Physics Quiz: Investigate Current And Magnetic Fields

What this quiz covers

This quiz focuses on Investigate Current And Magnetic Fields, giving you a quick way to practice the rules, question types, and explanations that matter most for Physics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. It increases (correct answer)
2. It decreases
3. It reverses direction
4. It stays the same because only the number of turns matters

Explanation: This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields. 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 strength inside is B = μ₀ n I, proportional to current I and turns per length n. In this setup, the solenoid produces an axial magnetic field when current flows, and increasing the current from 0.5 A to 1.5 A (while n stays the same) increases the field strength proportionally. Choice A is correct because it accurately explains that increasing current increases the magnetic field strength, as B is directly proportional to I. Choice D incorrectly claims the field stays the same, confusing it with only turns mattering, when actually both current and turns affect strength. 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 (through solenoid). Always remember that magnetic field lines form closed loops, fields from currents are strongest near the wire and weaken with distance, and increasing current increases field strength without changing direction.

Question 2

1. It reverses direction (correct answer)
2. It stays the same direction but becomes weaker
3. It becomes zero because reversing current cancels the magnetic field
4. It rotates $90^\circ$ to become parallel to the wire

Explanation: 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.

Question 3

1. Both sides experience forces into the page (⊗)
2. Left side: out of the page (⊙); Right side: into the page (⊗)
3. Left side: into the page (⊗); Right side: out of the page (⊙) (correct answer)
4. Both sides experience forces upward (↑)

Explanation: 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.

Question 4

1. Compass 1 (2 cm away) because the magnetic field is stronger closer to the wire (correct answer)
2. Compass 2 (6 cm away) because the magnetic field is stronger farther from the wire
3. Both compasses deflect the same amount because the current is the same
4. Neither compass deflects because only permanent magnets produce magnetic fields

Explanation: This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields and how field strength varies with distance. 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, and the strength is B = μ₀I/(2πr), decreasing inversely with distance r from the wire. In this setup, current flows upward through the wire, creating circling magnetic fields that deflect compasses, with stronger deflection where the field is stronger; since Compass 1 is at 2 cm (smaller r) and Compass 2 at 6 cm (larger r), Compass 1 experiences a stronger field and larger deflection. Choice A is correct because it accurately explains that the magnetic field is stronger closer to the wire, leading to larger deflection at 2 cm. Choice B incorrectly claims the field is stronger farther away, when actually the field weakens with distance as 1/r. To solve current-magnetic field problems, identify whether you're dealing with current creating a magnetic field, then apply the appropriate right-hand rule and consider field strength variations with distance or current. 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.

Question 5

1. They repel each other
2. They attract each other (correct answer)
3. There is no force because the currents are equal
4. They only attract if the currents are in opposite directions

Explanation: 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.

Question 6

1. Upward (↑) (correct answer)
2. Downward (↓)
3. To the right (→)
4. Into the page (⊗)

Explanation: 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 and the current in the wire flows out of the page; using the force right-hand rule: point your fingers to the right, point your thumb out of the page, and your palm faces upward—this means the wire experiences a force pushing it upward. Choice A is correct because it correctly uses the force right-hand rule: fingers in field direction (right), thumb in current direction (out), palm in force direction (upward). Choice D gets the perpendicularity wrong, stating the force acts into the page, 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 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).

Question 7

1. $0^\circ$ (current parallel to $\vec{B}$)
2. $30^\circ$
3. $60^\circ$
4. $90^\circ$ (current perpendicular to $\vec{B}$) (correct answer)

Explanation: 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.

Question 8

1. It reverses direction (clockwise field becomes counterclockwise, or vice versa) (correct answer)
2. It stays the same direction but becomes stronger
3. It becomes parallel to the wire instead of circling it
4. It disappears because reversing current cancels the magnetic field

Explanation: 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).

Question 9

1. To the right (→)
2. Upward (↑) (correct answer)
3. Downward (↓)
4. Into the page (⊗)

Explanation: 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 magnetic field points to the right (→) and the current in the wire 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 B is correct because it correctly uses the force right-hand rule: fingers in field, thumb in current, palm in force direction upward. Choice C gets the perpendicularity wrong, stating the force acts downward, 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.

Question 10

1. The left end (correct answer)
2. The right end
3. Both ends are north poles
4. Neither end; a solenoid produces no magnetic poles

Explanation: This question tests understanding of the relationship between electric current and magnetic fields, specifically how current creates magnetic fields. 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). The current flows counterclockwise when viewed from the left end of the solenoid, so using the right-hand rule, curl your fingers in the direction the current flows around the coil, and your thumb points toward the left end, indicating this is the north pole of the electromagnet where field lines emerge, while the opposite end is the south pole where field lines enter. Choice A is correct because it accurately applies the right-hand rule: fingers curl in current direction, thumb points to north pole at the left end. Choice B misapplies the right-hand rule by curling fingers the wrong way, leading to a prediction of right end as north when the correct is left. 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.

