# Finding the force of a magnetic field using an air core solenoid

1. Feb 24, 2012

### wingnutto

1. The problem statement, all variables and given/known data

Ok, I need help, but it is for a project my son is working on. I am homeschooling him (13) and he is entered in a local science fair. He has designed an air core solenoid shock absorber for a bike. It more or less works, and we are working our way through the forces generated and trying to come up with some predictive formulas to explain why it works and how to scale up. I am at the extent of my capacity, so maybe someone here can lend a guiding hand.

We have an air core solenoid, 22 ga. magnet wire, about 200 turns, 1.5 cm diameter, 10cm axial length. At 18v it's pushing about 12amps, and yes, it gets hot fast.

One of the things we are trying to understand is efficiency. I'd like to show him how to calculate the force this magnetic field can produce at the cost of how much power (216 Watts here - with a lot being wasted in heat). Can you express magnetic force in terms of Newtons? We know if we let it suck an N48 3/4x1.5" magnet, it's pulling with a force of about 14 Newtons for about a centimeter, but this complicates it I think by adding the force of the magnet into the equation.

My first thought was to calculate the MMF of the solenoid in ATs, but this appears to dead end. I have yet to find a way to translate AT into anything useful.

I have Bio-Savart, Lorentz, Webers and Henries swimming around in my head. Any suggestions on how to make sense of this would be most welcomed!

2. Relevant equations

My question exactly.

3. The attempt at a solution

Haven't gotten that far. I knew Amps*turns would be too simple!