Lenz's Law

[BillyBoBob]

Lenz's Law states:-

" The direction of the induced e.m.f is such that it will tend to establish a current which opposes the motion or change of flux inducing the e.m.f "

I understand Fleming's Right Hand Rule, and Maxwells Corckscrew Rule.

Can somone please describe Lenz's Law.

AS IF YOU WERE TALKING TO A CHILD.

I'm having real issues getting it. On youtube I've seeen the magnets moving through copper tubes, and I have the use of a solinoid with an iron core with metal discs including one with a cut in it.
I guess my main issue is understanding how D.C and A.C effect magnetic flux on the rings.

Many thanks

 

[jaresquire]

Changing magnetic flux can induce an emf which can cause a current to flow. This current will create a magnetic field which can either oppose or assist the field that created it.
If it assisted the field then the field will increase indefinitely - impossible as would require energy to be created. Therefore the induced field will always act against the field that creates it.
Lenz's law can save a lot of work with vectors trying to find the direction of an induced current.

 

[Adam W]

You know how a motor can be used as a generator?

Well if you power up a motor to make it spin, when it's spinning round it's working like a generator as well, but the voltage it produces [back emf] goes in the opposite direction to the voltage you're putting in, like 2 people pushing a car, but pushing in opposite directions.

 

[pushrod]

when you calculate an emf using the rate of change of flux X number of turns in the coil, lenz's law is the reason it is a negative value - because it is opposing the supply voltage.
With an inductance this voltage gives a current that is always resisting the supply current - it is like a resistance to the current and is called the inductive reactance (Xl) and is measured in ohms.

Xl =2 x pi x f x L

Edit: with regard to
"I guess my main issue is understanding how D.C and A.C effect magnetic flux on the rings"
direct current will produce a stationary magnetic field and an alternating current will produce one that is constantly growing , collapsing and then changing direction. So the flux will constantly be cutting thru any surrounding conductors (including the one making it) and inducing the back emf. The magnetic field from DC will not do this as it is stationary (apart from when it firsts grows when switched on and collapses when switched off)