1. Gravitational field
| s19 qp41 Q1(a) s19 qp42 Q1(a) |
| Two point masses are separated by a distance x in a vacuum. State an expression for the force F between the two masses M and m. State the name of any other symbol used. |
| \[ F = G\frac{Mm}{x^2} \] where G is the (universal) gravitational constant |
| s19 qp42 Q1(c) |
| Suggest why the total force between the spheres may not be equal to the force calculated using Newton’s law of gravitation. |
| Any one from: •law applies only to point masses/spheres are not point masses •radii of spheres not small compared with separation •spheres may not be uniform •the masses are not isolated •force between L and rod •spheres may be charged/may be electrostatic force (between spheres) 点评：此题较松，且只给1分；第一点最容易想到。 |
| w16 qp43 Q7(a) |
| Explain what is meant by a field of force |
| region (of space) where a force is experienced by a particle (mass for gravitational field, charge for electric field, current-carrying wires,magnets for magnetic field) |
| s18 qp42 Q1(a) |
| A gravitational field may be represented by lines of gravitational force. State what is meant by a line of gravitational force. |
| direction of force on a (small test) mass or path in which a (small test) mass will move 此题较为冷门，问的是line |
| m16 qp42 Q1(a) w19 qp41 Q1(a) |
| State Newton’s law of gravitation. |
| force proportional to product of the (two) point masses and inversely proportional to the square of their separation either reference to point masses or separation <<‘size’ of masses |
| w16 qp42 Q1(a) |
| Define gravitational field strength. |
| force per unit mass. \[ \vec{g} = \frac{\vec{F}}{m} \] |
| w18 qp42 Q1(a) |
| Explain why, at the surface of a planet, gravitational field strength is numerically equal to the acceleration of free fall |
| accceleration = F/m, field strength =F/m so the two quantities are equal |
| w18 qp41 Q1 (a)(i) m19 qp42 Q1 (a)(i) |
| State what is meant by gravitational potential at a point. |
| work done per unit mass in moving (bringing) the mass from infinity to the point |
| m19 qp42 Q1 (a)(ii) |
| Use your answer in (i) to explain why the gravitational potential near an isolated mass is always negative. |
| (gravitational) force is attractive (gravitational) potential at infinity is zero decrease in potential energy as masses approach or displacement and force in opposite directions |
| m19 qp42 Q1 (c) |
| Rockets on the satellite are fired for a short time. The satellite’s orbit is now closer to the surface of the planet. State and explain the change, if any, in the kinetic energy of the satellite. |
| v2∝ 1/r, (r smaller) so v greater and EK greater \[ G\frac{Mm}{r^2} = m\cdot \frac{v^2}{r} \] \[ E_k = \frac{GMm}{2r} \] |
| w18 qp41 Q1(a)(ii) |
| Suggest why, for small changes in height near the Earth’s surface, gravitational potentialis approximately constant. |
| near Earth’s surface change in height ≪ radius or height much less than radius of the Earth potential inversely proportional to radius and radius is approximately constant (so potential approximately constant) \[ \phi = -\frac{GM_E m}{(R_E+h)} \] |
| w16 qp42 Q1 (c) |
| Suggest quantitatively why it may be assumed that the Sun is isolated in space from other stars. |
| force would have little effect on (large) mass of Sun, therefor would cause an acceleration of Sun of 1.0 × 10–14 ms–2/very small/negligible acceleration. or many stars all around the Sun, therefore net effect of forces/fields is zero |
2. Circular Motion
| s16 qp42 Q1(a) |
| Explain why the centripetal force acting on both stars(binary-star system) has the same magnitude. |
| gravitational force provides/is the centripetal force same gravitational force (by Newton Third Law) or Universal Law of Gravitation. |
| w16 qp41 Q1 (b)(i) w16 qp43 Q1 (b)(i) w19 qp42 Q4 (a) w19 qp43 Q4(a) |
| A satellite in geostationary orbit appears to remain above the same point on the Earth and has a period of 24 hours. State two other features of a geostationary orbit. |
| 1. equatorial orbit/orbits (directly) above the equator 2. from west to east 3. has a period of 24 hours, (an Earth day/as the self-rotation of the Earth) 4. one particular orbital radius |
| m19 qp42 Q4 (c) |
| Suggest one advantage and one disadvantage of the use of geostationary satellites compared with polar-orbiting satellites for communication between points on the Earth’s surface. |
| Advantange: No tracking required Disadvantage: longer time delay |
3. Simple Harmonic Oscillation
| s16 qp41 Q3(a) |
| State, by reference to displacement, what is meant by simple harmonic motion. |
