A straight wire segment of length L is placed between the poles of a magnet where the external magnetic field B points upward (↑). The wire carries conventional current into the page (⊗). In which direction does the wire experience a magnetic force?
Answer: To the left (←)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current-carrying wires. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular); the force direction is given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F—this is the motor principle that makes electric motors work. In this setup, the magnetic field points upward and the current in the wire flows into the page; using the force right-hand rule: point your fingers upward (B), point your thumb into the page (I), and your palm faces to the left, meaning the wire experiences a force pushing it to the left. Choice A is correct because it correctly uses the force right-hand rule: fingers in field direction (upward), thumb in current direction (into page), palm facing left, accurately explaining the observable effect based on the current-field relationship. Choice B gets the perpendicularity wrong, stating the force acts along the field direction (to the right, opposite left), when the force must be perpendicular to both the current and the field, which is why the right-hand rule uses perpendicular orientations (fingers, thumb, palm all at right angles). To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Always remember that magnetic field lines form closed loops (no isolated poles), fields from currents are strongest near the wire and weaken with distance, force on current is strongest when current is perpendicular to field (zero force if parallel), and reversing either current or field direction reverses the resulting field pattern or force direction.
A current-carrying wire segment is placed between the poles of a horseshoe magnet where the magnetic field is uniform and points to the right (→). The wire carries current out of the page (⊙). Using the right-hand rule for force on a current in a magnetic field, what is the direction of the force on the wire?
Answer: Upward (↑)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current-carrying wires using the right-hand rule. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with the force direction given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F. The magnetic field points to the right (→) and the current flows out of the page (⊙), so using the force right-hand rule: point your fingers to the right (field direction), point your thumb out of the page (current direction), and your palm faces upward—this means the wire experiences a force pushing it upward (↑). Choice A is correct because it accurately uses the force right-hand rule: fingers point right (field), thumb points out of page (current), palm faces up (force direction), demonstrating the perpendicular relationship between all three vectors. Choice D incorrectly predicts the force acts into the page, which would mean the force is parallel to one of the input vectors (perpendicular to current but parallel to where the field would need to point for this result), violating the fundamental principle that magnetic force is perpendicular to both current and field. To solve current-magnetic field problems involving forces, always use the force right-hand rule (not the field-creation rule): fingers along B-field, thumb along current I, palm faces force F, ensuring all three directions are mutually perpendicular. Remember that this perpendicular relationship is what makes electric motors work—the force on current-carrying wires in a magnetic field causes rotation when the wires are arranged in a loop.
Two long parallel wires are mounted 2 cm apart so their interaction can be observed. Wire 1 carries current upward (↑). Wire 2 also carries current upward (↑). What is the magnetic interaction between the two wires?
Answer: They attract each other
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields and how fields exert forces on other currents. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule; two parallel currents in the same direction attract because the field from one exerts a force on the other toward it. In this setup, both wires carry current upward, so the magnetic field from Wire 1 at Wire 2 circles in a way that, using the force right-hand rule, produces an attractive force, and similarly for Wire 2 on Wire 1. Choice B is correct because it accurately predicts that parallel currents in the same direction attract each other. Choice D reverses cause and effect, claiming attraction only for opposite directions, when actually same directions attract and opposite repel. To solve current-magnetic field problems, identify whether you're dealing with current creating a magnetic field or current in an external field experiencing a force, then apply the appropriate right-hand rule. Practice visualizing in 3D: for parallel wires with same current direction, the fields and forces lead to attraction; reversing one current leads to repulsion.
A long straight wire carries current I to the right (→). To investigate the direction of the magnetic field produced by the current, you look at the wire from above and use the right-hand rule (thumb along current). What is the direction of the magnetic field lines around the wire when viewed from above?
