How do Railguns work?


Can someone explain the physics around railguns.
Do they use Electromagnetic waves for propulsion or?

In: 1

So basically if you have a wire and run a current through it it generates a magnetic field.

You take two wires parallel to each other with current in opposite directions the magnetic fields repel and the wires try and push themselves apart.

In a more general sense if you have a circle of wire and current running through it (note that if current is flowing in a circle opposite sides have opposite flows of current) the magnetic field generated tries to force the circle to expand. Or again in a general sense to increase the loop area. A square shaped circuit would try and stretch into a circle to increase the area inside for example.

In a railgun the loop is formed from the power source (usually capacitors) a rail, the projectile, the opposite rail and back to the power source.

Since the rails are fixed in place, (and to some extent the actual power wires to the rails and the power supply itself) the sliding projectile is the only spot the circuit has to expand. So with lots of current your projectile gets thrown forward by magnetic forces till it leaves the front of the barrel.

Correct. A rail gun uses two rails, one positively and one negatively charged, to guide and accelerate a projectile. The projectile contacts the rails and completes the circuit. This differs from a Gauss Cannon which uses a series of coils wrapped around a barrel which activate in series to accelerate the object.

>Do they use Electromagnetic waves for propulsion or?

Kind of, but not in the way you understand them

Rainguns are a bit quirky.

Take a conductive sled or projectile, put it on two conductive rails, then run high current up the rail, across the projectile, and down the other rail and understand that charges moving through a magnetic field get pushed to the side and you’ve got yourself a railgun.

The current flowing up the rail makes a strong magnetic field circling around it, and the rail with current flowing down it creates a field circling the other direction. As the electrons move across the projectile they get pushed on by the magnetic fields they are leaving and heading towards and these fields push the electron sideways relative to its movement and down the rails. Applied across an insane number of electrons this results in quite a bit of force pushing the projectile down the rails.

The electric force and magnetic force are both parts of the electromagnetic force (one of the 4 fundamental forces). You can’t have electricity without magnetism and vice versa.

When an electric charge moves, it creates a magnetic field, and in order to have a magnetic field, you need charges to move.

When current moves through a wire, it creates a magnetic field that loops around the wire. This follows whats known as the eoght hand rule. Take your right hand, if you put your thumb in the direction of the current, your fingers loop around the wire in the direction of your fingers.

The idea with a rail gun is if we have a magnetic field going through a projectile, we can run current through that projectile, and the magnetic field produced by that current will interact with the other magnetic field and we get a magnetic repulsion.

The formula is F = IL×B. The force is equal to the current, times the length of wire (width of the projectile in this case) cross product (vector mathematics you don’t need to understand) with the magnetic field.

The direction of the force is also determined the the right hand rule, if you point your thumb in the direction of the current, and have your fingers extend in the direction of the magnetic field, your palm points in the direction of the force.

The cross product is a way to multiply vectors and its at its maximum when the two vectors are perpendicular to each other, and 0 when the vectors are parallel. Most of the time you can replace it with sin(θ) where θ is the angle between the two vectors.

Let’s say we have a rain gun it produced a magnetic field of 1000T, launches a projectile of 1cm wide, with a current of 1A. And let’s say the barrel is 1m long and projectile is 1kg.

Assuming the magnetic field is perpendicular to the current (which makes the most sense because it would be the most efficient) the force will be F = 1A * 10^-2 m * 10^3 T = 10N

The projectile will be launched with a energy of W = F•d = 10N * 1m = 10J

That will be a velocity of v = sqrt(2 KE/m) = sqrt(2 * 10J/1kg) = 4.47 m/s