AP Physics 2 Quiz: Magnetic Fields

What this quiz covers

This quiz focuses on Magnetic Fields, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 2.

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 is zero at $P$ because magnetic fields exist only at the wire's surface.
2. It points upward because field lines follow the current's path.
3. It points toward the wire at $P$.
4. It points out of the page at $P$. (correct answer)

Explanation: This question tests understanding of magnetic fields. Magnetic fields are invisible properties of space created by moving charges or permanent magnets that exert forces on other moving charges or magnetic materials. For a long straight wire carrying current, the magnetic field forms concentric circles around the wire, with the field direction determined by the right-hand rule: point your right thumb along the current direction, and your fingers curl in the direction of the magnetic field. With current flowing upward and point P to the right of the wire, the field at P points out of the page. Choice C incorrectly assumes field lines follow the current's path, confusing the field pattern with current flow. To determine field direction around a current-carrying wire, always use the right-hand rule rather than assuming fields point along or toward the current.

Question 2

1. It is maximum because the fields from the two wires add in the same direction there.
2. It depends on whether a compass is placed at $M$ to create a field.
3. It is zero because the fields from the two wires are equal in magnitude and opposite in direction there. (correct answer)
4. It is nonzero only on the surfaces of the wires, so it is zero at $M$.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector fields created by moving charges that can superpose when multiple sources are present. For parallel wires carrying current in the same direction, each wire creates a circular magnetic field pattern around itself. At the midpoint M between two wires carrying equal upward currents, the left wire creates a field pointing out of the page while the right wire creates a field pointing into the page. Since these fields have equal magnitudes but opposite directions at M, they cancel completely, resulting in zero net field. Choice B incorrectly assumes fields from same-direction currents always add constructively, missing the geometry of the situation. When analyzing fields from multiple sources, always determine the field direction from each source separately before adding vectors.

Question 3

1. $B_A$ depends on whether a compass is placed at $A$.
2. $B_A$ equals $B_B$ because both points are in air.
3. $B_A$ is four times $B_B$. (correct answer)
4. $B_A$ is one-fourth of $B_B$.

Explanation: This question tests understanding of magnetic fields. Magnetic fields around current-carrying wires decrease with distance according to the inverse relationship B = μ₀I/(2πr), where r is the perpendicular distance from the wire. For the same current of 5.0 A, point A at 1.0 cm experiences a field B_A = μ₀I/(2π×0.01), while point B at 4.0 cm experiences B_B = μ₀I/(2π×0.04). Since point B is four times farther from the wire than point A, the magnetic field at B is one-fourth that at A, making B_A four times larger than B_B. Choice B incorrectly assumes the medium (air) determines field strength rather than distance, missing the fundamental inverse relationship. When comparing magnetic fields at different distances, remember that field strength is inversely proportional to distance: quadrupling the distance reduces the field to one-quarter.

Question 4

1. $B_P$ is nonzero only at the wire's surface.
2. $B_P$ is one-third of $B_Q$.
3. $B_P$ is three times $B_Q$. (correct answer)
4. $B_P$ equals $B_Q$ because the current is the same.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are regions of space where magnetic forces can be detected, created by moving charges or permanent magnets. For a long straight wire carrying current, the magnetic field magnitude at distance r is given by B = μ₀I/(2πr), showing an inverse relationship with distance. Since point P is at 2.0 cm and point Q is at 6.0 cm (three times farther), and both experience the field from the same 3.0 A current, B_P = μ₀I/(2π×0.02) and B_Q = μ₀I/(2π×0.06), making B_P three times larger than B_Q. Choice B incorrectly assumes field strength is constant regardless of distance, missing the fundamental inverse relationship between field strength and distance from the source. When comparing magnetic fields at different distances from a current-carrying wire, always apply the inverse proportionality: doubling the distance halves the field, tripling the distance reduces it to one-third.

