Year 13 OCR Physics: Common Misconceptions and Correction Methods | Year 13 OCR 物理：常见误区与纠正方法

📚 Year 13 OCR Physics: Common Misconceptions and Correction Methods | Year 13 OCR 物理：常见误区与纠正方法

This article highlights frequent misunderstandings in Year 13 OCR Physics and provides clear corrections to help you avoid losing marks. Mastering these nuances will strengthen your exam technique and conceptual clarity.

本文列出了Year 13 OCR物理常见的误区，并提供了清晰的纠正方法，帮助你避免失分。掌握这些细微之处将提升你的考试技巧和概念清晰度。

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1. Thermal Physics: Internal Energy, Heat and Temperature | 热力学：内能、热量与温度

Many students mistakenly equate temperature with internal energy and treat heat as a substance contained within an object.

许多学生错误地将温度等同于内能，并把热量视为物体所包含的一种物质。

Correction: Internal energy is the sum of the random kinetic and potential energies of all molecules. Temperature relates only to the average random kinetic energy. Heat is energy in transit due to a temperature difference, not a property stored in an object.

纠正：内能是所有分子无规则动能与势能的总和。温度仅与分子的平均无规则动能有关。热量是由于温差而传递的能量，并不是物体内储存的属性。

Another common error is thinking that the temperature of a substance always rises when it absorbs energy. However, during a phase change, internal energy increases while kinetic energy remains constant, so temperature stays the same.

另一个常见错误是认为物体吸收能量时温度一定会升高。然而在相变过程中，内能增加而动能保持不变，因此温度不变。

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2. Circular Motion: Centripetal Force is Not a New Force | 圆周运动：向心力不是一种新力

Students often add a ‘centripetal force’ to free-body diagrams as if it were an extra force like tension or gravity.

学生经常在受力分析图中额外添加“向心力”，好像它是与拉力或重力并列的一种力。

The centripetal force is simply the resultant force directed towards the centre of the circle. It must be provided by existing forces such as friction, tension, gravitational attraction or the normal component. There is no separate ‘centripetal’ force label in a diagram.

向心力只是指向圆心的合力。它必须由已有的力来提供，例如摩擦力、拉力、万有引力或法向分力。在受力图中并没有单独的“向心力”标示。

Additionally, some learners think that at constant speed the acceleration is zero; however, the direction of velocity changes continuously, giving a centripetal acceleration of magnitude v²/r or ω²r.

此外，有些学习者以为匀速圆周运动中速度大小不变则加速度为零；但实际上速度方向不断变化，从而产生大小为 v²/r 或 ω²r 的向心加速度。

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3. Simple Harmonic Motion: Acceleration and Displacement | 简谐运动：加速度与位移的关系

It is common to assume that the acceleration is zero when the displacement is maximum, because the velocity is momentarily zero. In SHM, the defining equation a = −ω²x shows that acceleration is maximum at the extreme positions and zero at equilibrium.

学生常认为当位移最大时加速度为零，因为速度瞬时为零。在简谐运动中，定义式 a = −ω²x 表明在端点位置加速度最大，在平衡位置加速度为零。

The total energy of an SHM system is constant, but some students think kinetic energy is maximum when potential energy is zero only for horizontal spring oscillators. In a vertical spring-mass system, gravitational potential energy must be considered, yet the total mechanical energy still remains constant if we include all forms.

此外，简谐运动系统的总能量恒定，但有些学生认为只有水平弹簧振子才在势能为零时动能最大。在垂直弹簧质量系统中，必须考虑重力势能，但若将所有形式的能量计入，总机械能仍然保持不变。

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4. Gravitational Fields: g and Gravitational Potential | 引力场：重力加速度与引力势

A frequent mistake is using g = 9.81 m s⁻² for all positions in orbital mechanics or thinking that gravitational potential is the same as gravitational potential energy.

一个常见错误是在轨道力学中对所有位置都使用 g = 9.81 m s⁻²，或者认为引力势与引力势能是同一个物理量。

In a radial field, g = GM/r², so it decreases with altitude. The gravitational potential V at a point is the work done per unit mass to bring a test mass from infinity to that point; it is negative and given by V = −GM/r. Gravitational potential energy is U = mV. Students often forget the negative sign, leading to incorrect energy calculations.

在径向场中，g = GM/r² ，因此随着高度增加而减小。引力势 V 是将单位质量从无穷远移到该点所做的功；它是一个负值，表达式为 V = −GM/r。引力势能 U = mV 。学生经常忘记负号，导致能量计算出错。

Also, many confuse the graphs of g against r and V against r. The area under a g-r graph gives the change in potential, a detail easily overlooked.

此外，很多人混淆了 g-r 图和 V-r 图。g-r 图下的面积代表势的变化，这一细节容易被忽视。

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5. Capacitors: Time Constant and Exponential Decay | 电容器：时间常数与指数衰减

Many believe that a capacitor is fully charged or discharged after one time constant τ = RC. In reality, after t = τ the charge falls to about 37% of its initial value, and it takes approximately 5τ to be considered fully charged or discharged.

许多人认为经过一个时间常数 τ = RC 后电容器就已完全充电或放电。实际上，在 t = τ 时，电量降至初始值的约 37%，通常需要 5τ 才能被认为完全充放电。

Another misconception is misreading Q-t and I-t graphs: the current decays exponentially, but the charge on a charging capacitor rises exponentially towards Q₀, following Q = Q₀(1 − e^(-t/τ)). Students sometimes apply the discharge formula to both quantities. Be careful to distinguish charging and discharging equations.

另一个误区是误读 Q-t 和 I-t 图：电流呈指数衰减，但充电时电容器电荷则按 Q = Q₀(1 − e^(-t/τ)) 呈指数上升趋近 Q₀。学生有时将放电公式错误地用在两种情形。必须仔细区分充电和放电方程。

When calculating energy stored, E = ½QV = ½CV² = ½Q²/C, many forget that this energy is not entirely available to do external work if the capacitor is still connected in a circuit with resistance.

在计算储存能量 E = ½QV = ½CV² = ½Q²/C 时，许多人忘记如果电容器仍连接在有电阻的电路中，这些能量并不全部可用于对外做功。

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6. Electric Fields: E and V Relationship, Equipotentials | 电场：场强与电势的关系，等势面

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