The Open Door Web Site
HOME PAGE BIOLOGY CHEMISTRY PHYSICS ELECTRONICS HISTORY HISTORY of SCI & TECH MATH STUDIES LEARN FRENCH STUDY GUIDE PHOTO GALLERY
ATOMIC and NUCLEAR ELECTRICITY and MAGNETISM MEASUREMENTS MECHANICS OPTICS PRACTICAL WORK QUESTIONS RELATIVITY THERMAL PHYSICS WAVES
ELECTRICITY and MAGNETISM
Google
Custom Search
Simple A.C. Generator
When a current flows through a conductor in a magnetic field, it experiences a force.  
The relation between the directions of the current, field and force can be remembered by using Fleming's left hand rule.
   
Anybody who has been paying attention and has understood the significance of Lenz's law might not be surprised to find that, for the situation in which we move a conductor through a magnetic field in order to generate an emf, an opposite rule can be used.  
Can you guess what it's called? Yes... Fleming's right hand rule!  
   
 
The aid to memory is the same but remember to use your right hand this time.  
As always in electro-magnetism, we are again talking about conventional current.  
   
Consider a coil of wire, rotating in a uniform magnetic field, of flux density B as shown below.  
 
When the coil is in the position shown in the diagram, side 2 is moving down and side 1 is moving up.  
We can use Fleming’s right hand rule to decide that end q will (at that instant) be the positive terminal of the generator.  
Remember, when using the rule, that conventional current flows away from positive towards negative outside the source of electrical energy, the generator in this case.  
When the coil has rotated through half a turn, and p will be positive.  
Therefore, a coil rotating in a uniform magnetic field will have an alternating emf induced in it, of frequency equal to the frequency of rotation of the coil.  
To connect the coil to, for example, a light bulb, carbon brushes make contact with brass slip rings as shown in the next diagram.  
 
   
Let the coil have N turns.  
The flux density has magnitude B.  
The cross-sectional area of the coil is A.  
Let the angle between the normal to the plane of the coil and the field be α  
Then the flux linkage between the field and the coil is given by  
 
If the angular velocity of the coil is ω then α=ωt  
This is assuming that at t = 0, the plane of the coil is perpendicular to the flux lines, ie at 90° to the position shown above.  
 
So, using the Faraday-Neumann law we have  
 
The term in the above equation, represents the slope of a graph of cosωt against t.
 
and, it can be shown that the slope of a graph of  cosωt against t is equal to -ωsinωt  
Therefore, the induced emf in the coil is given by  
 
This means that a graph of induced emf against time would look like this  
 
   
Below the graph are the positions of the coil relative to the magnetic field which correspond to maximum induced emf and zero induced emf.  
SITE MAP
WHAT'S NEW
ABOUT
PRIVACY
COPYRIGHT
SPONSORSHIP
DONATIONS
ADVERTISING
 

© The Open Door Team
2016
Any questions or
problems regarding
this site should be
addressed to
the webmaster

© David Hoult 2017

Hosted By
Web Hosting by HostCentric

 
SiteLock
 
 
Electricity and Magnetism Index Page