- #1
zenterix
- 708
- 84
- Homework Statement
- Consider the figure below, which shows an infinite wire carrying a current ##I_1## and a semicircular conducting loop with current ##I_2## placed near the wire.
- Relevant Equations
- What happens to to the semicircular loop?
Please see the image further below for the definition of the coordinate system.
The magnetic dipole moment of the loop is
$$\vec{\mu}=\mu\hat{k}\tag{1}$$
The magnetic field for an infinite wire depends only on the distance from the wire.
For the specific configuration above, we have
$$\vec{B}(y)=B(y)\hat{k}=\frac{\mu_0I}{2\pi y}\hat{k}\tag{2}$$
Note that I calculated this field considering an infinite wire and a point ##P## above it. That was a 2d problem (more on why I am saying this further below in my question).
Then
$$\vec{\tau}=\vec{\mu}\times \vec{B}=0$$
That is, there is no torque on the loop.
$$\vec{F}=\nabla(\vec{\mu}\cdot \vec{B})$$
$$=\nabla(\mu B(y))$$
$$=\mu\frac{\partial B}{\partial y}\hat{j}$$
Since the partial derivative above is negative, the loop translates towards the wire.
The problem I just solved online entailed reaching the two conclusions above, namely that the loop experiences no torque and translates towards the wire.
However, I am a little bit confused about some things.
I simply applied the formula I just learned: ##\vec{F}=\nabla(\vec{\mu}\cdot \vec{B})##, which comes from the force being the negative of the gradient of potential energy (the latter being ##-\vec{\mu}\cdot \vec{B}##).
However,
The way I solved the problem, I considered ##\vec{B}## to be a function of ##y## which is position in the ##\hat{j}## direction.
But doesn't ##\vec{B}## have a non-zero derivative relative to ##z## as well, ie in the ##\hat{k}## direction?
After all, ##y## is a function of ##z##.
There is something wrong with my interpretation of the equation ##\vec{F}=\nabla(\vec{\mu}\cdot \vec{B})## I believe.