How Does a Rail Gun Work Without an External Magnetic Field?

[jsmith613]

http://www.brightstorm.com/science/physics/magnetism/faradays-law-lenzs-law/#

Start at about 7:10 and watch until about 8:20.
I can see that an emf and hence a current will be induced in the rod between the two rails BUT if there is no external magnetic field (the magnetic field generated by the rod due to the current flowing in it is irrelvant) then why would the rod experience a force...

the only explanation I can think is that the change in magnetic flux is NOT instantaneous...therefore as it is reducing (might take 0.1 ms to completely dissapear) there is still some flux to apply a force to the rod...is this correct?

 

[Simon Bridge]

Um - the example in the video has an external field.
When you switch the field off, the flux through the loop goes to zero - creating an EMF, which gives the rod (and everything else) a kick.

The transient effect for these things is usually quite complicated and a bit beyond the level of the video. For instance, as you have noted, the induced current also has a magnetic field.