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A magnetic field points up from the floor in the laboratory. A proton and an electron are
shot into this field parallel to the floor. Looking along the direction of travel, the particles
will curve:
A) Proton to the right, Electron to the left.
B) Proton and Electron to the left.
C) Proton to the left, Electron to the right.
D) Proton and electron to the right.
E) None of these.
Remember that the proton has charge +e and the electron has charge –e. Using
the right hand rule you find F=qvxB, but q changes the direction of the force
vector for a negative particle.
A magnetic field points up from the floor in the laboratory. A proton and an positron
(positively charged anti-electron) are shot into this field parallel to the floor with the
same velocity. The ratio of the radii of the curves,proton to positron, will be
approximately:
A) 1 B) 1/1000 C) 1000 D) 1/2000 E) 2000
The mass of the proton is approximately 1000 MeV/c2, the mass of the electron is
approximately 0.5 MeV/c2, the radius of curvature goes like r=mv/qB. Since qB is
the same, the relevant part to look at is mv. The particles have the same velocity,
so r scales like m, r_proton/r_electron=m_proton/m_electron=2000.
A magnetic field points up from the floor in the laboratory. A proton and an positron
(positively charged anti-electron) are shot into this field parallel to the floor with the
same momentum. The ratio of the radii of the curves,proton to positron, will be
approximately:
A) 1 B) 1/1000 C) 1000 D) 1/2000 E) 2000
This time we are told the momentum is the same. Remember that p=mv, so if mv
is the same and qB are the same, the radii are the same.
A strong magnetic field points in the same direction as a beam of electrons. The electrons
will be:
A) Deflected up, B) Deflected Down, C) Deflected Left, D) Deflected right,
E) Unaffected
The B field and particle velocity are in the same direction, so sinθ is zero.
A strong magnetic field points in the same direction as a beam of electrons. Some
electrons scatter in a target, which changes their direction to 45 degrees with respect to
the beam. These electrons will now travel:
A) In a curve away from the beam.
B) In a curve towards the beam.
C) In a left-handed helix around the beam.
D) In a right-handed helix around the beam.
E) They remain unaffected.
This is a tricky question. If you look in the direction of the beam (particles are
moving away from you) then the field is also pointing away from you. A particle
that scatters down at 45 degrees will have a force exerted on it. Now, vxB is
pointing towards the right, but these are electrons so qvxB points to the left, We
thus get a clockwise moving helix, which we call a “left handed” helix.