# Lenz's Law requirements (1 Viewer)

#### mrpotatoed

##### Active Member
For lenz's law to take place, there must be a current produced in the conductor to create a magnetic field to oppose that of the magnet. If then, the conductor is not a complete circuit (eg piece of wire) that is moved relative to a magnet, there will be an induced EMF but no current. Therefore, lenz's law will not apply to this scenario, right?

#### nightweaver066

##### Well-Known Member
Lenz's law always applies so long as there is an induced EMF.

For example, you move a north pole towards a wire. It experiences change in magnetic flux => induced emf (Faraday's law) => eddy currents in this case. They circulate in such a way to produce a south pole to oppose the change in magnetic flux (Lenz's law).

#### Kaido

##### be.
For lenz's law to take place, there must be a current produced in the conductor to create a magnetic field to oppose that of the magnet. If then, the conductor is not a complete circuit (eg piece of wire) that is moved relative to a magnet, there will be an induced EMF but no current. Therefore, lenz's law will not apply to this scenario, right?
Pretty sure you just stated Lenz's law, LOL

#### mrpotatoed

##### Active Member
Lenz's law always applies so long as there is an induced EMF.

For example, you move a north pole towards a wire. It experiences change in magnetic flux => induced emf (Faraday's law) => eddy currents in this case. They circulate in such a way to produce a south pole to oppose the change in magnetic flux (Lenz's law).
How do you know if eddy currents are produced, and what exactly are they? Teacher kind of skimped over eddy currents so im not too sure how they work.

#### nightweaver066

##### Well-Known Member
How do you know if eddy currents are produced, and what exactly are they? Teacher kind of skimped over eddy currents so im not too sure how they work.
Eddy currents are induced currents that are circular.

The induced current must flow in such a way to oppose the initial change in flux. Say if you're moving a north pole towards the end of a solenoid where both ends are connected, then current can flow throughout the solenoid and through this circuit to produce the opposing magnetic flux (not eddy currents).

Now what if you're moving the magnet towards the front of the solenoid (not at either ends)? Then circular currents will be induced within each of the wires to produce the opposing magnetic flux, and these are eddy currents.

#### mrpotatoed

##### Active Member
So eddy currents are induced currents in an open circuit then, or in a non-conductive path? And back EMF is another word for induced EMF?

#### PhysicsMaths

##### Active Member
For lenz's law to take place, there must be a current produced in the conductor to create a magnetic field to oppose that of the magnet. If then, the conductor is not a complete circuit (eg piece of wire) that is moved relative to a magnet, there will be an induced EMF but no current. Therefore, lenz's law will not apply to this scenario, right?
Yes. If the conductor is not connected to an external circuit, only a potential difference (excess and deficiency of electrons on the ends), and hence an EMF will be induced within the conductor. The induced current cannot move through the conductor no there is essentially no magnetic field created to oppose the conductor's motion.

#### PhysicsMaths

##### Active Member
So eddy currents are induced currents in an open circuit then, or in a non-conductive path? And back EMF is another word for induced EMF?
No, back EMF is a term used to explain the opposing force or opposing change in flux created by the induced current (that creates the magnetic field)

#### mrpotatoed

##### Active Member
Yes. If the conductor is not connected to an external circuit, only a potential difference (excess and deficiency of electrons on the ends), and hence an EMF will be induced within the conductor. The induced current cannot move through the conductor no there is essentially no magnetic field created to oppose the conductor's motion.
Thanks, nightweaver said there will be eddy currents though, which repel the magnet, is this true?

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#### Kaido

##### be.
No, back EMF is a term used to explain the opposing force or opposing change in flux created by the induced current (that creates the magnetic field)
Pretty sure this is not true. emf induces a current, not the other way around.

(Or is it the current induced that gives rise to a 'backwards' emf? )

#### Kaido

##### be.
Thanks, nightweaver said there will be eddy currents though, which repel the magnet, is this true?
Yeah, eddy currents produce their own mag field that opposes or 'repel' (though oppose is a better word) the change in mag. flux

#### PhysicsMaths

##### Active Member
Pretty sure this is not true. emf induces a current, not the other way around.

(Or is it the current induced that gives rise to a 'backwards' emf? )
No, I stated that the induced current creates the opposing force, or opposing change in flux, not the EMF.

#### mrpotatoed

##### Active Member
Just as a side question (don't want to make another thread for it) can anyone tell me how to do question 2.9.6 of the physics dot point book? Using the right hand palm rule I get (a) right but the other three wrong.

#### PhysicsMaths

##### Active Member
Just as a side question (don't want to make another thread for it) can anyone tell me how to do question 2.9.6 of the physics dot point book? Using the right hand palm rule I get (a) right but the other three wrong.
What it looks like in those questions is that F represents the 'applied force' on the conductor, and so, the force due to the induced current is in the opposite direction. So basically, reverse the direction of F and then use RHP rule.
e.g. for b, the new F is to the right, and current directed down the page, so mag. field lines are going into the page

#### el_manu

##### Member
Lenz’s Law, states that the magnetic field created from an induced emf will be in the opposite direction of the
change in flux. In generators. Lenz’s Law says which way the current will flow so that an opposite magnetic field
to the flux is created.