Magnetism and Motion of Charged Particles (4C)
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MCAT Chemical and Physical Foundations of Biological Systems › Magnetism and Motion of Charged Particles (4C)
A positively charged particle moves to the left through a uniform magnetic field directed upward (both in the plane of the page). No electric field is present. Based on the right-hand rule, the magnetic force on the particle points:
To the left
Into the page
Out of the page
Upward
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. A positive particle moves left through an upward field, both in the plane. The right-hand rule shows force into the page, as velocity left and field up yield cross product inward. A distractor like choice B fails by choosing the opposite direction, often from reversing the rule's curl. Confirm with vector calculation: v left (-x), B up (+y), v × B into (-z). This method transfers, emphasizing right-hand rule for consistent force direction predictions.
An electron enters a uniform magnetic field region and follows a circular path. If the electron’s charge were hypothetically changed to $+e$ while keeping the same mass, speed, and entry direction relative to $\vec B$, which change is most consistent with the observed trajectory?
The electron would move straight because positive charges are unaffected
The electron would curve in the opposite direction with the same radius
The electron would spiral inward because the force becomes tangential
The electron would curve in the same direction but with a larger radius
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. Hypothetically changing electron charge to +e reverses force direction, curving oppositely with same radius (same m, |q|, v). The path flips direction but keeps size. A distractor like choice A fails by keeping direction same, ignoring sign change. Verify reversed q flips F. This, with right-hand rule, illustrates charge effect on trajectory.
In a cathode-ray tube test, electrons move to the right and enter a region of uniform magnetic field directed downward (in the plane of the page). Neglect electric fields. Which direction is most consistent with the electron beam’s initial deflection?
Upward
Out of the page
No deflection because electrons are too light
Into the page
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. Electrons move right with field downward in the plane, neglecting electric fields. For negative charge, the force is out of the page, causing initial deflection outward. A distractor like choice B fails by choosing the opposite, often from not reversing for negative q. Verify with v right (+x), B down (-y), q<0 yielding +z force. This method, incorporating right-hand rule with sign, applies to beam deflection experiments.
In a vacuum chamber, a proton ($q=+1.60\times10^{-19}\ \text{C}$) enters a region of uniform magnetic field $\vec B=0.50\ \text{T}$ directed into the page. The proton’s velocity at entry is $\vec v$ to the right (in the plane of the page), and electric fields are negligible. Which direction will the proton initially accelerate due to the magnetic field?
Upward (toward the top of the page)
To the right (no deflection; continues straight)
Downward (toward the bottom of the page)
Into the page (parallel to $\vec B$)
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. In this scenario, a proton with positive charge enters a uniform magnetic field directed into the page with velocity to the right. Using the right-hand rule for positive charge, pointing fingers to the right (velocity) and curling them into the page (field direction), the thumb points upward, indicating the initial force and acceleration are upward. A common distractor like choice D fails because the force is always perpendicular to the field, not parallel, as magnetic forces do not act along the field lines. To verify, apply the right-hand rule consistently: for positive charges, the force direction ensures circular motion in the plane perpendicular to B. This rule can be transferred to predict deflections in various orientations, reinforcing that the cross product determines the perpendicular force direction.
A charged particle enters a region where a uniform magnetic field is present, and the particle’s path is observed to remain perfectly straight with unchanged speed. Which condition is most consistent with this observation (neglecting gravity and collisions)?
The magnetic field is extremely strong
The particle is neutral ($q=0$)
The particle has very small mass
The particle’s velocity is perpendicular to $\vec B$
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. For straight path with unchanged speed, despite being called charged, the observation implies neutral q=0, as force requires q ≠ 0 and sinθ ≠ 0. This condition prevents deflection, consistent with zero force. A distractor like choice B fails by suggesting perpendicular velocity causes straight motion, when it maximizes curving. Verify q=0 gives F=0 always. This reasoning, with right-hand rule absent for q=0, distinguishes neutral from charged paths.