Answer: Counterclockwise circles around the wire
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows to the right through the wire; using the right-hand rule, point your thumb to the right, and your fingers curl counterclockwise when viewed from above (looking against the current direction), showing that the magnetic field circles the wire counterclockwise. Choice B is correct because it accurately applies the right-hand rule: thumb in current direction, fingers curl in counterclockwise direction when viewed from above. Choice A misapplies the right-hand rule by pointing thumb in the wrong direction or using left hand, leading to a prediction of clockwise when the correct is counterclockwise. To solve current-magnetic field problems, identify whether you're dealing with current creating a magnetic field, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire). Common errors to avoid: (a) using left hand instead of right (gives opposite direction), (b) thinking field lines are straight or radiate outward (they circle perpendicular to the wire).
A long straight vertical wire passes through a hole in a table. Conventional current I=2.0A flows upward (out of the table). A small compass is placed on the table 5 cm east (to the right) of the wire to investigate the magnetic field produced by the current. In which direction does the magnetic field B from the wire point at the compass location (ignoring Earth's field)?
Answer: Toward the north (up the page)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows upward (out of the table) through the vertical wire, and using the right-hand rule, pointing your thumb upward shows that your fingers curl counterclockwise when viewed from above, meaning at the compass location 5 cm east (to the right), the magnetic field points northward (up the page). Choice A is correct because it accurately applies the right-hand rule: thumb in current direction (upward), fingers curl to show the field at the east position pointing north. Choice B misapplies the right-hand rule by using the left hand or curling fingers the wrong way, leading to a prediction of southward direction when the correct direction is northward. To solve current-magnetic field problems, identify whether you're dealing with current creating a magnetic field, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire). Always remember that magnetic field lines form closed loops, fields from currents are strongest near the wire and weaken with distance, and reversing current reverses the field direction.
A long straight wire carries current upward (↑). A student maps the magnetic field direction around the wire by moving a compass to different locations. Viewed from above the wire, in which direction do the magnetic field lines circle the wire?
Answer: Counterclockwise
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields and the pattern of field lines around a straight wire. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows upward through the wire, so using the right-hand rule, point your thumb upward, and your fingers naturally curl counterclockwise when viewed from above, showing that the magnetic field circles the wire in this direction. Choice B is correct because it accurately applies the right-hand rule: thumb pointing up (current direction) results in fingers curling counterclockwise when viewed from above, which is the direction of the magnetic field lines circling the wire. Choice C incorrectly claims the field is straight upward parallel to the current, when actually the magnetic field must circle perpendicular to a straight current-carrying wire—field lines never run parallel to the current that creates them. To solve current-magnetic field problems, remember that magnetic field lines form closed loops (no isolated poles), and for a straight wire carrying current, these loops are circles centered on the wire, perpendicular to the current direction. Practice visualizing in 3D: if current goes up through a vertical wire, the magnetic field circles horizontally around it in the counterclockwise direction when viewed from above.
In a lab demo, a long straight vertical wire carries a steady current I=2.0A upward (↑). A small compass is placed 3 cm east (to the right) of the wire to investigate how current creates a magnetic field. When the current is switched on, which direction should the compass needle (its north end) deflect due to the magnetic field from the wire (ignore Earth's field)?
Answer: Out of the page (⊙)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields and how to apply the right-hand rule to predict field direction. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows upward through the vertical wire, so using the right-hand rule, point your thumb upward, and your fingers naturally curl counterclockwise when viewed from above, showing that the magnetic field circles the wire in this direction. At the location of the compass (3 cm east/right of the wire), the field points out of the page (⊙), which explains why the compass needle's north end deflects in this direction to align with the field. Choice D is correct because it accurately applies the right-hand rule: with thumb pointing up (current direction), fingers curl counterclockwise from above, placing the field out of the page at the eastern position. Choice C incorrectly predicts the field points into the page, which would be the field direction on the west side of the wire, not the east side where the compass is located. To solve current-magnetic field problems, identify whether you're dealing with current creating a magnetic field or current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines that circle around the wire.
Two long parallel wires are 3 cm apart. Wire 1 carries current upward (↑). Wire 2 carries current upward (↑) as well. This setup is used to investigate how the magnetic field from one current exerts a force on another current. What is the direction of the force on wire 2 due to wire 1?
