# Odd experiment results (electromagnetics)

Okay, a bit of background. So I'm working on a linear accelerator (you might know it as a Gauss rifle), and I had worked through the equations I thought I needed (Magnetic force, displacement, etc.)

When I tested, it didn't work. I wondered if there was a short in my electromagnet coils, so I used a compass to see if there was a magnetic field. I found that there was a magnetic field, so I got my multimeter and checked the amperage through the coil. That too, was right where it should be. I was using CVCC device to limit the current, so I took that off and shorted the power source straight across the coils. Nothing. Now I'm just not sure what's wrong. Does anyone have any ideas?

I'm using the below to move a piece of plate metal and a nail 4.5 cm:

• 300 wraps of 22 AWG gauge magnet wire

• 5 Amps of current

• The surface area of the plate I am trying to move is about $5\text{cm}^2$

• The distance between the plate and the coils is about 4.5cm

• The mass of the plate, and what it is attached to is about 20g

• The electromagnet pulls the plate against a spring with 350g compression*

*the problem is not that the electromagnet cannot pull against the spring, it is that the electromagnet does not move the plate at all.

The equations I was using were $$F = (NI)^2\mu_0A/2g^2$$ for the force exerted on the plate in Newtons, where N is the number of turns, I is the current, A is the area of the plate, and g is the gap separating plate and coils.

I then rearranged $F=ma$ to $\frac{F}{m}=a$ and used $F = (NI)^2\mu_0A/2g^2$ for force. I also used the mass of the plate plus the compression weight of the spring, for mass.

With the above equations and the list of values, the electromagnet should exert 0.349N. With my mass + spring, it should accelerate the plate at almost 1m/s (0.943m/s). Given that it only has to travel 4.5cm, it should do that really quickly.

Honestly, I don't know what went wrong. I don't suppose anyone can point out if I missed something glaringly obvious?

*pictures of electromagnet:

Electromagnet

Plate (extended)

Plate (compressed)

• Give it time, dude. – DanielSank Oct 4 '14 at 23:44
• @DanielSank Yes, I know. Actually, I meant "Does anyone see anything glaringly obvious, like I'm using the wrong equations?" – CoilKid Oct 5 '14 at 2:26
• What material is the plate made of? metal is not enough, its magnetic properties are important. Easy to check with a small permanent magnet. – anna v Oct 5 '14 at 3:35
• Your very first problem is that the magnetic path is wide open. Your magnetic field lines are going all over the place, but not where you need them: trough the plate. This is not how one would build an efficient electromagnet. An efficient design will include a yoke that returns the magnetic field lines on the shortest path possible. Take a look at how the professionals are building magnets that can produce serious force: wagner-magnete.de/en/content500.html. – CuriousOne Oct 5 '14 at 3:46
• @annav Sheet metal. I checked before I glued it in, it is ferromagnetic. – CoilKid Oct 5 '14 at 4:42