In an electron optics experiment, electrons enter a uniform magnetic field region with speed $v$. The field magnitude is doubled from $B$ to $2B$ while keeping the same entry speed and direction (velocity perpendicular to $\vec B$). What change is most consistent with the observed path radius?
Radius is unchanged because magnetic fields do no work
Path becomes straight because increased $B$ aligns velocity with the field
Radius doubles because the force is stronger
Radius halves because the stronger field increases centripetal force
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. In the electron optics experiment, doubling B while keeping v perpendicular increases the force, tightening the circular path. The radius halves because r = mv / qB, inversely proportional to B. A distractor like choice A fails by suggesting stronger force enlarges radius, misapplying force-radius relation in centripetal motion. Verify by recalculating r with 2B, confirming halving. This reinforces applying the radius formula and right-hand rule for direction in varying field strengths.
A singly charged positive ion enters a magnetic field region and follows a circular arc. If the ion’s speed is increased while $B$ is unchanged and $\vec v \perp \vec B$, which change is most consistent with the observed curvature?
The radius increases (path becomes less curved)
The radius decreases (path becomes more curved)
The direction of curvature reverses
The ion stops curving and moves straight
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. A positive ion follows a circular arc, and increasing speed with v ⊥ B unchanged increases radius. The radius grows as r = mv / qB is proportional to v, reducing curvature. A distractor like choice B fails by predicting tighter path, confusing with field strength effects. Calculate r with higher v, confirming increase. This reinforces the formula and right-hand rule for consistent path analysis.
In a mass spectrometry prototype, ions of equal speed enter a uniform magnetic field perpendicular to their motion. Ion A has charge $+e$; Ion B has charge $+2e$; both have the same mass. Which statement is most consistent with the curvature of their paths?
Both curve equally because they have the same speed
Ion B curves less because higher charge increases inertia
Neither curves because magnetic fields only affect neutral particles
Ion B curves more (smaller radius) because magnetic force is larger
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. In mass spectrometry, ions with equal speed and mass but charges +e and +2e enter perpendicularly. Ion B with higher charge curves more (smaller radius) since r = mv / qB decreases with q. A distractor like choice A fails by confusing charge with inertia, when higher q increases force, tightening the path. Calculate r for both, confirming half radius for +2e. This reinforces the formula and right-hand rule for direction, transferable to isotope separation.
A negative ion enters a region with uniform magnetic field $\vec B$ into the page. At entry, the ion’s velocity is upward. Which direction is most consistent with the ion’s initial deflection?
No deflection because the ion is negative
To the right
Into the page
To the left
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. A negative ion enters with upward velocity into a field into the page. Adjusting the right-hand rule for negative charge, the force points to the right, causing initial deflection right. A distractor like choice A fails by applying the positive charge direction, ignoring sign reversal. Verify via q<0 reversing v × B direction to right. This check, combined with right-hand rule, aids in analyzing charged beams in various field orientations.
A proton and an electron enter the same uniform magnetic field with identical initial velocity vectors (same direction and speed), with $\vec v \perp \vec B$. Which outcome is most consistent with their subsequent motion in the field?
They curve in the same direction but with different radii
Only the proton curves because electrons are too light to be deflected
They curve in opposite directions
Both move straight because opposite charges cancel magnetic effects
Explanation
This question tests the understanding of the motion of charged particles in magnetic fields. The magnetic force on a charged particle is given by \(\vec{F} = q(\vec{v} \times \vec{B})\), which is perpendicular to both velocity and magnetic field, causing deflection without changing the particle's speed. Proton and electron with identical v ⊥ B curve in opposite directions due to opposite charges reversing force. Their paths differ in direction but may have different radii based on mass. A distractor like choice A fails by ignoring charge sign effect on direction. Verify opposite q reverses F direction. This, using right-hand rule per charge, predicts opposing curvatures.