Magnetism and Current-Carrying Wires
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AP Physics 2 › Magnetism and Current-Carrying Wires
A vertical wire carries current downward in a uniform magnetic field directed east. Which statement best describes the direction of the magnetic force on the wire?
It points downward.
It points north.
It points south.
It points east.
Explanation
This problem tests magnetism and current-carrying wires. The wire carries current downward (vertical) in an eastward magnetic field (horizontal). Using the right-hand rule, point fingers downward (current) and curl them east (field); your thumb points north. The force is horizontal and perpendicular to both the vertical current and horizontal field. Choice C (downward) incorrectly assumes force aligns with current, forgetting that magnetic force must be perpendicular to both I and B. When current and field are perpendicular, the force direction completes a right-handed coordinate system with them.
A straight wire carries current out of the page. A uniform magnetic field points upward. Which statement best describes the direction of the magnetic force on the wire?
It points to the left.
It points to the right.
It points upward.
It points out of the page.
Explanation
This problem tests magnetism and current-carrying wires. The current points out of the page while the magnetic field points upward. Using the right-hand rule, point fingers out of the page (current) and curl them upward (field); your thumb points to the left. The magnetic force is always perpendicular to both current and field vectors. Choice A (right) represents applying the right-hand rule backward, a common error when dealing with out-of-page directions. To avoid mistakes with 3D problems, physically orient your hand to match the geometry before applying the rule.
A straight wire runs north–south and carries 4.0 A north. Earth’s magnetic field at that point is 50 µT downward. Which statement best describes the direction of the magnetic force on the wire?
The force is toward the east.
The force is toward the north.
The force is toward the west.
The force is downward.
Explanation
This problem tests magnetism and current-carrying wires. The wire carries current north, and Earth's magnetic field points downward. Using the right-hand rule: point fingers north (current), curl them downward (field direction), and the thumb points west. The magnetic force on the wire is therefore toward the west. Choice A incorrectly suggests eastward force, which would result from reversing the right-hand rule or confusing the directions. Remember to visualize the three-dimensional orientation carefully when applying the right-hand rule to real-world scenarios.
A 0.15 m wire segment is vertical and carries $I=5.0\ \text{A}$ upward. A uniform magnetic field of $0.30\ \text{T}$ points into the page. Which statement best describes the direction of the magnetic force on the segment?
The force is into the page.
The force is to the left.
The force is to the right.
The force is upward along the wire.
Explanation
This problem tests magnetism and current-carrying wires. The wire carries current upward (vertical), and the magnetic field points into the page. Using the right-hand rule: point fingers upward (current direction), curl them into the page (field direction), and the thumb points to the right. Therefore, the magnetic force on the wire segment is to the right. Choice C incorrectly suggests the force is into the page, which represents the misconception of confusing the field direction with the force direction. Always apply the right-hand rule carefully: fingers along current, curl toward field, thumb shows force.
A horizontal wire carries conventional current to the left through a uniform magnetic field directed upward on the page. Which statement best describes the magnetic force on the wire?
It is directed to the left along the wire.
It is directed upward in the direction of $\vec{B}$.
It is directed into the page.
It is directed out of the page.
Explanation
This problem tests understanding of magnetism and current-carrying wires. When current flows to the left through an upward-pointing magnetic field, we apply the right-hand rule to find the force direction. Point your fingers left (current direction) and curl them upward (field direction) - your thumb points into the page, indicating the force direction. Choice D incorrectly assumes the force follows the magnetic field direction, a fundamental misconception about how magnetic forces work perpendicular to both current and field. Use the right-hand rule systematically: fingers along current, curl toward field, thumb shows force - the force is always perpendicular to both.
Two long parallel vertical wires are $4.0\ \text{cm}$ apart. The left wire carries $5.0\ \text{A}$ upward, and the right wire carries $5.0\ \text{A}$ downward. The wires will
attract each other because currents always attract.
move upward together because the force is along each wire.
experience no force because the currents are equal.
repel each other because the currents are opposite.