Question 5

1. It points downward in the plane of the page. (correct answer)
2. It is zero unless a test magnet is present.
3. It points to the right because magnetic field lines travel with the current.
4. It points upward in the plane of the page.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are properties of space around electric currents or magnets that exert forces on other moving charges or magnetic materials. For a wire carrying current into the page, the magnetic field forms circular patterns around the wire following the right-hand rule: point your right thumb into the page (current direction), and your fingers curl clockwise when viewed from above. At point N to the right of the wire, the clockwise field pattern means the field points downward in the plane of the page. Choice A incorrectly assumes magnetic fields travel with the current, confusing field direction with current flow. To find magnetic field direction, use the right-hand rule based on current direction, not assumptions about fields following currents.

Question 6

1. It points from left to right, from the north pole toward the south pole. (correct answer)
2. It points from right to left, from the south pole toward the north pole.
3. It points upward because magnetic field lines are paths that charges would follow.
4. It is zero because magnetic fields exist only at the poles' surfaces.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector fields in space created by magnets or electric currents that can exert forces on other magnets or moving charges. For a bar magnet, magnetic field lines emerge from the north pole and curve around to enter the south pole, forming closed loops. At point T above the magnet's midpoint, the field lines are curving from the north pole (left) toward the south pole (right), so the field points from left to right. Choice C incorrectly treats magnetic field lines as paths that charges follow, confusing field direction with particle motion. Remember that magnetic field lines show the direction a north pole would be pushed, not the path charges take.

Question 7

1. It points radially outward from the solenoid's axis.
2. It points in the −$x$ direction.
3. It exists only near the wire windings, not in the interior.
4. It points in the +$x$ direction. (correct answer)

Explanation: This question tests understanding of magnetic fields. Magnetic fields inside solenoids are uniform and parallel to the solenoid's axis, with direction determined by the right-hand rule applied to the coil windings. When viewing the solenoid from the +x end and seeing counterclockwise current, curl your right-hand fingers counterclockwise—your thumb points toward you, which is in the +x direction inside the solenoid. The magnetic field inside an ideal solenoid is uniform and axial, quite different from the complex field patterns outside. Choice D incorrectly assumes fields exist only at current locations, missing that magnetic fields fill the space around and within current configurations. For solenoids, use the right-hand rule with fingers following the current in the coils; your thumb indicates the field direction inside.

Question 8

1. $B_U = 2B_V$ because the field magnitude is inversely proportional to distance from the wire. (correct answer)
2. $B_U = 4B_V$ because the field magnitude is inversely proportional to $r^2$.
3. $B_U$ depends on the magnetic properties of an object placed at $U$.
4. $B_U = B_V$ because the current is the same at both points.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are regions in space where magnetic forces act, with strength depending on the source and distance from it. For a long straight wire, the magnetic field magnitude follows the relationship B = μ₀I/(2πr), showing an inverse proportionality to distance r. Since point U is at half the distance of point V from the wire (1.0 cm vs 2.0 cm), the field at U is twice as strong as at V: B_U = 2B_V. Choice B incorrectly applies an inverse square law, which governs electric fields from point charges but not magnetic fields from straight wires. Remember that magnetic field strength from a straight wire decreases linearly with distance, not quadratically.

Question 9

1. Point $R$, but only if a compass is placed there to detect it.
2. Points $R$ and $S$ have equal field magnitude because the current is constant.
3. Point $R$, because the field magnitude decreases with distance from the wire. (correct answer)
4. Point $S$, because the field spreads out and strengthens with distance.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are regions of space where magnetic forces can be detected, created by electric currents or permanent magnets. The magnetic field around a long straight wire decreases in strength with distance from the wire, following an inverse relationship: B ∝ 1/r, where r is the perpendicular distance from the wire. Since point R is closer to the wire (1.0 cm) than point S (3.0 cm), the magnetic field magnitude at R is stronger than at S. Choice D incorrectly assumes constant current means constant field everywhere, missing that field strength depends on distance from the source. When comparing magnetic field strengths, always consider the distance from the source—closer means stronger for a given current.

