No Electromagnetic Force in Moving Magnet & Solenoid

[brainyman89]

While a magnet is moved toward the end of a solenoid, a voltage difference is induced between the two ends of the solenoid wire and a current flows (case of closed circuit), however there is no existence of electromagnetic force (Laplace force) though we have current traversing the solenoid in a magnetic field. so why?

 

[uart]

What makes you think that there is no Laplace (Lorentz) force on the windings brainyman?

 

[brainyman89]

since if electromagnetic force exists, then the solenoid would move left or right and that doesn't exist?

 

[brainyman89]

in fact, i have read an answer that says: "there is a Laplace force at anyone point on the wire. However, because the coil is circular then the direction of the current is also circular, thus one side of the coil has a force in one direction and the other side has an equal force in the opposite direction. This does suggest that perhaps there is a net turning force for the whole solenoid. Therefore, the sum of all forces is zero so for this reason there is no effect of laplace force."

Is that right?

 

[uart]

No, there should be a net force on the coil that in the direction of the approaching magnet.

Say that you approach with a north pole then flux will be directed into the end of the coil, but because the flux is diverging there will also be a component directed radially outward through the coil. When I calculate the direction of the induced current and then apply $q v \times B$ to this I get a reaction force in the direction of the approaching magnet. In other words, the coil is repelled by the magnet.

 

[Dale]

I'm with uart on this one. If there is a closed circuit then there will be current flow and therefore a force on the windings. The premise of the OP seems flawed.

 

[brainyman89]

you mean that the force of repelling or attracting is the net summation of electromagnetic forces for the whole solenoid?

 

[Dale]

Yes. And don't forget the possibility of a torque in addition to an overall net force.

 

[brainyman89]

so can we generalize and say that whenever we have electric current and magnetic field, Laplace (Lorentz) force should definitely exist.

 

[Dale]

Yes. The Lorentz force and Maxwell's equations are the fundamental equations of classical electromagnetism, meaning that all other equations and effects can be derived from them.

 

[brainyman89]

in case we have two horizontal rails at a distance apart and connected to a battery, and a uniform magnetic field B is perpendicular to the plane of the rails. A rectilinear conductor slides along the rails.
we usually consider and study Laplace force on the free conductor that can slide on the rails. My question is: does Laplace force exists only on the free conductor or it exits on all rails and conductors of the circuit?

 

[Dale]

It exists on any wire which is in a magnetic field and carrying a current. The reason that you usually focus on the force on the sliding conductor is simply that that is the one which is doing work.

 

[uart]

in case we have two horizontal rails at a distance apart and connected to a battery, and a uniform magnetic field B is perpendicular to the plane of the rails. A rectilinear conductor slides along the rails.
we usually consider and study Laplace force on the free conductor that can slide on the rails. My question is: does Laplace force exists only on the free conductor or it exits on all rails and conductors of the circuit?

Yes the rails will try to push outwards (sideways). The rails have to be held firmly in place to prevent this.

 

[brainyman89]

with respect to Fleming right hand rule, is there a proof for the reason that makes the direction of Laplace force perpendicular to the plane containing B and I.

 

[Dale]

I would consider it a definition of the direction of B. In other words, we infer the existence of the B field by the presence of a force on a current-carrying conductor. The direction of the B field is defined as the direction perpendicular to the plane containing I and f.

 

[brainyman89]

Suppose we have a connecting wire connected to a resistor only. If we approach a magnet toward this wire, then will both exists:
1. induced emf thus induced current,
2. and Laplace force causing the wire to go in specific direction

That is to say, Could we get both emf and Laplace force by only giving variable flux?

 

[Dale]

Yes. In other words the variable flux can be used to do both electrical and mechanical work.

 

[brainyman89]

previously, i used to be taught that B, I and F are related together in such a way that if you provide any two elements of these three, you will definitely get the third, in other words, you can not get any element(or factor) unless the other two are provided. Is the statement accurate??

 

[Dale]

Yes.

 

[brainyman89]

SO how this could be accurate though we achieved to generate emf and mechanical force with only varying B

 

[brainyman89]

how this could be accurate though we achieved to generate emf and mechanical force with only varying B? i.e we provide only one factor and get the other two?

 

[Dale]

The geometry of the wire, resistor, and B, together with Maxwell's equations, provides an additional constraint which allows you to determine I. Then with I and B you can determine f.

 

[brainyman89]

in my previous example, we have a connecting wire connected to a resistor only. then we approach a magnet toward this wire, and we get both emf and Laplace force by only giving variable flux?

So we only provide one element that is B, however we get I and F, which contradicts the idea that says B, I and F are related together in such a way that if you provide any two elements of these three, you will definitely get the third, in other words, you can not get any element(or factor) unless the other two are provided.

 

[Dale]

So we only provide one element that is B

This is incorrect. You provided B and a means of determining I (wire/resistor geometry and composition) using Maxwell's equations. From B and I you obtain f. You provided both B and I, although the I is hidden in Maxwell's equations and requires some solving.

 

[brainyman89]

"although the I is hidden in Maxwell's equations and requires some solving"
didn't understand this statement, can u illustrate it simply.

 

[Dale]

See Ampere's law, the fourth equation listed here:
http://en.wikipedia.org/wiki/Maxwell's_equations#Table_of_.27microscopic.27_equations

One of the variables is the current. Given the geometry and properties of the resistor, wire and the fields you can solve for the current.

 

[brainyman89]

hence if rewrite this statement replacing I with emf, would it stays right: B, emf and F are related together in such a way that if you provide any two elements of these three, you will definitely get the third, in other words, you can not get any element(or factor) unless the other two are provided.

 

[brainyman89]

would it stays right?

 

[Dale]

No, you still need to get I, however for many materials there is a linear relationship between emf and I, called resistance.

 

[brainyman89]

if we have a solenoid suspended freely and no current is traversing it, and we approach a magnet toward it, would it repel or rotate?

can we consider a solenoid totally as a bar magnet when a current traverses it??

my another question is: when we are providing a wire, u r considering that we are providing current, this is confusing me there is a wide difference between a wire and a current?

thanks for answering

 

[Dale]

if we have a solenoid suspended freely and no current is traversing it, and we approach a magnet toward it, would it repel or rotate?

That is a good question, I guess it would depend on the exact geometry, but I don't know for sure.

can we consider a solenoid totally as a bar magnet when a current traverses it??

The magnetic field of a solenoid is very similar in shape to the magnetic field of a bar magnet, but I don't know what you mean by "totally".

my another question is: when we are providing a wire, u r considering that we are providing current, this is confusing me there is a wide difference between a wire and a current?

Yes, there is a wide difference between a wire and a current. However, in all of your examples so far you have a changing flux in a circuit and therefore an emf and therefore a current in the wire. If you made a different example then I would answer differently, but so far you have only made examples of wires with currents.

 

[brainyman89]

You provided B and a means of determining I (wire/resistor geometry and composition) using Maxwell's equations. From B and I you obtain f. You provided both B and I, although the I is hidden:

Did u mean in this statement that providing variable B equals providing B and I (i.e variation=current), hence i will get B and I thus producing F.

 

[Dale]

Did u mean in this statement that providing variable B equals providing B and I (i.e variation=current), hence i will get B and I thus producing F.

Essentially, but I would have said "providing variable B and wire/resistor geometry equals providing B and I".