# Difference in these two concepts of Magnetic Torque

## Summary:

Understanding the difference of magnetic force
I am confused with the concept of Torque handled differently in books,
Concept1: If a loop is placed in a magnetic field and the current flowing in the loop is ##I## there will be force and torque acting on the loop given by ##F = I \vec l \times \vec B ##. The torque is given by ##\tau =\vec \mu \times \vec B ##.
Concept2: If a current flows through the loop then it becomes an electromagnet and a North and South pole are developed and if another magnetic field is present then there will be another North and South pole. The similar poles repel each other and a torque is produced.
Are the two concepts same? Why there is no mention of electromagnet in the concept1? Please advise.

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Borek
Mentor
First of all: what is B?

Physicslearner500039
First of all: what is B?
B is magnetic field

Borek
Mentor
B is magnetic field
Electromagnet produces magnetic field, doesn't it?

Physicslearner500039
etotheipi
Gold Member
2019 Award
A current carrying loop has a magnetic moment ##\vec{m} = I\vec{A}##. It is correct that in an external magnetic field a couple ##\vec{\tau} = \vec{m} \times \vec{B}## is exerted on the loop.

If you like, you can think of a magnetic moment equivalently as two opposite magnetic poles of magnitude ##p## separated by a vector ##\vec{l}##, such that$$\vec{m} = p\vec{l}$$This is more natural for e.g. a bar magnet, but I suppose there is nothing stopping you from thinking about the current carrying loop in terms of magnetic poles.

Concept2: If a current flows through the loop then it becomes an electromagnet and a North and South pole are developed and if another magnetic field is present then there will be another North and South pole. The similar poles repel each other and a torque is produced.
Are you saying that, e.g. the external magnetic field is produced by another bar magnet, whose north pole (say) is repelling the effective north pole of the current carrying loop. I suppose that is a fine analogy, but it's probably not really helpful to think of current carrying loops in terms of magnetic poles.

Physicslearner500039