Question 10

1. It points toward $+x$ (to the right) inside the solenoid. (correct answer)
2. It points toward $-x$ (to the left) inside the solenoid.
3. It points radially outward because field lines follow the motion of charges in the wire.
4. It exists only at the solenoid's ends, so it is zero inside.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector fields in space created by electric currents or permanent magnets. Inside a solenoid, the magnetic field is approximately uniform and parallel to the solenoid's axis, with direction determined by the right-hand rule: curl your fingers in the direction of current flow around the coils, and your thumb points in the field direction. With counterclockwise current viewed from the left end, your thumb points to the right (+x direction) inside the solenoid. Choice C incorrectly assumes fields follow charge motion in the wire, confusing current path with field direction. For solenoids, remember that the interior field is uniform and axial, with direction given by the right-hand rule applied to the coil current.

Question 11

1. It points upward in the plane of the page.
2. It is zero unless a magnetic material is placed at the center.
3. It points out of the page.
4. It points into the page. (correct answer)

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector quantities in space produced by electric currents or permanent magnets that exert forces on other currents or magnets. For a current loop, the magnetic field at the center can be determined using the right-hand rule: curl your right fingers in the direction of current flow, and your thumb points in the field direction. With clockwise current in the plane of the page, your thumb points into the page at the center. Choice D incorrectly assumes magnetic fields require magnetic materials to exist, confusing field detection with field existence. To find the field direction at the center of a current loop, always use the right-hand rule with fingers following the current.

Question 12

1. It points to the left, toward the south pole. (correct answer)
2. It exists only inside the magnet, so it is zero there.
3. It points upward because field lines rise off the magnet.
4. It points to the right, away from the south pole.

Explanation: This question tests understanding of magnetic fields. Magnetic fields emerge from the north pole of a magnet and enter the south pole, forming continuous closed loops through and around the magnet. Outside a bar magnet, field lines point away from the north pole and toward the south pole. At a point on the axis to the right of the south pole, the field continues in the same direction it had between the poles—pointing left toward the south pole. Choice C incorrectly assumes field lines rise vertically off magnets, confusing the 3D nature of field patterns with a simplified 2D representation. Remember that magnetic field lines outside a magnet always point from north to south poles, continuing the pattern established between the poles.

Question 13

1. It points sideways away from the magnet because fields exist only at the poles.
2. It points downward because outside a magnet the field goes from north to south. (correct answer)
3. It is zero unless a compass is present to create a detectable field.
4. It points upward because magnetic field lines must start at the south pole.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector fields produced by magnets or currents that exert forces on other magnetic objects or moving charges. For a bar magnet, field lines form closed loops emerging from the north pole and entering the south pole. At point W beside the magnet halfway between poles, the field lines are curving from the north pole (top) toward the south pole (bottom), so the field points downward. Choice C incorrectly assumes fields exist only at the poles, missing that magnetic fields extend throughout space around the magnet. Remember that magnetic field lines are continuous curves showing field direction at every point, not just at the poles.

Question 14

1. The inner loop produces the larger field magnitude at the center. (correct answer)
2. The outer loop produces the larger field magnitude at the center.
3. Both loops produce equal field magnitude at the center because the currents are equal.
4. The field magnitude at the center depends on the object used to measure it.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector quantities in space that can be created by electric currents and combine through vector addition. For a circular loop, the magnetic field at the center has magnitude B = μ₀I/(2R), inversely proportional to the radius R. The inner loop (R = 2.0 cm) produces field B₁ = μ₀I/(4.0 cm) while the outer loop (R = 4.0 cm) produces B₂ = μ₀I/(8.0 cm), making the inner loop's contribution twice as large. Choice C incorrectly assumes equal currents produce equal fields regardless of geometry, missing the radius dependence. When comparing magnetic fields from current loops, remember that smaller loops produce stronger fields at their centers for the same current.

