magnetic field can't do work?

[stevvo111]

So came across this problem, BR section 8, passage 1, question 2.

Qeustion is how much work is done on an electron in moving it across the bar in figure 1 (figure 1 showing an into the page pointing mag field, which induces a voltage in the bar which is moving rightward, causing electrons to flow down toward the bottom of the bar).

Answer is evLB.

my concern is, doesn't the magnetic force not do any work? Or can it just not do work directly? The above answer appears to be F*d, with F being qvB... B is perpendicular to the motion of the electron which is just moving from north to south. Thus there is no way the magnetic force can do work on this electron.

But is this the case because the magnetic field here is inducing an emf, the emf can in turn do work, thus meaning the magnetic field can indeed do work indirectly given it induces an emf within some conducting material.

Thus the hard fast "rule, so to say, that magnetic fields cannot do work is technically wrong since in some cases, it can indeed do work...although indirectly.


^is that fair logic or am I crazy?

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[jetsfan1234]

yeah its the induced emf which is doing the work. in the problem, they give the induced emf as epsilon = BLv. Recall that the voltage unit is joules/coulomb. Multiply the emf by the charge (e) to get the work in joules: eBLv.

 

[gettheleadout]

By Faraday's Law the change in magnetic flux is actually the cause of the induced emf, not the magnetic field itself. The force of the magnetic field upon a charged particle never does work, as it is always perpendicular to the particle's motion.

To say that the magnetic field itself induces an emf and thus does work by proxy is not really legitimate, as far as I understand it.

 

[stevvo111]

By Faraday's Law the change in magnetic flux is actually the cause of the induced emf, not the magnetic field itself. The force of the magnetic field upon a charged particle never does work, as it is always perpendicular to the particle's motion.

To say that the magnetic field itself induces an emf and thus does work by proxy is not really legitimate, as far as I understand it.

^I guess it's confusing to relate Magnetic Force, which equals qvB, to the answer, which was eBLv where evB looks very very similar to the magnetic force equation.

But I see what you are saying,

I missed this question mainly because I did not thoroughly read the passage, had I done so, I would have realized Work=qV where q is e and V=BLv

 

[ilovemedi]

yeah its the induced emf which is doing the work. in the problem, they give the induced emf as epsilon = BLv. Recall that the voltage unit is joules/coulomb. Multiply the emf by the charge (e) to get the work in joules: eBLv.

Bumping this back from a year ago. I know Work = Change in PE = q*V. But why does EMF = the Voltage here? I thought EMF was = to the rate of change of the flux within loop. page 137 gives equation as -flux/change in time? How are we suppose to know this??