Physics behind magnetic security gates?

  • Thread starter Jimmy4455
  • Start date
  • #1
2
0
Hello, I recently walked through the gates of my local library with my hard drive magnets clipped onto my fingers, and I noticed that they were vibrating. I've been looking on the internet and I also found radio-frequency identification systems. Could someone please explain the physics behind these systems? Is it due to the motor effect?
 

Answers and Replies

  • #2
Simon Bridge
Science Advisor
Homework Helper
17,857
1,654
You mean the bit by the door that detects when you try to remove a book from the library without permission?
http://www.explainthatstuff.com/rfid.html


I don't know what you mean by "motor effect" - it works by unducig a current in the tag, which, in turn, produces it's own radio signal, which is picked up by a radio reciever. Ever used a crystal radio set?
 
  • #3
sophiecentaur
Science Advisor
Gold Member
25,074
4,761
Hello, I recently walked through the gates of my local library with my hard drive magnets clipped onto my fingers, and I noticed that they were vibrating. I've been looking on the internet and I also found radio-frequency identification systems. Could someone please explain the physics behind these systems? Is it due to the motor effect?
That's a bit bizarre! Whatever were you doing that for? A bet?
Radio frequency waves have a much high frequency than you can detect directly (of course) but there may have been some regular pulsing of the current (mains frequency, perhaps) in the frame which would produce a varying force on the magnets (motor effect). I find it surprising, though, that such high power levels would be needed for a simple close-up monitoring system. You could do a simple test, using the mains lead to a high current appliance like a heater. If you could separate the L and N conductors, you could see if a magnet placed near to just one of them, gives you the same sensation. You would need to do this carefully, of course and reinstate the cable afterwards. Health and safety rules, as usual.
 

Related Threads on Physics behind magnetic security gates?

Replies
10
Views
44K
  • Last Post
Replies
1
Views
583
  • Last Post
Replies
10
Views
3K
  • Last Post
Replies
5
Views
24K
  • Last Post
Replies
13
Views
6K
  • Last Post
Replies
12
Views
9K
  • Last Post
Replies
13
Views
4K
  • Last Post
Replies
21
Views
16K
  • Last Post
Replies
4
Views
4K
Top