Question 15

1. Into the page by the right-hand rule.
2. Out of the page by the right-hand rule. (correct answer)
3. Tangent to the loop, following the current's path.
4. Zero because the field exists only at the wire itself.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are produced by moving charges, and their direction follows specific rules based on the current configuration. For a circular current loop, use the right-hand rule: curl your fingers in the direction of current flow (counterclockwise when viewed from above), and your thumb points in the direction of the magnetic field at the center. Since the current flows counterclockwise in the page, your thumb points out of the page at the loop's center. Choice C incorrectly assumes the magnetic field follows the current's path, confusing field direction with charge motion—magnetic fields are perpendicular to current flow, not parallel. To find the field direction at the center of a current loop, curl your right-hand fingers along the current direction; your thumb shows the field direction.

Question 16

1. Downward, toward the north pole.
2. Upward, away from the north pole. (correct answer)
3. Sideways, because field lines circle around the poles.
4. Zero because the field exists only between the poles.

Explanation: This question tests understanding of magnetic fields. Magnetic field lines emerge from the north pole of any magnet and curve around to enter the south pole, forming continuous closed loops. At a point on the axis just above the north pole, the field lines are emerging from the pole and pointing upward, away from the magnet. This is true for any magnet configuration—field lines always exit north poles and enter south poles. Choice C incorrectly assumes field lines circle around individual poles like they do around current-carrying wires, confusing the field patterns of magnets with those of currents. Remember that magnetic field direction at any point near a magnet can be found by following the field line passing through that point, always flowing from north to south outside the magnet.

Question 17

1. It points north (+$y$) by the right-hand rule.
2. It points south (−$y$) by the right-hand rule. (correct answer)
3. It points east (+$x$), following the moving charges.
4. It is zero unless a compass is placed at the point.

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector quantities produced by moving charges, with direction determined by the right-hand rule: point your thumb along the current direction, and your fingers curl in the direction of the magnetic field lines. For a wire carrying current east (+x), at a point directly above (+z), wrap your right hand around the wire with thumb pointing east—your fingers curl from above the wire toward the south (-y direction). The magnetic field circles the wire in a specific rotational sense determined by the current direction. Choice C incorrectly assumes the field points in the same direction as the current flow, confusing magnetic field direction with charge motion direction. To find magnetic field direction around a straight wire, use the right-hand rule: thumb along current, fingers show field circulation pattern.

Question 18

1. Into the page (−$z$) by the right-hand rule.
2. Downward (−$y$), in the same direction as the current.
3. Out of the page (+$z$) by the right-hand rule. (correct answer)
4. Zero unless a moving charge passes through the point.

Explanation: This question tests understanding of magnetic fields. Magnetic fields circulate around current-carrying wires in a pattern determined by the right-hand rule: point your thumb in the current direction, and your fingers show how field lines wrap around the wire. For current flowing downward (-y), point your thumb down; at a point east of the wire (+x direction), your fingers curl from east toward north, then west, then south, meaning the field points out of the page (+z direction) at the eastern point. The field forms concentric circles centered on the wire, perpendicular to the current direction. Choice C incorrectly assumes magnetic fields point in the same direction as current flow, confusing field orientation with charge motion. Apply the right-hand rule systematically: thumb along current, fingers show field circulation, determining field direction at any point around the wire.

Question 19

1. It depends on the magnetic material placed at point $M$.
2. It equals the field from one wire because the other is farther.
3. It is doubled because the fields from the two wires add.
4. It is zero because the fields from the two wires cancel. (correct answer)

Explanation: This question tests understanding of magnetic fields. Magnetic fields are vector quantities that add according to vector addition rules, with both magnitude and direction crucial for determining the net field. When two parallel wires carry current in the same direction (both upward), the magnetic fields they produce circulate around each wire following the right-hand rule. At the midpoint between the wires, the field from the left wire points into the page while the field from the right wire points out of the page. Since the wires carry equal currents and the midpoint is equidistant from both, these fields have equal magnitudes but opposite directions, resulting in complete cancellation. Choice B incorrectly assumes fields always add constructively, ignoring that vector addition requires considering direction. When analyzing fields from multiple sources, always determine both magnitude and direction at the point of interest before combining vectors.