# Why does metal moving though a magnetic field slow down?

• I
• thedubdude
The leading and trailing edges of the induced eddy currents generate a magnetic field in a direction to push against the E/W motion of the metal and slow it down.

#### thedubdude

TL;DR Summary
A piece of metal moving West to East in a North to South magnetic field slows down...but how?
A piece of metal moving West to East in a North to South fixed magnetic field slows down...but how? Yes of course eddy currents are set up in the metal and these currents generate their own magnetic field which somehow slows down the moving metal piece...but how does this actually slow the moving metal down? I can't imagine a counter magnetic force , generated by the moving metal, that would slow the metal down, as the fixed North/South field is perpendicular to the West/East motion of the metal. And yes I understand that the induced eddy currents in the moving metal will generate heat which is then dissipated (radiated as infra red light), but what West/East force is exerted on the moving metal to slow it down?

Look up the laws of electromagnetic induction (the place is full of info at all levels). In particular, chase Lenz’s Law. Basically, any induced electric current is in a direction such as to ‘oppose its cause’.
It’s analogous to the reaction force that you get when you accelerate a mass . It sort of knows where you are trying to push it.

• topsquark
thedubdude said:
Summary: A piece of metal moving West to East in a North to South magnetic field slows down...but how?

And yes I understand that the induced eddy currents in the moving metal will generate heat which is then dissipated (radiated as infra red light), but what West/East force is exerted on the moving metal to slow it down?
Conservation of energy says that heat energy must come from somewhere, and the only two places are the fixed magnetic field, or the movement of the conductor in that field.

When the conductor moves through the fixed magnetic field, a voltage appears in the conductor, and a current flows, that heats the conductor. That is an electric generator with a short circuit.

The current that flows in the moving conductor, also makes a magnetic field, that reacts against the initial magnetic field, to create the force that slows the conductor. That is, an electric motor that provides a force opposing motion.

The conductor, moving in the initial field, caused a perpendicular (+90°) current to flow in the conductor, that in turn caused a perpendicular (+90°) magnetic field, to oppose the motion (+180°).
Turning 90° left, twice, is the same as reversing direction. i·i = i² = -1 .

• topsquark
Thank you all for your responses...however I still don't understand. I can't imagine any magnetic field that will slow down the moving metal. Clearly the moving metal generates its own magnetic field...but how does it couple to the North/South field to slow down? The N/S field is orthogonal to the direction of motion so how could coupling to it slow down the metal? And if the induced magnetic field in the metal doesn't couple to the N/S field , what does it couple to to slow down? Thanks.

You must think in three dimensions. North-South, East-West, and Up-Down.
https://en.wikipedia.org/wiki/Lorentz_force#Force_on_a_current-carrying_wire

If the magnetic field is N-S, and the movement is E-W. Then the induced voltage and current will be U-D. That is an electric current generator. The eddy current flows in a low resistance, so there is a high eddy current.

The induced electric eddy current generates a horizontal magnetic field that opposes the initial N-S magnetic field. That is the electric motor that causes the deceleration force.
https://en.wikipedia.org/wiki/Eddy_current_brake

Sorry, but I do not understand.
By Lenz Rule, a conductive loop will resist a change in magnetic flux.
But moving across magnetic field lines is not a change in magnetic flux! Equal number of field lines enter the loop on one side and leave on opposite side. So no electromotive force and no resistance?

So I studied the wikipedia page referenced. An I see how up and down magnetic fields are generated from the moving metal...but please please explain how up and down forces can cause deceleration (ie. a force vector) for the E/W motion?

thedubdude said:
please please explain how up and down forces can cause deceleration (ie. a force vector) for the E/W motion?
The direction of fall is just one way and the direction of the force is also just one way. No 'up and down' in this case.

thedubdude said:
explain how up and down forces can cause deceleration (ie. a force vector) for the E/W motion?
You do understand that the magnetic force on a moving charge is at right angles (90deg) to both the magnetic field and the (induced current) velocity ?

