# Very difficult Isaac Physics Question - Just need some pointers to get me on track.

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#1
Hi there.
Been stuck on a really hard Isaac physics question for the past hour and a half and I just want to get it out of the way as it's been annoying me.
H6.6 - Charged Particles in Electric Fields

Question: In a demonstration, electrons are going around in a 12cm diameter helix with the beam at 70∘ to the 0.0032T magnetic field. Calculate the speed of the electrons.

I just need a couple of pointers to get me going. I've tried a lot of answers but I have no idea if I'm on the right track at all.
1- I don't understand how the centripetal force is provided by the magnetic force provided by the magnetic field. Using the left hand rule, the direction of the magnetic field always seems to be perpendicular to the direction of motion (the angle only seems to be th current and the field and not the direction of motion). If they are perpendicular then how can they influence each other?
2 - I've already tried to use the formula F = BQvsin(theta) for this question, using F = mv^2/r and managing to derive an equation for the velocity where v = BQrsin(theta)/m. Using the values I know I obtained an answer of 3.2 X 10^7 ms^-1.
3 - When I inputted this answer, Isaac Physics gave me a convoluted hint:
"The motion of the electron can be split up into circular motion perpendicular to the field and linear motion parallel to it. Which component of the velocity of the electron will affect the centripetal force required for the circular motion?" Which I also don't understand very well. The velocity is always perpendicular to the force, right? So how can it have more than a single component?

I have been having real difficulty with this question and have no idea what to do next. Just a couple of pointers would be fine - Im not just pestering anyone for the answer to the question, as I feel that if I get the gist of the question I might be able to do it.

Any help would be greatly appreciated. Thanks a lot!
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2 years ago
#2
1. Exactly. The force have no effect on the speed, it has an effect on VELOCITY (ie the direction), because it give acceleration toward the "centre".

2. So yes it is perpendicular to the force, but what about the plane it's moving in?

You are taking the right direction, it's just that instead of thinking everything in the force direction (which is changing) consider the plane the particle is in (it's unchanging)
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