Register to reply

A rotating coil measuring the magnetic field

by lfused
Tags: coil, field, magnetic, measuring, rotating
Share this thread:
Aug1-08, 11:35 AM
P: 7
1. The problem statement, all variables and given/known data
A rotating coil is a common device for measuring magnetic fields. Consider a coil of area A and N turns that is rotated at angular frequency w in a magnetic field. The position of the coil is adjusted so as to produce a maximum induced current Imax which can be measured by using an appropriate ammeter. R is the total resistance of the coil circuit find the relationship between the unknown magnetic field and Imax

2. Relevant equations

E = -d(phi)/dt where phi = magnetic flux
phi = BA where B is the magnetic field
E= -N *d(phi)/dt

3. The attempt at a solution
I'm not really sure what to do with this problem at all because we didn't really go over how angular frequency related to magnetic flux in class. Since w= (2*pi*r)/T meaning w is related to the change in time I tried using the equation IR= -(BAN)/w but I'm not sure if that's right.. and I wouldn't know how to determine the Imax from that. I know taking a derivative can help determine max and min by setting the equation equal to zero... but I really have no clue what I'm doing.
Phys.Org News Partner Science news on
Pilot sites in energy from coffee waste show good results
Startups offer banking for smartphone users
Factor in naked mole rat's cells enhances protein integrity
Aug2-08, 09:50 AM
P: 208
The magnetic flux across the coil is

[tex]\Phi = NBA\cos \phi = NBA \cos \omega t\, .[/tex]

that is, varies sinusoidally. Taking the derivative, we find the induced EMF:

[tex]E = -\frac{d\Phi}{dt} = NBA\omega \sin \omega t\, .[/tex]

Ohm's law gives

[tex]I = \frac{E}{R} = \frac{NBA\omega}{R} \sin \omega t\, .[/tex]

The maximum value of current can now be clearly seen. So, you've almost guessed your equation right. Next time you could try using dimensional analysis to see if it's a legal expression or not.

Register to reply

Related Discussions
Radius of coil in magnetic field Introductory Physics Homework 5
A coil with turns given magnetic field, time find the emf induced in the coil Introductory Physics Homework 4
Magnetic Field at the Center of a Coil Introductory Physics Homework 3
Magnetic field through coil is given by mu*Current/2PI*r, right? Introductory Physics Homework 0
Magnetic field due to an elliptical coil Advanced Physics Homework 3