Simplifying Lenz's Law

I am having great difficulty grasping the concept of Lenz's law. As far as I am unware it is derived as:

"any current driven by an induced e.m.f opposes the change causing it"

I do not really understand what this means - help would be greatly appreciated :)
 
1,155
78
The law of Lenz can be described as follows:

Induced quantities, such as current (mmf), voltage (emf), flux, & torque, are always oriented in a direction opposite to the original quantities.

A motor is started. It's current draw is large when at a standstill. Once the rotor spins, mmf/emf are induced into the stator. This induced current opposes the original current. Hence the net current draw decreases.

If I pedal a bicycle type generator under no load, and I control the speed to 300 rpm, for example. I then load the generator with 50 watts. The added load current has a magnetic field which produces a an additional torque. This torque counters my pedaling torque.

Otherwise, a motor or generator would create its own supply of energy. Induced quantities, electrical as well as mechanical, always act in the *opposite direction* to the corresponding original quantities.

Is this helpful?

Claude
 
This Law is often demonstrated by moving a magnet towards the end of a long coil of wire. (Solenoid)
The result is that an emf is generated/induced in the coil.
If a current is able to flow in the coil, it flows in such a direction that the magnetic field it generates opposes the one that produced it- the one due to the magnet.
In practical terms, if you moved a N pole of a magnet towards the end of the coil, the current generated at the end of the coil would produce a N pole there - the N pole would be opposing the one producing it. (Like poles repel)
If you Google Lenz's Law demo you will find a better explanation of this than is possible in words, using some animations. There's also some stuff on YouTube.
 

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top