Answer: Toward wire 1 (attraction)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular). In this setup, both wires carry current upward (parallel currents in the same direction), so the magnetic field from wire 1 circles it counterclockwise (right-hand rule), and at wire 2's location, this field points leftward if wire 2 is to the right; applying the force rule to wire 2 (thumb up for current, fingers left for field), palm faces toward wire 1, indicating attraction. Choice B is correct because it correctly predicts that parallel currents in the same direction attract, based on the field from one exerting force on the other. Choice A reverses cause and effect, claiming repulsion, when actually parallel same-direction currents attract, while opposite directions repel. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines; for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Practice visualizing in 3D: if current goes up through a vertical wire, field circles horizontally around it; if field points horizontally right and current flows up, force pushes horizontally forward—all three directions are mutually perpendicular.
Two long parallel wires are 2 cm apart. Wire 1 carries current upward (↑). Wire 2 also carries current upward (↑). To investigate the interaction between currents and magnetic fields, a student observes that the wires move slightly when both currents are on. What is the direction of the force between the wires?
Answer: They attract each other
This question tests understanding of how currents interact through their magnetic fields, specifically the forces between parallel current-carrying wires. When electric current flows through a wire, it creates a magnetic field that circles the wire; a second current-carrying wire placed in this field experiences a force according to the motor principle, with the direction determined by applying the force right-hand rule to the second wire in the field of the first. For wire 1 with current up, the magnetic field at wire 2's location (2 cm away) can be found using the field right-hand rule: thumb up gives field into the page at wire 2; then for wire 2 with current up in this field pointing into the page, the force right-hand rule (fingers into page, thumb up) gives palm facing left—meaning wire 2 is pushed toward wire 1, and by Newton's third law, wire 1 is pulled toward wire 2. Choice B is correct because parallel currents in the same direction create magnetic fields that result in attractive forces between the wires—this is a fundamental result that parallel currents attract while antiparallel currents repel. Choice A incorrectly predicts repulsion, which would occur if the currents were in opposite directions (antiparallel), not when they're in the same direction as stated in the problem. To solve problems involving forces between current-carrying wires, remember that parallel currents in the same direction attract each other, while currents in opposite directions repel—this principle is used in the definition of the ampere. The force per unit length between parallel wires is F/L = (μ₀I₁I₂)/(2πd), showing the force depends on both currents and decreases with separation distance.
A student investigates how a straight wire's current affects a nearby compass. The compass is kept at a fixed distance from the wire. When the current is reversed (same magnitude, opposite direction), what happens to the compass deflection?
Answer: The deflection reverses direction
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields and affects nearby compasses. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. For comparing scenarios: when the current is reversed (flowing in the opposite direction), the right-hand rule applied with thumb now pointing in the opposite direction shows that the magnetic field also reverses to circle the wire in the opposite circulation, causing the compass needle to deflect in the opposite direction as well. Choice A is correct because it properly predicts that reversing current reverses field direction, accurately explaining the observable effect on the compass based on the current-field relationship. Choice C incorrectly predicts that reversing current affects only magnitude, when actually reversing current direction reverses field direction while increasing magnitude only increases strength without changing direction. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire, or through solenoid). Common errors to avoid: (a) using left hand instead of right (gives opposite direction), (b) confusing which right-hand rule applies (field-from-current vs force-on-current), (c) thinking force is parallel to current or field (it's perpendicular to both), (d) forgetting that field lines circle around wire not radiate outward, and (e) assuming the compass points toward the wire (it aligns tangent to field circles).
A wire segment lies in a uniform magnetic field and experiences a measurable sideways deflection when current flows, demonstrating that a magnetic field can exert a force on a current. If the current direction in the wire is reversed while the external magnetic field B stays the same, what happens to the direction of the magnetic force on the wire?
Answer: It reverses direction
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular); the force direction is given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F—this is the motor principle that makes electric motors work. When the current is reversed (flowing in the opposite direction), the right-hand rule applied with thumb now pointing opposite shows that the force also reverses direction, while the magnitude stays the same if θ is unchanged. Choice A is correct because it properly predicts that reversing current reverses force direction, as force is proportional to current direction. Choice B incorrectly predicts that reversing current only affects strength but not direction, when actually reversing current direction reverses force direction while magnitude depends on |I|. To solve current-magnetic field problems, identify whether you're dealing with current in an external field experiencing a force, then apply the appropriate right-hand rule: for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Always remember that force on current is strongest when current is perpendicular to field (zero force if parallel), and reversing either current or field direction reverses the resulting force direction.
