Gauss Rifle: Energy Origins?

[Wenzel745]

Hello world!

I'm working on the Gauss Rifle project and plan on turning it in by this Friday (the 31st). While writing my research paper I came across a bit of a problem: Where does the energy come from?

The Conservation of Energy Law states that in a closed system the energy in is always equal to energy out. In this project you have a closed system with magnets and steel balls. You push the first steel ball in say with a velocity of 0.25m/s and through the series of accelerators the final ball exits closer to 5m/s. How can this be possible? If the magnets are just as magnetic before and after the experiment, where does this energy come from?

I hope you guys can help me with this, thanks in advance!

-Wenzel

PS: Please include sources or topics for me to research in your reply!

[rmarz]

Wenzel745 - The energy, or work is hiding in plain sight. It's sort of an entropy trick question. The actual energy was borrowed as the magnets came together violently and propelled the ball bearings and the "energy bill" was paid when you had to exert work to pull the magnets apart.

Rick Marz

[Wenzel745]

So, just to clarify, the energy was borrowed in the collision of the magnets together when the ball hits them? Or when the magnets were created? And if it is during the collision, how does that work with solid magnets that are not two halves of a whole, but rather just the whole?

-Wenzel

[rmarz]

Wenzel745 - Maybe I confused you a bit. The magnets posessed potential energy just prior to their collision (energy expended and transferred). As long as the magnets were sticking together the potential energy between the two of them was zero. When you expend energy to seperate them, you have restored the potential energy for the next cycle.

Let's say the magnets were never attached to each other prior to the experiment. They still had a magnetic attraction as they got close together and at a critical point started to accelerate and ended in the collision. Energy would have to be added to seperate them and start the cycle again. I guess magnets are a special class of subject due to their unique attractive peoperties, but in the end, energy is expended to restore them to their beginning state.

Rick Marz

[theborg]

wenzel745,

Welcome to Science Buddies and thanks for the great question. You are correct about the conservation of energy, however, you have to take into account the force applied to the steel balls from the magnets themselves, as Rick Marz points out. Think of the magnetic field/force of the magnets the same way as the force of gravity. When the ball is just outside the influence of the magnets, there is a potential energy (PE), similar to the PE of a ball sitting at the top of a cliff just before it roles off. As you start the first bearing rolling at an initial velocity, 0.25 m/s, it is accelerated toward the magnetic stages trading the PE for kinetic energy (KE)...much like the ball falling off the cliff, accelerating due to the force of gravity...so that when the bearing strikes the magnet stage, it's at some velocity greater than 0.25 m/s. During collision, much of that KE is transfered through the magnet stage to the steel bearing on the other side. That bearing has to overcome the pull of the magnet to break free and be sent down toward the next stage. To see the effects of an energy transfer during an elastic collision, checkout a "Newton's Cradle". The 2nd bearing should have an initial velocity greater than 0.25 m/s, but somewhat less than the impact velocity of the 1st bearing. Then it will be accelerated toward the second magnet stage repeating the trade between PE and KE. As mentioned in the previous posts, the remaining energy bill is then "paid" when the "trigger" bearings are pulled away from the magnets and PE is restored. I hope this helps clarify some things.

A good discussion of PE vs KE as well as magnetism can be found at the following site.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

[Wenzel745]

Thank you both very much for your replies, I think I understand what you're saying now. Thanks!