Help with HSC Past Paper Question - F=qvBsin(theta) and F=mv^2/r (1 Viewer)

[axekick]

I get that you set the equations equal, which leaves r=mv/qBsin40, but the question only works when you don't use sin40.

Why is this? Isn't the sin40 part of the force exhibited on the electron, therefore shouldn't it be included?

It's my first post, so be nice

Thanks

 

[jyu]

View attachment 25386

I get that you set the equations equal, which leaves r=mv/qBsin40, but the question only works when you don't use sin40.

Why is this? Isn't the sin40 part of the force exhibited on the electron, therefore shouldn't it be included?

It's my first post, so be nice

Thanks

The given angle is irrelevant, vector v is still perpendicular to vector B.

 

[axekick]

The given angle is irrelevant, vector v is still perpendicular to vector B.

Does that count for all questions such as this? I get what you're saying; but does this mean you NEVER use the angle if it's perpendicular?

I've never come across this, and if this was the case, I would have assumed they would put an answer that would distract us from this answer.

Just need a little clarification, but thanks for your quick reply.

 

[Rathaen]

The sin(theta) in the formula is used to separate the component of the electron's velocity through the magnetic field from the component that is perpendicular to the magnetic field. In this case the given angle isn't relevant because it's an angle in a different plane which doesn't cause the vector to deviate from the perpendicular to the magnetic field. This isn't to say that you're not using the angle of the vector to the magnetic field, because the angle that you actually DO use is 90 degrees (because it's perpendicular to the field lines), which gives sin(90) = 1, effectively contributes nothing to the calculation and thus is usually left out when you actually do the working.

The angle that's been given in this case is probably to separate the people who understand the formula for the radius of curvature from those who don't, basically.

 

[jamesischool]

shouldn't this be in ideas to imp? lol - i just got this question going through a past paper and remembered seeing it on here

 

[Lucas_]

shouldn't this be in ideas to imp? lol - i just got this question going through a past paper and remembered seeing it on here

Yeah, what he said ^