BR magnetic field

[anbuitachi]

Hi, this question concerns the cyclotron but the concept is the same as normal question. But anyway. The concept i'm confused about is magnetic field doing work.

So in this question, the Uniform magnetic field is causing the ion to travel in a circular path in the cyclotron. If i understand magnetic field correctly, only magnetic field perpendicular to the traveling charge produce a force on the ion... yet one of the questions asks for the work done in this case as it makes the ion travel in circles - which is of course the work done is zero because only parallel force does work... If thats the case, when does magnetic fields do work on a charge? I'm confused about this...

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[addo]

As far as I know, a magnetic field does no work on a moving charge, for exactly the reason you stated.

 

[MLT2MT2DO]

Hi, this question concerns the cyclotron but the concept is the same as normal question. But anyway. The concept i'm confused about is magnetic field doing work.

So in this question, the Uniform magnetic field is causing the ion to travel in a circular path in the cyclotron. If i understand magnetic field correctly, only magnetic field perpendicular to the traveling charge produce a force on the ion... yet one of the questions asks for the work done in this case as it makes the ion travel in circles - which is of course the work done is zero because only parallel force does work... If thats the case, when does magnetic fields do work on a charge? I'm confused about this...

This whole passage was difficult to understand for me. But as far as the work, if the magnetic field lines move left to right, or right to left, the charged ion still has to travel perpendicular to get to the circumference of the cyclotron (since the ion doesn't get "spit" out until it is on the borders of the cyclotron). So work has to be done.

 

[BerkReviewTeach]

Hi, this question concerns the cyclotron but the concept is the same as normal question. But anyway. The concept i'm confused about is magnetic field doing work.

So in this question, the Uniform magnetic field is causing the ion to travel in a circular path in the cyclotron. If i understand magnetic field correctly, only magnetic field perpendicular to the traveling charge produce a force on the ion... yet one of the questions asks for the work done in this case as it makes the ion travel in circles - which is of course the work done is zero because only parallel force does work... If thats the case, when does magnetic fields do work on a charge? I'm confused about this...

The cyclotron has two different types of fields acting on the particle. The electric field accelerates it (doing work on it) while the magnetic field turns it around (doing no work on it), so it can pass through the electric field again and again.

It might make more sense if we follow a charged particle through a few cycles. Let's say a cation starts on the left side of the E field. It will speed up as it moves left to right, gaining kinetic energy (from the work done on it by the E field). Once it is beyond the E field, the magnetic field on the right causes it to do a 180 and start moving right to left. It passes through the E field again (which now must point in the opposite direction as before) right to left where it again speeds up. Once it leaves the E field, the B field on the left must turn it around by 180, but this time because it is moving faster than before, the semi-circle is bigger. Now it starts the process over again, moving left to right through the E field and building up more speed. After it leaves that field, the B field on the right turns it around so it can pass through the E field again from right to left.

Each pass through the E field speeds it up some more, until eventually it is going so fast that the radius is too big for the B field to turn it around completely before it flies out of the exit tube.

Overall, the E field did work on it to speed it up and thereby give it more kinetic energy. The B field simply turned it back around so it could keep passing through the E field until it had enough speed to leave. The B field just turns it around and doesn't give it any speed. The B field does no work on it.