Explanation
This problem tests understanding of magnetism and current-carrying wires. When two parallel wires carry current, they create magnetic fields that interact with each other's currents. The left wire (current up) creates a magnetic field that circles it - at the right wire's location, this field points out of the page. Using the right-hand rule on the right wire (current down, field out), the force points to the right, away from the left wire. Choice A incorrectly states that currents always attract, missing that opposite currents in parallel wires repel. Apply the two-step process: first find the field from one wire at the other's location, then determine the force using that field.
A horizontal wire carries current to the right. The magnetic field at the wire is also to the right. Which statement best describes the magnetic force on the wire segment?
It is directed to the right along the wire.
It is directed into the page by the right-hand rule.
It is directed upward because $\vec{B}$ points right.
It is zero because $\vec{I}$ is parallel to $\vec{B}$.
Explanation
This problem tests understanding of magnetism and current-carrying wires. When current and magnetic field are parallel (both to the right), the magnetic force on the wire is zero because F = IL × B involves the cross product of parallel vectors. The force magnitude is proportional to sin(θ) where θ is the angle between current and field - when θ = 0° (parallel), sin(0°) = 0, so no force exists. Choice B incorrectly applies the right-hand rule to parallel vectors, not recognizing that no perpendicular force can result from parallel current and field. Check the angle between current and field first - if parallel or antiparallel, the magnetic force is always zero.
Two long parallel wires are $6.0\ \text{cm}$ apart. The left wire carries $2.0\ \text{A}$ upward, and the right wire carries $8.0\ \text{A}$ upward. The right wire experiences a force that is
away from the left wire because the currents are unequal.
toward the left wire because the currents are in the same direction.
upward along the wire because magnetic forces act along current.
zero because only equal currents exert forces.
Explanation
This problem tests understanding of magnetism and current-carrying wires. When parallel wires carry current in the same direction (both upward), they attract each other regardless of the current magnitudes. The left wire creates a magnetic field that circles it - at the right wire's location, this field points out of the page. Applying the right-hand rule to the right wire (current up, field out), the force points to the left, toward the other wire. Choice B incorrectly assumes unequal currents cause repulsion, missing that attraction/repulsion depends only on relative current directions, not magnitudes. For parallel wires: same direction currents attract, opposite directions repel.
A vertical wire carries $1.5\ \text{A}$ downward in a uniform magnetic field of $0.30\ \text{T}$ directed out of the page. Which statement best describes the magnetic force on the wire?
It is directed downward along the wire.
It is directed to the right.
It is directed out of the page.
It is directed to the left.
Explanation
This problem tests understanding of magnetism and current-carrying wires. A vertical wire carrying current downward in a magnetic field directed out of the page experiences a force perpendicular to both. Using the right-hand rule: point fingers downward (current), curl them out of the page (field direction), and your thumb points to the right, indicating the force direction. Choice C incorrectly assumes magnetic forces act along the wire, confusing magnetic force with other types of forces that can act longitudinally. Always apply the right-hand rule: the magnetic force is perpendicular to both current and field, never along either direction.
A straight horizontal wire carries $3.0\ \text{A}$ to the right in a region where $\vec{B}=0.20\ \text{T}$ is directed into the page. Which statement best describes the magnetic force on the wire segment in the field?
It is directed downward on the page.
It is directed into the page.
It is directed to the right along the wire.
It is directed upward on the page.
Explanation
This problem tests understanding of magnetism and current-carrying wires. When a current-carrying wire is placed in a magnetic field, it experiences a force given by F = IL × B, where the direction is determined by the right-hand rule. With current flowing to the right and magnetic field into the page, point your fingers right (current direction) and curl them into the page (field direction) - your thumb points upward, indicating the force direction. Choice C incorrectly assumes the force follows the field direction, a common misconception that magnetic force acts parallel to the magnetic field. Remember to use the right-hand rule: fingers point along current, curl toward field, thumb shows force direction.