题型分析 Question Types

类型 A: 定义题 Definitions

要求用文字表述物理量定义。

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Example 1 (9702_s20_qp_41 Q8a): Define the tesla.

• B1 magnetic field normal to current (1)
• B1 newton per ampere (1)
• B1 newton per metre (1)

Example 2 (9702_w20_qp_41 Q9a): Define magnetic flux.

• M1 flux density × area (1)
• A1 where flux is normal to area (1)

Example 3 (9702_s21_qp_41 Q9a): State what is meant by a magnetic field.

• M1 region where there is a force exerted on (1)
• A1 a current-carrying conductor / a moving charge / a magnetic material (1)

类型 B: 磁场对载流导体的力 + 力矩平衡

结合力矩计算磁场大小。

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Example 1 (9702_s20_qp_41 Q8c): 导线平衡实验求 $B$

条件: 导线长 0.85 cm, 力臂 5.6 cm, $\Delta I = 1.2$ A, 砝码质量 $1.3 \times 10^{-4}$ kg

• C1 change = $mg \times (\Delta)L$
• A1 $= 3.3 \times 10^{-5}$ N m
• C1 change = $B \times (\Delta)I \times L \times x$
• C1 $3.3 \times 10^{-5} = B \times 1.2 \times 0.85 \times 10^{-2} \times 5.6 \times 10^{-2}$
• A1 $B = 0.058$ T

Example 2 (9702_s23_qp_41 Q6c): 平行导线间力

$B$ at wire Q due to wire P = 2.6 mT, current in Q = 5.0 A

• C1 $F = BIL$
• A1 force per unit length $= 2.6 \times 10^{-3} \times 5.0 = 0.013$ N m$^{-1}$

Example 3 (9702_w24_qp_41 Q7c): 平行导线间的磁力

需解释: 每根导线处于另一根产生的磁场中，电流垂直于磁场，所以有力。

• B1 (each) wire sits in the (magnetic) field created by the other
• B1 current (in one wire) is perpendicular to (magnetic) field (due to other wire) so force acts

类型 C: Hall Probe 霍尔探头

计算 Hall 电压或解释为何半导体优于金属。

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Example 1 (9702_w20_qp_41 Q8c): 计算 Hall 电压

$n = 1.3 \times 10^{29}$ m$^{-3}$, $t = 0.10$ mm, $B = 4.6 \times 10^{-3}$ T, $I = 6.3 \times 10^{-4}$ A

• C1 $V_H = \frac{BI}{ntq}$
• A1 $= 1.4 \times 10^{-12}$ V

Example 2 (9702_w20_qp_41 Q8d): 为什么 Hall probe 用半导体而非金属

• B1 semiconductors have a (much) smaller value for $n$
• B1 $V_H$ for semiconductors is (much) larger so more easily measured

Example 3 (9702_s21_qp_41 Q9c): 识别 $n$ 和 $t$

• B1 $n$: number density of charge carriers
• B1 $t$: PV or QT or SW (the thickness)

类型 D: 法拉第电磁感应 EMF 计算与作图

从 $B$-$t$ 图求感应电动势，或画 $E$-$t$ 图。

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Example 1 (9702_s20_qp_41 Q9a): 350 匝线圈，磁场变化

• C1 e.m.f. $= \frac{\Delta B \times A \times N}{t}$
• A1 $= 45 \times 10^{-3} \times \pi \times (1.8 \times 10^{-2})^2 \times 350 / 0.20 = 0.080$ V
• B1 0 to 0.2 s: horizontal line at 0.080 V
• B1 0.2 to 0.4 s: zero
• B1 0.4 to 0.8 s: horizontal line at 0.040 V (opposite polarity)

Example 2 (9702_w20_qp_41 Q9b): 变压器感应电动势解释

• B1 (alternating) current creates changing (magnetic) flux
• B1 core links (magnetic) flux with secondary coil
• B1 changing flux (in secondary) causes induced e.m.f.
• B1 rate of change of flux is not constant so e.m.f. is not constant

Example 3 (9702_w22_qp_41 Q5b): 放电过程中相邻导线感应电动势

• B1 current in wire P gives rise to a magnetic field
• B1 as current (in P) changes, wire Q cuts (magnetic) flux (of wire P)
• B1 cutting flux causes induced e.m.f. (across Q)

类型 E: 楞次定律与涡电流解释

解释电磁感应中的能量损失或阻尼效应。

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Example 1 (9702_s20_qp_41 Q9b): 旋转磁铁在铝盘上产生力矩

• B1 disc cuts flux lines (of the magnet)
• B1 (by Faraday's law) e.m.f. is induced in the disc
• B1 e.m.f. causes (eddy) currents in the disc
• B1 current in the magnetic field (of the magnet) causes force on disc

Example 2 (9702_s21_qp_41 Q10b): 磁铁旁的金属环阻尼振荡

• B1 ring cuts (magnetic) flux and causes induced e.m.f.
• B1 (induced) e.m.f. causes (eddy/induced) currents
• M1 currents cause magnetic field (around ring)
• A1 two fields interact to cause resistive/opposing force

Example 3 (9702_s21_qp_41 Q10c): 切开环后振荡更多

• B1 current cannot pass all the way around the ring
• B1 (induced) currents smaller
• B1 smaller resistive force (so more oscillations)

类型 F: 磁感线图 Magnetic Field Patterns

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Example 1 (9702_s23_qp_41 Q6b): 画长直电流的磁场线

• B1 concentric circles around the wire
• B1 spacing between circles increases with distance from wire
• B1 arrows showing direction of field is clockwise

Example 2 (9702_s23_qp_41 Q6a): 定义磁场

• M1 region where a force acts on
• A1 a current-carrying conductor / a moving charge / a magnetic material