A long straight vertical wire passes through a hole in a table. Conventional current I=2.0A flows upward (out of the table surface). A small compass is placed on the table 8 cm east of the wire. In this setup, what direction will the compass needle point due to the magnetic field produced by the current (ignore Earth's field)?
Answer: Toward the north
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields around a straight wire. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows upward through the vertical wire, so using the right-hand rule, point your thumb upward (out of the table), and your fingers curl counterclockwise when viewed from above, meaning at the compass location 8 cm east of the wire, the magnetic field points northward. Choice B is correct because it accurately applies the right-hand rule: thumb in current direction (upward), fingers curl showing the field at the east position points north, explaining why the compass needle aligns northward. Choice D misapplies the right-hand rule by confusing the field direction with radial outward lines, leading to a prediction of eastward when the correct direction is northward as the field circles the wire perpendicularly. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire, or through solenoid). Always remember that magnetic field lines form closed loops (no isolated poles), fields from currents are strongest near the wire and weaken with distance, force on current is strongest when current is perpendicular to field (zero force if parallel), and reversing either current or field direction reverses the resulting field pattern or force direction.
A wire segment of length L carries current I and is placed in a uniform magnetic field B. In one trial, the wire is oriented so the current is parallel to the magnetic field (current and B point in the same direction). What is the magnetic force on the wire in this case?
Answer: Zero, because the force is perpendicular to both I and B
This question tests understanding of how magnetic fields exert forces on current-carrying wires, specifically the special case when current is parallel to the magnetic field. A current-carrying wire placed in an external magnetic field experiences a force with magnitude F = BIL sin(θ), where θ is the angle between the current direction and the magnetic field direction—this sine dependence means the force is maximum when current and field are perpendicular (θ = 90°, sin θ = 1) and zero when they are parallel (θ = 0°, sin θ = 0). In this case, with the current parallel to the magnetic field (both pointing in the same direction), θ = 0°, so sin(0°) = 0, resulting in zero magnetic force on the wire regardless of the current magnitude or field strength. Choice B is correct because it recognizes that when current and magnetic field are parallel, there is no magnetic force—the force requires a perpendicular component between current and field to exist. Choice A incorrectly suggests there's a maximum force when current and field are parallel, confusing the parallel case (zero force) with the perpendicular case (maximum force), and misunderstanding that current and field don't "reinforce" each other but rather interact through the cross product which is zero for parallel vectors. To solve force problems, remember that magnetic force on a current depends on the sine of the angle between current and field: F = BIL sin(θ), which is why the force is always perpendicular to both current and field (cross product), and why parallel current and field produce no force. This principle explains why charged particles can travel along magnetic field lines without deflection (when velocity is parallel to field) but spiral around field lines when they have a perpendicular velocity component.
A rectangular wire loop is placed in a uniform magnetic field B directed to the right (→). The loop is oriented so that the left vertical side carries current upward (↑) and the right vertical side carries current downward (↓). What best describes the forces on the two vertical sides due to the magnetic field?
Answer: Left side: into the page (⊗); right side: out of the page (⊙)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current-carrying wires in a loop. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular); the force direction is given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F—this is the motor principle that makes electric motors work. For the left side with current upward and field to the right, using the force right-hand rule: point your fingers to the right (B), point your thumb upward (I), and your palm faces into the page—meaning force into the page; for the right side with current downward, thumb downward reverses the force to out of the page. Choice C is correct because it correctly uses the force right-hand rule for each side: left into the page, right out of the page, accurately predicting opposite forces that could cause rotation. Choice B misapplies the right-hand rule by pointing thumb in the wrong direction for each side, leading to reversed predictions for the force directions. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Common errors to avoid: (a) using left hand instead of right (gives opposite direction), (b) confusing which right-hand rule applies (field-from-current vs force-on-current), (c) thinking force is parallel to current or field (it's perpendicular to both), (d) forgetting that field lines circle around wire not radiate outward, and (e) assuming the compass points toward the wire (it aligns tangent to field circles).
