Q21. Figure 5 shows three situations in which an electron moves at velocity v th
ID: 1950289 • Letter: Q
Question
Q21.
Figure 5 shows three situations in which an electron moves at velocity v through a uniform magnetic field B and experiences a magnetic force BF. Determine which situation(s) are physically reasonable for the orientations of the vectors.
The correct answer is none of them.
With images like figure b, applying the right hand rule is easy (and I know it's wrong because it's an electron hence Fb needs to be in the opposite direction) but figures like a & c are difficult to apply the right hand rule (as I know it) in. What's the actual direction of Fb in images a & c?
My answers: a) positive x-axis (b) negative z-axis (c) positive x-axis.
Is that right? Thanks.
Explanation / Answer
You can also use vector product and reverse sign due to negative charge In a) v can be represented as (-x ) i + (y) j B can written as (X) i So v cross B has direction= [(-x ) i + (y) j]*[(X) i] =-(yX)k So force is along positive z axis b) B can be represented as (-x ) i + (y) j v can written as (X) i So v cross B has direction =[(X) i]*[(-x ) i + (y) j] =[Xy k] So force is along negative z axis
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