SpriteKit Magnetic/Electric fields producing no results - ios

Question

I'm playing around with some of the new SpriteKit tools and I've run into a frustrating problem.
As you know, iOS 8 introduced the SKFieldNode class which enables the user to create custom "force fields" which affect other SKNodes. I have gotten great results using the springField and the radialGravityFields, however I have yet to figure out how to use the magneticField and electricField types.
Let me explain.
The following code produces absolutely no effects on node4, the SKSpriteNode which I would like to be affected by the SKFieldNode.
SKSpriteNode *node4 = [SKSpriteNode spriteNodeWithTexture:[SKTexture textureWithImageNamed:#"Red"] size:CGSizeMake(25.0, 25.0)];
node4.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:12.5];
node4.position = CGPointMake(300.0, 250.0);
node4.physicsBody.dynamic = YES;
node4.physicsBody.charge = 30;
node4.physicsBody.linearDamping = 3;
node4.physicsBody.affectedByGravity = NO;
node4.physicsBody.collisionBitMask = 0;
node4.physicsBody.mass = 1;
SKFieldNode *centerNode = [SKFieldNode magneticField];
centerNode.position = CGPointMake(150.0, 200.0);
centerNode.strength = 100000000000;
centerNode.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:25];
centerNode.physicsBody.charge = -30;
centerNode.physicsBody.dynamic = NO;
centerNode.falloff = 0;
The node is drawn on screen, yet stays completely static. As you can see, this happens even with a falloff of 0 and a strength of a 10000000000.
Changing the line:
SKFieldNode *centerNode = [SKFieldNode magneticField];
Into:
SKFieldNode *centerNode = [SKFieldNode springField];
Causes the centerNode to act as a springField, and sends the node4 all over the place.
Using an electricField, rather than a magneticField, yields no results either.
Before someone asks, yes, I am adding both nodes to the scene, I just didn't include those lines.
Can anyone see where I'm going wrong?

Answer 1

There's not enough code here for me to test it fully, but I see two likely problems up front:
If node4 isn't moving to start with, a magnetic field has no effect on it. SpriteKit's magnetic field models the second half of the Lorentz force equation (F = qv ⨯ B), where the force on a particle relates to its charge and velocity.
You're setting node4's linear damping to greater than the maximum. The linearDamping property takes a value between 0 and 1, where 1.0 fully arrest's a body's motion. Values greater than 1 are probably clamped to 1.0, so your body shouldn't be moving even if hit with tremendous force.
Probably unrelated: you don't need to add a charged physics body to the field node unless you want that node to be affected by other electric/magnetic fields.

Answer 2

I have the same problem only when I create nodes by writing code. If you work with SpriteKit Editor, it works well!
You can add a new sks file and then unarchive it to SKScene by this code:
extension SKScene {
class func unarchiveFromFile(file : NSString) -> SKNode? {
if let path = NSBundle.mainBundle().pathForResource(file, ofType: "sks") {
var sceneData = NSData(contentsOfFile: path, options: .DataReadingMappedIfSafe, error: nil)!
var archiver = NSKeyedUnarchiver(forReadingWithData: sceneData)
archiver.setClass(self.classForKeyedUnarchiver(), forClassName: "SKScene")
let scene = archiver.decodeObjectForKey(NSKeyedArchiveRootObjectKey) as SKScene
scene.size = GameKitHelper.sharedGameKitHelper().getRootViewController().view.frame.size
archiver.finishDecoding()
return scene
} else {
return nil
}
}
}
Open the sks file and drag sprites on the scene, it really works well.