A straight wire runs north–south on a table and carries conventional current from south to north. A compass is placed 4 cm directly above the wire (same vertical plane). When the current is reversed (now flowing from north to south), what happens to the direction of the magnetic field at the compass location due to the wire?
Answer: It reverses direction (clockwise field becomes counterclockwise, or vice versa)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. When the current is reversed (flowing in the opposite direction), the right-hand rule applied with thumb now pointing opposite shows that the magnetic field also reverses to circle the wire in the opposite circulation (clockwise instead of counterclockwise, or vice versa), causing the compass needle to deflect in the opposite direction as well. Choice A is correct because it properly predicts that reversing current reverses field direction. Choice B incorrectly predicts that reversing current affects only magnitude, when actually reversing current direction reverses field direction while increasing magnitude only increases strength without changing direction. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire, or through solenoid). Common errors to avoid: (a) using left hand instead of right (gives opposite direction), (b) confusing which right-hand rule applies (field-from-current vs force-on-current), (c) thinking force is parallel to current or field (it's perpendicular to both), (d) forgetting that field lines circle around wire not radiate outward, and (e) assuming the compass points toward the wire (it aligns tangent to field circles).
A straight wire in a uniform magnetic field experiences a force given qualitatively by F=BILsin(θ), where θ is the angle between the current direction and the magnetic field direction. To investigate how angle affects the force, a student rotates the wire while keeping B, I, and L the same. At which angle θ is the magnetic force magnitude on the wire the greatest?
Answer: 90∘ (current perpendicular to B)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular). In this setup, the force magnitude depends on sin(θ), so as the student rotates the wire, the force is zero at θ=0° (parallel) and maximum at θ=90° (perpendicular), where sin(90°)=1. Choice D is correct because it properly predicts that the force is greatest when current is perpendicular to the field, maximizing sin(θ). Choice A incorrectly claims the force is greatest when parallel, when actually the force is zero if parallel since sin(0°)=0. To solve current-magnetic field problems, identify whether you're dealing with current in an external field experiencing a force, then apply the formula F = BIL sin(θ) and note maximum at 90°. Always remember that force on current is strongest when current is perpendicular to field (zero force if parallel), and reversing either current or field direction reverses the resulting force direction.
A solenoid is connected to a DC power supply so that conventional current circulates around the coils. A student wraps their right-hand fingers in the direction of the current around the solenoid. In this setup, what does the right-hand rule predict about the magnetic field inside the solenoid?
Answer: The magnetic field points in the direction of the right-hand thumb along the solenoid's axis
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields in a solenoid. For a solenoid (coil of wire), the field resembles a bar magnet with field lines emerging from one end (north pole) and entering the other (south pole), and the direction is given by the right-hand rule: curl your fingers in the direction the current flows around the coil, and your thumb points through the center toward the north pole. In this setup, the current circulates around the coils, so using the right-hand rule, curl your fingers in the direction of the current, and your thumb points along the axis inside the solenoid, indicating the direction of the magnetic field. Choice A is correct because it accurately applies the right-hand rule: fingers curl in current direction, thumb points along the field inside the solenoid. Choice C misapplies the right-hand rule by using the left hand or reversing the thumb, leading to a prediction of opposite direction when the correct direction is along the thumb. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire, or through solenoid). Always remember that magnetic field lines form closed loops (no isolated poles), fields from currents are strongest near the wire and weaken with distance, force on current is strongest when current is perpendicular to field (zero force if parallel), and reversing either current or field direction reverses the resulting field pattern or force direction.
A current-carrying straight wire is oriented vertically and carries current downward (↓). A student uses a magnetic field sensor to map the direction of B around the wire at points on a circle centered on the wire. When viewed from above, what is the direction of the magnetic field lines around the wire?