Question 11

1. It triples (correct answer)
2. It stays the same
3. It becomes one-third as large
4. It reverses direction but keeps the same magnitude

Explanation: 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 wire is perpendicular to the field (θ=90°), and increasing the current from 1.0 A to 3.0 A directly triples the force magnitude since F is proportional to I, with B and L constant. Choice A is correct because it accurately explains the observable effect based on current-field relationship, with force magnitude scaling linearly with current. Choice D incorrectly predicts that increasing current reverses direction, when actually increasing magnitude only increases strength without changing direction, while reversing current would reverse the force. 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.

Question 12

1. To the left (←) (correct answer)
2. To the right (→)
3. Upward (↑)
4. Into the page (⊗)

Explanation: 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.

Question 13

1. Upward (↑) (correct answer)
2. Downward (↓)
3. To the left (←)
4. Into the page (⊗)

Explanation: 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).

Question 14

1. The magnetic field points in the direction of the right-hand thumb along the solenoid’s axis (correct answer)
2. The magnetic field forms circles around the solenoid’s axis, not along it
3. The magnetic field points opposite the right-hand thumb along the solenoid’s axis
4. There is no magnetic field inside a solenoid carrying DC current

Explanation: 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.

Question 15

1. Upward (↑) (correct answer)
2. Downward (↓)
3. To the right (→)
4. Into the page (⊗)

Explanation: 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.

Question 16

1. The field is stronger at 6 cm than at 2 cm
2. The field is the same at 2 cm and 6 cm
3. The field is stronger at 2 cm than at 6 cm (correct answer)
4. The field direction changes with distance but the strength does not

Explanation: This question tests understanding of how the magnetic field strength created by a current-carrying wire varies with distance from the wire. When electric current flows through a straight conductor, it creates a magnetic field that forms concentric circles around the wire, with field strength that decreases with distance according to an inverse relationship: B ∝ 1/r, where r is the distance from the wire. At 2 cm from the wire, the magnetic field sensor is closer to the source of the field, while at 6 cm it is three times farther away, meaning the field at 6 cm will be weaker than at 2 cm—specifically, it will be one-third as strong due to the inverse relationship. Choice C is correct because it accurately reflects this inverse relationship between magnetic field strength and distance: the field is stronger at 2 cm (closer to wire) than at 6 cm (farther from wire). Choice A reverses this relationship, incorrectly suggesting the field gets stronger with distance, which violates the fundamental principle that fields from localized sources weaken as you move away from the source. To solve problems involving magnetic field strength around wires, remember that the field strength decreases with distance (B ∝ 1/r for a straight wire), while the field direction at any point is tangent to the circular field lines as determined by the right-hand rule. This distance dependence explains why magnetic effects are most noticeable close to current-carrying wires and why electromagnetic devices often use coils to concentrate multiple wires' fields in a small region.

Question 17

1. The force reverses direction (correct answer)
2. The force stays the same direction but becomes larger
3. The force becomes zero because reversing current cancels the magnetic field
4. The force rotates to point along the magnetic field

Explanation: This question tests understanding of how reversing current direction affects the magnetic force on a wire in an external magnetic field. A current-carrying wire in a magnetic field experiences a force perpendicular to both the current and field directions, with the force direction given by the right-hand rule: fingers along field B, thumb along current I, palm faces force F—reversing either the current or the field (but not both) reverses the force direction. Initially with current to the left (←) and field upward (↑), the force right-hand rule gives: fingers up (field), thumb left (current), palm faces into the page; when current reverses to the right (→) while field stays upward (↑), the rule gives: fingers up (field), thumb right (current), palm faces out of the page—the force has reversed from into the page to out of the page. Choice A is correct because it accurately predicts that reversing the current direction while keeping the magnetic field constant results in a reversal of the force direction, which is why the wire will deflect in the opposite direction. Choice C incorrectly claims the force becomes zero because reversing current cancels the magnetic field, confusing the external magnetic field (which is maintained by the magnet and doesn't depend on the wire's current) with the field created by the wire itself (which does reverse but isn't relevant to the force calculation). To solve problems involving forces on currents in magnetic fields, remember that the force direction depends on both current and field directions through the right-hand rule, so reversing either one (but not both) reverses the force. This principle is fundamental to electric motors, where reversing current direction reverses the torque direction, allowing bidirectional motor control.

Question 18

1. Toward the north (up the page)
2. Toward the south (down the page)
3. Into the page (⊗)
4. Out of the page (⊙) (correct answer)

Explanation: 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.

Question 19

1. Clockwise
2. Counterclockwise (correct answer)
3. Straight upward, parallel to the current
4. Straight downward, opposite the current

Explanation: 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.

Question 20

1. It becomes one-third as large
2. It becomes three times as large (correct answer)
3. It becomes $1.0\,\text{N}$ regardless of $B$ and $L$
4. It reverses direction because the current is larger

Explanation: 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). In this setup, the wire is perpendicular to the field (θ=90°), so increasing the current from 0.5 A to 1.5 A (a factor of 3) directly increases the force magnitude by the same factor, while direction remains unchanged since current direction isn't reversed. Choice B is correct because it accurately applies the force formula: F proportional to I, so tripling I triples F when B and L are constant. Choice D incorrectly predicts that increasing current reverses direction, when actually reversing current direction reverses force 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 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.