| Any one from: • viscosity of liquid • friction within the liquid • viscous drag • friction/resistance between walls of tube and liquid hint: damping comes from the resistance force that oppose the direction of displacement, you shall explain with details from the questions. |
| w17 qp41 Q2(a) |
| State, by reference to simple harmonic motion, what is meant by angular frequency. |
| \[ \omega = 2\pi \cdot f \] or \[ \omega = \frac{2\pi}{T} \] |
| w19 qp43 Q4(a)(i) |
| State what is meant by the displacement of the mass on the spring. |
| distance from a (reference) point in a given direction |
| w19 qp43 Q4(a)(ii) |
| Suggest how Fig. 4.2 shows that the mass is not performing simple harmonic motion. |
| line is not straight or gradient is not constant \[ \vec{a}= -\omega^2 \cdot \vec{x} \] |
| w19 qp42 Q4(a) |
| State (i) what is meant by damping. (ii) the evidence provided by Fig. 4.2 that the motion of the ball is damped. |
| (i) loss of energy (ii) amplitude (of oscillations) decreases (with time) |
| w18 qp42 Q4 (c)(i) |
| The oscillations shown in Fig. 4.3 are damped. Suggest one cause of this damping. |
| acceleration/force proportional to displacement (from fixed point) acceleration/force and displacement are in opposite directions |
| s19 qp42 Q3 (c)(i) |
| Suggest and explain the change, if any, in the period of oscillation of the mass. A numerical answer is not required. |
| period is shorter/lower Any one from: •greater spring constant/stiffness •(restoring) force is greater (for any given extension) •acceleration is greater (for any given extension) •greater energy/maximum speed (for a given amplitude) 基本公式为： \[ T = 2\pi \sqrt{\frac{m}{k}} \] 并联的两根弹簧会带来更大的spring constant，从而导致周期减少。其三个原因当中如果不说括号部分的内容都不严谨。但是评分标准比较宽容仁慈。 |
4. Ideal Gas and Thermodynamics
| w18 qp42 Q2 (a) m19 qp42 Q2 (a)(i) |
| State what is meant by an ideal gas. |
| gas that obeys equation pV = constant ×T symbols p,V and T explained. e.g. P – the pressure of the gas V – the volume occupied by the gas(container) T – the temperature, in thermodynamic scale (kelvin), of the gas |
| m19 qp42 Q2 (a)(ii) |
| The pressure p of an ideal gas having density ρ is given by the expression \[ p =\frac{1}{3} \rho \left<c^2\right> \] State the meaning of <c2> |
| mean-square-speed (of atoms/molecules) or the mean value of the squares of speeds for all atoms/particles in the ideal gas. 不能说root mean squared speed； |
| s16 qp42 Q2(a) i |
| State what is meant by the Avogrado Constant NA |
| number of atoms/nuclei in 12 grams of carbon-12 footnote: 为什么用atoms和nuclei没有区别 |
| s16 qp42 Q2(a) ii |
| State what is meant by the mole |
| amount of substance which contains N_A particles/molecules/atoms / which contains the same number of particles/atoms/molecules as there are atoms in 12g of carbon-12 |
| m16 qp42 Q2(a) (i) w18 qp41 Q2(a) |
| State what is meant by internal energy, |
| sum of kinetic of random distribution and potential energy of atoms/molecules ver2: sum of potential and kinetic energies (of molecules/atoms/particles) (energy of) molecules/atoms/particles in random motion |
| s19 qp42 Q2(a)(ii) |
| State what is represented by a negative value of ΔU. |
| decrease in internal energy |
| m16 qp42 Q2 (a)(i) |
| State the basic assumption of the kinetic theory of gases that leads to the conclusion that there is zero potential energy between the molecules of an ideal gas. |
| no forces (of attraction or repulsion) between molecules |
| m19 qp42 Q2 (b)(iii) w19 qp41 Q2(a) |
| One of the assumptions of the kinetic theory of gases is related to the volume of the atoms. State this assumption. Explain whether your answer in (ii) is consistent with this assumption |
| assumption: volume of atoms negligible compared to volume of container/cylinder evidence: the volume of atoms is much smaller than the volume occupied. 4e-8 << 6.8e-3 |
| s16 qp42 Q3(a) |
| Explain what is meant by the statement that two bodies are in thermal equilibrium. |
| no net energy transfer between the bodies / bodies are at the same temperature |
| m16 qp42 Q3(a) |
| Define specific heat capacity. |
| the (thermal) energy per unit mass to raise the temperature of a substance by one degree |
| w18 qp42 Q3(a) s19 qp41 Q3(a) |
| Define specific latent heat of fusion. Define specific latent heat. |
| (thermal) energy per unit mass (to cause change of state) (energy transfer during) change of state between solid and liquid at constant temperature (thermal) energy per (unit) mass (to change state) (heat transfer during) change of state at constant temperature |