Answer: Clockwise circles around the wire
This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields around a straight wire. When electric current flows through a conductor, it creates a magnetic field around the conductor—for a straight wire, the field forms concentric circles around the wire with direction given by the right-hand rule: point your thumb in the direction of conventional current (positive to negative), and your fingers curl in the direction of the magnetic field lines. In this setup, current flows downward through the vertical wire; using the right-hand rule, point your thumb downward (into the page when viewed from above), and your fingers naturally curl clockwise, showing that the magnetic field circles the wire in this direction at points around it. Choice B is correct because it accurately applies the right-hand rule: thumb in current direction (downward), fingers curl in clockwise direction when viewed from above, explaining the mapped field pattern. Choice A misapplies the right-hand rule by using the opposite direction for thumb or hand, leading to a prediction of counterclockwise when the correct direction is clockwise. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for current creating field, thumb points along current and fingers curl along field lines (circles around wire, or through solenoid). Practice visualizing in 3D: if current goes up through a vertical wire, field circles horizontally around it; if field points horizontally right and current flows up, force pushes horizontally forward—all three directions are mutually perpendicular.
A straight horizontal wire runs left-to-right and carries conventional current I to the right (→). The wire is placed in a uniform external magnetic field B directed into the page (⊗). In which direction is the magnetic force F on the wire segment in the field?
Answer: Upward (↑)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current-carrying wires. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular); the force direction is given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F—this is the motor principle that makes electric motors work. In this setup, the magnetic field points into the page and the current in the wire flows to the right; using the force right-hand rule: point your fingers into the page (B), point your thumb to the right (I), and your palm faces upward, meaning the wire experiences a force pushing it upward. Choice A is correct because it accurately uses the force right-hand rule: fingers in field direction (into page), thumb in current direction (right), palm facing upward, properly predicting the observable upward deflection based on the current-field relationship. Choice D confuses the direction of the magnetic field (which is into the page) with the direction of the force (which is perpendicular to both I and B), when the question specifically asks about force direction, incorrectly suggesting force along the field. To solve current-magnetic field problems, identify whether you're dealing with (1) current creating a magnetic field or (2) current in an external field experiencing a force, then apply the appropriate right-hand rule: for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Common errors to avoid: (a) using left hand instead of right (gives opposite direction), (b) confusing which right-hand rule applies (field-from-current vs force-on-current), (c) thinking force is parallel to current or field (it's perpendicular to both), (d) forgetting that field lines circle around wire not radiate outward, and (e) assuming the compass points toward the wire (it aligns tangent to field circles).
A rectangular wire loop is placed in a uniform magnetic field to investigate a simple motor effect. The magnetic field B points to the right (→). At the instant shown, conventional current in the left vertical side of the loop is upward (↑) and in the right vertical side is downward (↓). Which statement best describes the forces on the two vertical sides due to the magnetic field?
Answer: Left side: into the page (⊗); Right side: out of the page (⊙)
This question tests understanding of the relationship between electric current and magnetic fields, specifically how magnetic fields exert forces on current. A current-carrying wire placed in an external magnetic field experiences a force perpendicular to both the current direction and the field direction, with magnitude F = BIL sin(θ) where B is field strength, I is current, L is wire length in field, and θ is angle between current and field (maximum force when perpendicular); the force direction is given by the right-hand rule: point your fingers along the magnetic field B, point your thumb along the current I, and your palm faces the direction of force F—this is the motor principle that makes electric motors work. The magnetic field points to the right; for the left side with current upward, using the force right-hand rule: fingers right, thumb up, palm into the page—for the right side with current downward: fingers right, thumb down, palm out of the page. Choice C is correct because it correctly uses the force right-hand rule to predict opposite forces: into on left, out on right. Choice B misapplies the right-hand rule by pointing thumb in the wrong direction for each side, leading to reversed force directions. To solve current-magnetic field problems, identify whether you're dealing with current in an external field experiencing a force, then apply the appropriate right-hand rule: for force on current, fingers point along external field B, thumb points along current I, and palm faces force F direction. Practice visualizing in 3D: if field points horizontally right and current flows up, force pushes horizontally forward—all three directions are mutually perpendicular.
