# Homework Help: Magnet and wire interaction

1. Feb 16, 2016

### LotusTK

Im not too sure about the (ii) part of this question. The mark scheme is very poor too and gives very little information (which is always the bloody case with A level mark schemes).

I dont know what i can say about how the two fields interact.

Can someone tell me what i am missing in my answer below? What bits are correct and what bits are wrong?

The rod exerts a magnetic force upwards on the rod, since there is moving charge in the form of electrons passing through the rod, and these electrons are pushed upwards by the magnetic field and cause the rod to feel an upwards force as a result. The electrons exert an equal and opposite force down on the magnet, adding to the force on the electronic balance, hence increasing the reading.

The total force acting on the mass balance is: mg + BIL

2. Feb 16, 2016

### Staff: Mentor

That's about right. You didn't need to mention electrons as the charge carriers for the current, simply mentioning that the current though the rod produces a magnetic field that interacts with the field of the magnet should be sufficient. The rod being held in place by clamps prevents the magnet from expelling the rod, so the force will remain so long as the current flows.

Which of Newton's laws might you use to describe the fact that the rod's field pushes the magnet down with the same force that the magnet's field pushes the rod upwards?

3. Feb 16, 2016

### LotusTK

I was mentioning the electrons since they are the actual physical things that feel the force due to the magnets magnetic field.

Im not too sure how the field of the magnet and the field produced by the current interact, would they create a "resultant" field or something?

The equal and opposite part of my answer was the reference to the 3rd law, but i should really identify it as a 3rd law pair to be as specific as possible

4. Feb 16, 2016

### Staff: Mentor

Yes, the fields will sum.