Current Carrying Wire In A Magnetic Field

You will be familiar with the basic notion of a magnetic field, in which magnetic materials experience a magnetic force.  However it is worth revising some of the basic ideas that you will have come across in early secondary school.







You will be familiar with the motor effect.  If we put a current carrying wire in a magnetic field, we see that there is a force.



As we pass a current through the wire, there is a force that acts on the wire at 90o to the direction of the magnetic field.  This is given by Fleming’s Left Hand Rule with which you will be familiar.



We can work out the force that is exerted on the wire quite simply.  Experiment shows us that the force is proportional to:

This is summed up in a simple formula:


 F = BIl


[B magnetic field strength; I – current in A; l – length in m]


The term B is called the magnetic field strength, or the flux density, and is measured in Tesla, T.  The magnetic flux density can be thought of as the concentration of field lines.  We can increase the force by increasing any of the terms within the equation.  If we coil up the wire, we increase its length within the magnetic field.


Worked Example

A current of 8.5 A flowing through a magnetic field is found to exert a force of 0.275 N.  The length of wire in the magnetic field is 5 cm.  What is the value of the magnetic field?
Formula first:  F = BIl.

Ž B =  F  = ___0.275 N ___ = 0.647 T

         Il       8.5 A ´ 0.05 m


Question 1

In a demonstration of the above equation, the length of wire in a magnetic field is 0.05 m.  When a current of 2.5 A flows, a force of 0.01 N is shown.  What is the magnetic field strength?


This relationship holds true as long as the current is at 90o to the magnetic field.  If the wire is at an angle to the field, the relationship takes this into account by changing to:


F = BIl sin q