# Forces acting on bar magnet inside a charged solenoid

• Recoil
In summary: If it's attracting something, it's probably because it has like poles pointing in opposite directions. So, in a situation like this, the current is causing the like poles to repel each other.
Recoil
This is not a specific homework problem. It is a lack of understanding in theory relating to a lab report I have to write.

We (my group and I) wrapped a 40 cm PVC pipe in copper wire (very densely and for almost the entire length), applied a current to it, and dropped a permanent magnet through the tube. The current was applied such that the magnetic field of the solenoid (as we modeled it) would point upwards, opposite gravity.

By connecting two extra coils of wire at each end to a special data capture instrument, we could record the induced current in these coils as a function of time (giving us an accurate way of measuring the descent time of the magnet).

When we applied increasing levels of current, the data (and observation) showed very clearly that the magnet was slowing - at one point it even stopped at the top of the tube (until we pushed it).

That's the context - the problem is, then, what is the force opposing the magnets motion? (Not numerically but conceptually)

I originally thought that the eddy currents induced in the solenoid (by the passing magnet) would affect the drop time (thus explaining the force opposing the magnets motion). However, when no current was applied, the drop time of the magnet was very similar to what we would expect from classical kinematics (little change), and I don't believe eddy currents explain why a greater applied current affects drop time.

Any assistance is much appreciated.

## The Attempt at a Solution

Hi Recoil.

You are asking how or why does the magnetic field of a solenoid interact with the field of a permanent magnet?

https://www.miniphysics.com/ss-magnetic-field-due-to-current-in-a-solenoid.html

Recoil
NascentOxygen said:
Hi Recoil.

You are asking how or why does the magnetic field of a solenoid interact with the field of a permanent magnet?

Yes, I am wondering why and how the permanent magnet passing through the magnetic field of the solenoid is experiencing a repulsion (thus slowing it's descent time).

How about drawing a bar magnet in the middle of the solenoid and sketching in some field lines like Nascent0 did?

Would you have the same question about two permanent magnets repelling each other?

Recoil
Recoil said:
Yes, I am wondering why and how the permanent magnet passing through the magnetic field of the solenoid is experiencing a repulsion (thus slowing it's descent time).
Did you find that your magnet experienced a repulsion if you did one of the following:
(i) reversed the orientation of the permanent magnet, or
(ii) swapped the wires connecting to the DC voltage supply?

Recoil
Merlin3189 said:
How about drawing a bar magnet in the middle of the solenoid and sketching in some field lines like Nascent0 did?

Would you have the same question about two permanent magnets repelling each other?

I did not think of it that way - so it is being decelerated as a result of like poles repelling each other - which would explain why the deceleration increased with current, since the magnetic field inside the solenoid increased?

NascentOxygen said:
Did you find that your magnet experienced a repulsion if you did one of the following:
(i) reversed the orientation of the permanent magnet, or
(ii) swapped the wires connecting to the DC voltage supply?

I believe we did observe a difference when swapping the orientation of the magnet, but did not test thoroughly - we didn't test the effect of swapping the wires... That might have helped a lot.

Of course, no sooner had i pressed send than I realized that the repulsion I'd thought about does not apply "in the middle of the solenoid", only at the ends!
But you would have noticed the repulsion from the bar entering the solenoid and detected it reaching the end before it was greatly accelerated out of the solenoid.

The permanent magnet in the solid copper pipe experiences drag all the way down, because it is continuously generating current just in front and just behind it all the way through - being repelled in front and attracted behind.

Recoil
Merlin3189 said:
Of course, no sooner had i pressed send than I realized that the repulsion I'd thought about does not apply "in the middle of the solenoid", only at the ends!
But you would have noticed the repulsion from the bar entering the solenoid and detected it reaching the end before it was greatly accelerated out of the solenoid.

The permanent magnet in the solid copper pipe experiences drag all the way down, because it is continuously generating current just in front and just behind it all the way through - being repelled in front and attracted behind.

I see, I am wondering if you have an resources I could read up on this? I would need to formalise this theory for my lab report.

I'm afraid I don't know of any useful explanation I could direct you to.
I am coming to it simply in a qualitative way, based on the notion that a bar magnet is like a solenoid (externally.) So the force between a solenoid and an external bar magnet is like the force between two bar magnets (which I think you may find explanations of.) Inside a solenoid there is a uniform field, so I'd expect there to be no net force on the bar magnet when it is parallel to the field. When it is not parallel, there would be a torque trying to make it become oriented with its field in the same direction and sense as the solenoid field. When it is parallel to the solenoid field, but its field is in the opposite sense, then it could be in unstable equilibrium and would turn around at the first opportunity. (Maybe some of the slowing you observe is due to the magnet being pushed out of line and rubbing against the plastic tube?)

All of this can be seen qualitatively by using Faraday's idea of field lines and I don't suppose anyone is expecting any quantitative calculations.from you.

When we applied increasing levels of current, the data (and observation) showed very clearly that the magnet was slowing - at one point it even stopped at the top of the tube (until we pushed it).
is consistent with the idea that the repulsion happens only at the end of the solenoid. Once you push it past this point, I presume it traveled through the remainder of the solenoid without stopping, then accelerated away from the other end..

Recoil
It probably would have been more informative had the PVC pipe been sat horizontally on the lab bench, allowing students to guide the bar magnet into, then through, and out of either end of the horizontal solenoid free of the confounding effect of gravity. That way investigators would be able to feel the force the bar magnet experiences at each point throughout its travel over the full length of the solenoid and beyond.

Recoil and Merlin3189

## 1. What is a bar magnet?

A bar magnet is a permanent magnet that has a north pole and a south pole. It is made of a material, such as iron or steel, that has magnetic properties.

## 2. How does a bar magnet interact with a charged solenoid?

A bar magnet inside a charged solenoid will experience a force due to the magnetic field created by the solenoid. The direction of this force will depend on the direction of the magnetic field and the orientation of the bar magnet's poles.

## 3. What are the forces acting on a bar magnet inside a charged solenoid?

The forces acting on a bar magnet inside a charged solenoid include the magnetic force from the solenoid's magnetic field, as well as any gravitational or mechanical forces that may be present.

## 4. How does the strength of the solenoid's magnetic field affect the forces on the bar magnet?

The strength of the solenoid's magnetic field will directly affect the magnitude of the force on the bar magnet. A stronger magnetic field will result in a greater force on the magnet.

## 5. Can the forces on a bar magnet inside a charged solenoid be controlled?

Yes, the forces on a bar magnet inside a charged solenoid can be controlled by adjusting the strength and orientation of the solenoid's magnetic field. This can be done by changing the current flowing through the solenoid or by adjusting the shape and placement of the solenoid's coils.

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