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)

Questions 1 - 10
1

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 curve in the same direction but with a larger radius

The electron would spiral inward because the force becomes tangential

The electron would move straight because positive charges are unaffected

The electron would curve in the opposite direction with the same 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.

2

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.

3

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 particle is neutral ($q=0$)

The particle has very small mass

The particle’s velocity is perpendicular to $\vec B$

The magnetic field is extremely strong

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.

4

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 decreases (path becomes more curved)

The ion stops curving and moves straight

The direction of curvature reverses

The radius increases (path becomes less curved)

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.

5

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?

To the right

Into the page

No deflection because the ion is negative

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.

6

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?

Only the proton curves because electrons are too light to be deflected

Both move straight because opposite charges cancel magnetic effects

They curve in the same direction but with different radii

They curve in opposite directions

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.

7

A lab setup uses a uniform magnetic field to steer a beam of singly charged ions. The ions enter with $\vec v$ to the right and $\vec B$ upward (both in the plane of the page). Which path is most consistent with the magnetic field’s influence on positive ions?

Curving out of the page

Curving upward in the plane of the page

Continuing straight to the right with no curvature

Curving 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. Positive ions enter with v right and B up, both in-plane, yielding force out of page. The path curves out of the page for positive charges. A distractor like choice A fails by choosing into, often rule misapplication. Verify v right (+x), B up (+y), +q gives +z curve. This check, using right-hand rule, predicts steering in setups.

8

A beam of protons enters a region of uniform magnetic field. The magnetic force provides the centripetal force for circular motion. Which statement is most consistent with the proton’s speed while it remains in the field (neglecting collisions)?

Speed decreases because the magnetic force opposes motion

Speed increases because the magnetic force does positive work

Speed remains constant because the magnetic force is perpendicular to velocity

Speed oscillates because the force alternates between parallel and antiparallel

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 protons in circular motion within the field, the force is centripetal, always perpendicular to velocity. The speed remains constant because magnetic forces do no work, preserving kinetic energy. A distractor like choice A fails by claiming positive work, misapplying that perpendicular forces change speed like friction. Confirm with work = F · ds = 0 since F ⊥ v. This principle, tied to the right-hand rule, explains constant speed in magnetic deflections universally.

9

An electron ($q=-1.60\times10^{-19}\ \text{C}$) is injected upward into a region with uniform magnetic field $\vec B=0.20\ \text{T}$ directed to the right. The electron’s velocity is perpendicular to $\vec B$, and no electric field is present. Which path is most consistent with the magnetic field’s influence?

A circular path in the plane perpendicular to $\vec B$

A parabolic trajectory upward due to constant magnetic acceleration

A straight-line path upward because magnetic fields do no work

A spiral that speeds up because the magnetic field increases kinetic energy

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. Here, an electron with negative charge is injected upward into a uniform magnetic field to the right, with velocity perpendicular to the field and no electric field present. The perpendicular force results in a circular path in the plane perpendicular to B, as the magnetic force provides the centripetal force for uniform circular motion. A distractor like choice D fails because magnetic fields do no work and cannot increase kinetic energy, a common misapplication thinking forces along velocity. As a transferable check, confirm the path is circular when v is perpendicular to B, with radius r = mv / |q|B. Applying the right-hand rule adjusted for negative charge (reversing the force direction) helps predict the sense of rotation in such systems.

10

In a lab demonstration, a singly charged positive ion enters a uniform magnetic field region. The ion’s velocity has a component parallel to $\vec B$ and a component perpendicular to $\vec B$ (no electric field). What outcome is most consistent with the ion’s subsequent motion while inside the field?

Motion that quickly stops because the magnetic force opposes velocity

Straight-line motion because the parallel component cancels the perpendicular component

Helical motion with constant speed along the field direction

Parabolic motion because the magnetic force is constant in a fixed direction

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 lab demonstration, a positive ion enters a uniform magnetic field with velocity components parallel and perpendicular to B, and no electric field. The parallel component remains unaffected (zero force), while the perpendicular component causes circular motion, combining into helical motion with constant speed along the field. A distractor like choice D fails because the magnetic force is not constant in direction; it varies, unlike gravity causing parabolas, a common confusion with other fields. For verification, decompose velocity: the parallel part gives linear motion, perpendicular gives circling. This approach transfers to analyzing complex paths, using the right-hand rule to determine the helix handedness based on charge sign.

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