# Electromagnetic door locks: what's the theory of operation?

1. Mar 3, 2009

### mdhurt34

Hello,

I do not have much of a physics background, so I am sure this is a rather elementary question.

(Guidance/pointers to other resources would be greatly appreciated.)

My main question: What is the theory of operation behind electromagnetic door locks?

They appear to basically be an electromagnet attracting a metal plate, but on casual inspection, seem pretty strong without requiring any sort of large power source or obviously fancy controller circuitry.

See: http://en.wikipedia.org/wiki/Magnetic_lock and http://sdcsecurity.com/class.aspx?id=1

What happens through out the "system" as force is progressively applied to the door, and the lock eventually fails? Are there changes in fields, charges, voltage, current,... anything?

Also, I have some sub-questions inspired by the above query, (and my lack of fundamental understanding): :-)

1. What does the classic (and yet still practically built & used) electromagnet "device" look like, and what are it's components?
2. What exactly do those pretty "flux lines" in popular magnetic illustrations actually mean?
3. How would you calculate the "force" acting on a stationary ball bearing at both 10 cm and 20 cm away from our classic electromagnet?
4. What happens in a system as a simple ball bearing enters the field generated by the above classic electromagnet powered by a simple DC power source? (what starts to change with regard to: force, position, current, voltage, etc.)
5. What must happen for a "constant force" to be imparted to a ball bearing as it travels through some defined region of the electromagnet's field? (For simplicity, let's assume it's headed directly towards the magnet, and only due to the apparent attracting force.)

Thank you,
:)

Last edited by a moderator: Apr 24, 2017