# Force due to magnet and current carrying wire

1. Feb 2, 2012

### sgstudent

When a current carrying wire is placed into a magnetic field there will be a force. Using Fleming's Left Hand rule. They also said that by increasing the number of turns of the coil will increase the force. But how does this increase the force?

Also, by placing a soft iron rod inside the coil will also increase the force. But how does it increase the force? I know that the magnetic field lines will be concentrated but I'm unsure how it increases the force.

My level of education is secondary 4 and sitting for the GCE O levels
Thanks for the help!

2. Feb 3, 2012

### Simon Bridge

If I read you correctly - by increasing the amount of current within the field.

You can build these as motors and see it happen.

3. Feb 3, 2012

### gsal

When you have a conductor in a magnetic field, the force is proportional to the current and magnetic field, right? So, if you increase the current the force increases...when you say that "increasing the number of turns of the coil will increase the force" you are effectively increasing the current...this would be equivalent to not increasing the number of turns and simply increasing the current...by the way, when you talk about a current carrying conductor in a field is one thing where you assume that the "return" current is far away outside the magnetic field...so, just be careful when you imagine that "coil" you are talking about...if you imagine the entire coil inside the magnetic field, then, the force on one side of the coil opposes the one from the other side and you got zero net force.

4. Feb 3, 2012

### Simon Bridge

... through the center of mass. However, you can still get movement. In the simple motor, both coils are inside the field. The force is opposite for opposite currents creating a couple.

If you increase the number of turns you increase the couple.

Though in a really simple motor, you are essentially correct - you use one pole close and arrange to run a current for half each rotation and rely on the far side of the coil being far from the pole.

I would imagine that a long coil as in an inductor would not jump when between the poles of a magnet. That was the first picture I had reading the question then thought: "hang on..."

5. Feb 3, 2012

### ZealScience

Because basically this kind of interaction is the about magnetic field caused by currents. every turn is contributing to certain amount of magnetic field, so more turns means more contribution and therefore stronger force.

6. Feb 3, 2012

### sgstudent

Oh ok I get it. Thanks you guys, but for the second one, how does it work?

7. Feb 4, 2012

### gsal

Before getting too deep into magnetics (there are several other threads that do), basically, the second one works in a similar way as the first one...in the first case, you increased the current; on the second, you "increase" the magnetic field...or at least the concentration of the magnetic field in the neighborhood of interest (near the conductor). Iron is a "better conductor" of magnetic flux than air.