Lenz's law

I just need some clarification on a few things.
So Lenz's law states that emf acts in the opposite direction to the change in flux or flux linkage.
Does that mean that emf has a direction and is a vector?
Or is it more correct to say that the induced emf induces a current which acts to oppose the change? And if this is a better description how should you explain it when it's not a complete circuit so no current flows?

Reply 1

uberteknik

Original post by G.Y

E.M.F. is a scalar quantity which is confusing because force is a vector. In fact, emf is the work done by the electromagnetic field per unit charge and work has magnitude but not direction.

Original post by G.Y

Or is it more correct to say that the induced emf induces a current which acts to oppose the change? And if this is a better description how should you explain it when it's not a complete circuit so no current flows?

It's a better description. The induced current generates a magnetic field which opposes any change in the field.

It's a consequence which preserves the energy conservation laws.

(edited 6 years ago)

Reply 2

G.Y

Original post by uberteknik

E.M.F. is a scalar quantity which is confusing because force is a vector. In fact, emf is the work done by the electromagnetic field per unit charge and work has magnitude but not direction.

It's a better description. The induced current generates a magnetic field which opposes any change in the field.

It's a consequence which preserves the energy conservation laws.

Thank you but does that mean when the circuit isn't complete, for example just a magnet going into a coil, that no magnetic field is actually generated to oppose the change?

Reply 3

uberteknik

Original post by G.Y

Thank you but does that mean when the circuit isn't complete, for example just a magnet going into a coil, that no magnetic field is actually generated to oppose the change?

A field is generated momentarily as the magnet enters or leaves the coil. The inherent charge in the conductor rushes to one end and bunches up rather like the charge on the plates of a capacitor. The charge imbalance between the ends of the conductor gives rise to a potential difference.

If the magnet is longer than the coil length, then the magnetic field within the coil is constant and hence no charge will flow. An opposing magnetic field cannot be generated without charge movement.

Reply 4

Eimmanuel

Study Forum Helper

Original post by G.Y

I would add “extra” info to your queries though I think uberteknik had done a good job.

The direction that we are talking in emf is something like the direction of current. Recall that current is a scalar quantity.

We can deduce the “direction of induced emf” considering the rate of change of magnetic flux linkage. This is described in detail in the university physics by Young & Freedman, you can look into it. Or you can follow the link and see the info - Direction of Induced emf (page 5 -7 )

https://www.scribd.com/doc/135877738/Ch29-Electromagnetic-Induction

Original post by G.Y

….Or is it more correct to say that the induced emf induces a current which acts to oppose the change? And if this is a better description …

I would not say it is more correct. They are “equivalent”. From my experience, students tend to find Lenz’s law stated in induced current more easy to understand.

The induced current in a loop is in the direction that creates a magnetic field that opposes the change in magnetic flux through the area enclosed by the loop.
The above statement is taken from Serway Physics textbook.

Original post by G.Y

….how should you explain it when it's not a complete circuit so no current flows?

What you have described is what known as motional emf. You can find the detail in the above link on page 14 – Motional Electromotive force.

Note that motional emf and the induced emf “caused by” changing magnetic flux linkage can be equivalent or different (this is not required to be known by A level students). If there is a changing magnetic flux linkage through a closed circuit and the closed circuit is also in motion, the total emf induced is equal to the sum of motional emf and induced emf due to changing magnetic flux linkage.

Original post by uberteknik

.....It's a consequence which preserves the energy conservation laws.

I hope the following paragraphs would not cause a stir of arguments.

I know that a lot of physics references or textbooks mention that Lenz’s law is a consequence of conservation of energy. I don’t think so. Most of the examples that the physics books stated or described make use of contradiction argument which I think have not really shown it correctly (in my opinion). I agree with what Moses had described.

Re_ [Phys-L] Lenz's law and conservation of energyjpg_Page2.jpg

Re_ [Phys-L] Lenz's law and conservation of energyjpg_Page2.jpg

If you are an A level Physics student, please check with your teacher what the syllabus agrees on. I know that IB physics expects students to learn that Lenz law is a consequence of conservation of energy. I usually tell my students to learn what the syllabus expects and use them for the exams but they should know what could be “bad” about the explanation.

Hope that I did not cause more confusion.