Now I get it...Thank you. The leading and trailing edges of the induced eddy currents generate a magnetic field in a direction to push against the E/W motion of the metal and slow it down. Correct?

Maybe I'm confusing myself, I'm thinking its not the magnetic field generated by the eddy currents (since that is N/S) as much as it is the expression of the Lorentz force itself as you explained: quoting Wikipedia:

" It says that the electromagnetic force on a charge q is a combination of a force in the direction of the electric field E proportional to the magnitude of the field and the quantity of charge, and a force at right angles to the magnetic field B and the velocity v of the charge"

So the force on the eddy currents flowing in forward and back edges of the moving metal have to come in the direction of motion, as that is the only 90 degree angle left to be in quadrature to the N/S magnetic field and the directions of the eddy currents on the front and back edges.

Is this more accurate?

I think so. Hard to have this conversation without a blackboard! But the changing B field induces the currents which then feel the force from the field. The geometry works out as braking force because of resistance in conductor..

hutchphd said:
I think so. Hard to have this conversation without a blackboard! But the changing B field induces the currents which then feel the force from the field. The geometry works out as braking force because of resistance in conductor..
But moving across a B field is not changing B field!

snorkack said:
But moving across a B field is not changing B field!
I remember being told that "it's the rate at which magnetic field lines are being cut". If you're not in favour of field lines then that won't be a reasonable explanation but it's another way of looking at it. There's no equivalent with lines of gravitational field or electric field so that could be a problem but magnetism is always an odd one.

Baluncore said:
When the conductor moves through the fixed magnetic field, a voltage appears in the conductor, and a current flows, that heats the conductor. That is an electric generator with a short circuit.
Does not follow.
When a piece of wire moves through fixed magnetic field, a voltage appears in the conductor, but there is no short circuit. The ends of wire get charged, but they are not connected, so no circuit and no current.
When it is a current loop that moves through the field, again a voltage appears in both arms, and the ends of the loop get charged; but since the voltage is equal in both arms, again no current.

snorkack said:
When it is a current loop that moves through the field, again a voltage appears in both arms, and the ends of the loop get charged; but since the voltage is equal in both arms, again no current.
Yes but this is true only for a fixed uniform field of infinite extent. If the angle changes (generator) or the magnet moves (maglev or magnetic brake) then locally there are time dependencies which cause currents.

hutchphd said:
Yes but this is true only for a fixed uniform field of infinite extent. If the angle changes (generator) or the magnet moves (maglev or magnetic brake) then locally there are time dependencies which cause currents.
But practically uniform and infinite field is what is implied by merely "N-S magnetic field" - like Earth magnetic field, that locally is nearly uniform. And this does not slow down the movement of the current loop.
On two levels. First, a conducting loop moving in uniform field will not have a current induced. The reason is that there is a voltage induced in a conductor, even in uniform field, but two arms of loop moving at equal speed through equal mmagnetic fields have equal induced voltages, which cancel out. On the second level, if you did have a current in the loop for some other reason, then the current would experience a force in magnetic field, but the two arms have equal and opposite currents (because of continuity of charge in a circuit) and therefore in equal magnetic field equal and opposite forces, which cancel out.

Indeed, when the current loop moves through changing magnetic field then the movement is slowed down. For example look at the case of moving into a stronger magnetic field. Again two levels. First level, there are still opposite voltages induced in two arms of the loop - but since the leading arm of the loop is in the stronger magnetic field, it has the stronger induced voltage, which is not completely canceled by the voltage induced in the trailing arm, so a current gets induced. Second level, both arms still have equal current because of circuit continuity, but because the leading arm is in the stronger field, it experiences te stronger force which is not fully compensated by the force on the trailing arm. And by the Lenz rule, the direction of the induced current is such that the direction of the force created by the induced current slows down the movement of the loop.

Are you trying to teach me E and M or ask a question? If the latter, please frame it as a specific question.