9702-2020-M/J-41

[NaughtyChas]

解析

1(a)

Gravitational force: Force on mass in a gravitational field

同理可得：Magnetic force: Force on a moving charge in a magnetic field

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2(a)

Internal energy of the system:

• Sum of total potential energy and kinetic energy of all the molecules in the system
• reference to random motion of the molecules

Why change in internal energy of an ideal gas is equal to the change in total kinetic energy of its molecules?

• No intermolecular forces in ideal gas so no potential energy
• All internal energy equal to kinetic energy

一个铁球从真空中坠落。判断一下有没有内能的变化。

首先结论是肯定没有的，然后就是这个题有三种不同的证明思路：

1.

• No air resistance so no work done
• No thermal energy transferred to no change in internal energy 2.
• Decrease in PE = increase in KE
• No thermal energy 3.
• PE of molecules does not change
• KE of molecules does not change

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4(a)

解释Specific acoustic impedance很简单，想着z = ρc就行了 Product of the density of the medium and the speed of the wave + transmitting in the medium

一定要加上在介质中传播这个小尾巴。

解释Specific acoustic impedance是如何影响折射率的：简单粗暴直接解释

• 差距大折射率接近1
• 差距小折射率接近0

Attenuation: Loss of amplitude of wave.

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5(c)

两个带相等电荷量的球体在靠的足够近的情况下，两球体的中间点的场强不一定等于2E。因为球体靠的足够近，因此就可能导致球体上的电荷受到力的作用而变得分布不均。这种情况下场强就无法完全遵守Q/4Πεr^2了。

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8(a)

Define the tesla:

• Newton per ampere
• Magnetic field normal to current