Relativistic particle in uniform magnetic field (solution check)

AI Thread Summary
The discussion centers around a solution for a relativistic particle in a uniform magnetic field, where the original poster derived a period expression for the particle's motion. The examiner marked the solution as incorrect, leading to confusion about the validity of the calculations. Participants highlighted the importance of dimensional analysis and the potential differences in unit systems, specifically Gaussian versus SI units. Ultimately, it was clarified that the official answer included a factor of gamma, indicating the need for careful consideration of relativistic effects. The original poster acknowledged a misunderstanding in relating velocity to the period, confirming the examiner's correction.
Adgorn
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Homework Statement
A relativistic particle with mass ##m## and positive charge ##q## moves in the ##[yz]## plane in a circle of radius ##R## under the influence of a uniform magnetic field ##B=B_0 \hat x##. Find the period of the motion.
Relevant Equations
##\frac {d\overrightarrow p} {dt}=q \frac {\overrightarrow v} {c}\times \overrightarrow B_0##
My solution was as follows:

$$\frac {d\overrightarrow p} {dt}=q \frac {\overrightarrow v} {c}\times \overrightarrow B_0$$
The movement is in the ##[yz]## plane so ##|\overrightarrow v\times \overrightarrow B_0|=vB_0##, therefore: $$\biggr |\frac {dp} {dt}\biggr |= \frac {qvB_0} {c}.$$ On the other hand, $$p=\gamma m v=\gamma m R \omega$$ and since the force is perpendicular to the velocity, hence to the momentum: $$\biggr|\frac {dp} {dt}\biggr|=p \omega=\gamma mR \omega^2.$$ Comparing the expressions, I got $$ \frac {qR \omega B_0} {c}=\frac {mR \omega^2} {\sqrt {1-\frac {R^2 \omega^2} {c^2}}}$$
which after some algebra yielded $$T=\frac {2\pi} {\omega}=2\pi \sqrt {\frac {m^2c^2} {q^2B_0^2}+\frac {R^2} {c^2}}.$$ My examiner marked this as false without explaining why, but I can't find any fault with this solution, so is it correct?
 
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Adgorn said:
$$T=\frac {2\pi} {\omega}=2\pi \sqrt {\frac {m^2c^2} {q^2B_0^2}+\frac {R^2} {c^2}}.$$ My examiner marked this as false without explaining why, but I can't find any fault with this solution, so is it correct?
The answer must reduce to the non-relativistic result in the case of ##v << c##.
 
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Adgorn said:
My solution was as follows:

$$\frac {d\overrightarrow p} {dt}=q \frac {\overrightarrow v} {c}\times \overrightarrow B_0$$
Are you sure? Did you try dimensional analysis on that?

Also, see https://en.wikipedia.org/wiki/Lorentz_force
 
PeroK said:
The answer must reduce to the non-relativistic result in the case of ##v << c##.
LaTeX pet peeve … ##v \ll c## 😇
 
Adgorn said:
I got ...$$T=\frac {2\pi} {\omega}=2\pi \sqrt {\frac {m^2c^2} {q^2B_0^2}+\frac {R^2} {c^2}}.$$
This looks correct if you want to express ##T## in terms of the radius ##R##. You will often see ##T## expressed in terms of the speed ##v## of the particle: $$T = 2 \pi \frac{\gamma mc}{qB_0} = 2 \pi \frac{ mc}{qB_0\sqrt{1-v^2/c^2}} $$
 
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TSny said:
Maybe using Gaussian or Heaviside-Lorentz units.
The thing is that the examiner does not seem to…
 
Orodruin said:
The thing is that the examiner does not seem to…
How can you tell? [Edited to correct a typo]
 
TSny said:
I can you tell?
In SI units the low velocity angular frequency would be ##qB/m##, not ##qB/mc## because that would have the wrong dimensions.
 
  • #10
Orodruin said:
In SI units the low velocity angular frequency would be ##qB/m##, not ##qB/mc## because that would have the wrong dimensions.
But how do we know if the class is using SI units? In Gaussian units I believe the low velocity limit would be ##qB/mc##.
 
  • #11
TSny said:
But how do we know if the class is using SI units?
I inferred that they are not using Gaussian or Heaviside-Lorentz units. If they were the answer would not be marked wrong as you already mentioned.
 
  • #12
Orodruin said:
I inferred that they are not using Gaussian or Heaviside-Lorentz units. If they were the answer would not be marked wrong as you already mentioned.
OK. Maybe the OP will get back to us about the units.
 
  • #13
Also what might fooled your teacher is that the two expressions (yours and @TSny at post #5) seem not to be equivalent, but they are (up to a factor of ##c##, check other posts about unit system ) if you do the algebra and replace $$v=\omega R,\omega=\frac{B_0cq}{\sqrt{B_0^2q^2R^2+m^2c^4}}$$
 
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  • #14
Delta2 said:
Also what might fooled your teacher is that the two expressions (yours and @TSny at post #5) seem not to be equivalent, but they are if you do the algebra and replace $$v=\omega R,\omega=\frac{B_0cq}{\sqrt{B_0^2q^2R^2+m^2c^4}}$$
A possibility but ##v## was not given so answering with a ##\gamma## in there seems a bit weird to say the least. At the very least the examiner should be aware that there may be more than one equivalent way of writing the correct result and particularly in SR it tends to happen at a fairly high rate. This was one of the first things I learned when I started teaching SR: Many times looking at a final result that looks different from the one you computed at first glance, they are actually the same.

Of course, we don’t know if the examiner knows this or if the exam was corrected by a TA marking down everything that is not a carbon copy of the model solution.
 
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  • #15
TSny said:
OK. Maybe the OP will get back to us about the units.
Orodruin said:
I inferred that they are not using Gaussian or Heaviside-Lorentz units. If they were the answer would not be marked wrong as you already mentioned.

The class is using Gaussian units, sorry for not being clear on that.

It looks like the examiner was wrong, I just wanted to be sure. The official answer was indeed ##\gamma T## where ##T## is the non-relativistic answer, which I could not match with my answer because I mistook ##v = \frac R T## for ##v = \frac {2 \pi R} T##. Thanks for the help everyone!
 
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