| w19 qp42 Q3(b)(ii) |
| Explain why, although the power of the heater is changed, the rate at which thermal energy is transferred from the surroundings to the ice is constant. |
| temperature difference (between apparatus and surroundings) does not change |
| s18 qp42 Q3(a) |
| During melting, a solid becomes liquid with little or no change in volume. Use kinetic theory to explain why, during the melting process, thermal energy is required although there is no change in temperature. |
| (during melting,) bonds between atoms/molecules are broken potential energy of atoms/molecules is increased no/little work done so required input of energy is thermal 此题比较冷门，需要涉及到断键是化学知识，但是也可以说是the separation of molecules will be increased. |
| s16 qp41 Q2(a) |
| An ideal gas is assumed to consist of atoms or molecules that behave as 6.hard, identical spheres that are in continuous motion and 7.undergo elastic collisions. State two further assumptions of the kinetic theory of gases. |
| 1.time of collisions negligible compared to time between collisions 2. intermolecular forces (except during collisions) 3. random motion (of molecules) 4. large numbers of molecules (gas particles) 5.(total) volume of molecules negligible compared to volume of containing vessel / (average)mean separation large compared with size of molecules there are 7 basic assumptions in total, the question has listed 2. |
| w19 qp42 Q2(a) |
| Smoke particles are suspended in still air. Brownian motion of the smoke particles is seen through a microscope. Describe: (i) what is seen through the microscope (ii) how Brownian motion provides evidence for the nature of the movement of gas molecules. |
| (i) specks of light moving haphazardly / randomly / in a random way (ii) (gas) molecules collide with (smoke) particles or random motion of the (gas) molecules causes the (haphazard) motion of the smoke particles or causes the smoke particles to change direction Brownian motion是气体分子的运动，烟雾颗粒的，虽然随机，但并不是布朗运动； 对布朗运动的正确解释是爱因斯坦的卓越贡献，证明了分子概念的正确性； 另外的模型是花粉在水中的随机运动；解释完全一致；此题出现频率非常低。 |
5. Electric Field
| w17 qp22 Q5 (a) |
| Define the coulomb |
| coulomb is ampere * second |
| w19 qp42 Q5 (a) |
| State what is meant by an electric field. |
| region of space where charge experiences an (electric) force |
| s19 qp41 Q5 (a) s19 qp43 Q5(a) |
| State what is meant by electric field strength. |
| force per unit charge (force on) positive (test) charge \[ \vec{E} = \frac{\vec{F}}{+q} \] |
| w19 qp41 Q6(a) |
| State an expression for the electric field strength E at a distance r from a point charge Q in a vacuum. State the name of any other symbol used. |
| \[ \vec{E} = \frac{Q}{4\pi \epsilon_0\cdot r^2} \] where ε0 is permittivity of free space 此题答案仅给了该版本；但是个人认为用k，也是没有问题的。主要是试卷的formula sheet当中k被用作是Boltzmann Constant，所以避免混淆没有提及 \[ \vec{E} = \frac{kQ}{r^2}, k=\frac{1}{4\pi\epsilon_0} \] where k is the Coulomb’s constant。 注意和上一条题目的对比，Q和q有何不同，为啥公式不一致；是什么定律嫁接了这两个不同的公式，以及适用范围。你如果可以回答我以上的问题，那么就真的懂了。 |
| w18 qp41 Q6 (a)(i) w18 qp42 Q6 (a)(i) s19 qp42 Q6 (a) w19 qp42 Q9(a) |
| Define electric potential at a point. |
| work done per unit charge in moving positive charge from infinity to the point 与gravititional potential有类似，也有区别 |
| w18 qp41 Q6 (a)(ii) |
| State the relationship between electric potential and electric field strength at a point. |
| field strength = potential gradient “-” sign included or directions discussed exp: 第一分为M1，电场强度是电势对距离的导数。 \[ E(x) = -\frac{\mathrm{d} V(x)}{\mathrm{d} x} \] x, 是距离场源的位置，V(x)是该点处的电势函数； 第二分为A1，因为电场是矢量，需说明电场的方向与电势增加的方向相反，即负号的含义；可以用类似的方式陈述： the direction of the electric field always orginates from the higher electric potential to lower potential |
6. Capacitor
| m16 qp42 Q7 (a) w16 qp42 Q7 (a)(i) m19 qp42 Q6 (a) |
| Define capacitance. Define the capacitance of a parallel-plate capacitor |
| ver1-1 point capacitance = charge/potential difference ver2 charge/potential (difference) or charge per (unit) potential (difference) ver3 – 2 points charge/potential difference charge on one plate, p.d. between the plates |
| w17 qp42 Q7 (a) |
| Suggest why, when the capacitor is connected across the terminals of a battery, the capacitor stores energy, not charge. |
| 1 points for charge exp equal and opposite charges on the plates, so there is no resultant charge. 1 point for energy exp +Ve and -Ve charges separated, so energy is stored |
| s19 qp41 Q7 (a) |
| State two different functions of capacitors in electrical circuits |
| Any valid two points : • to store (electrical) energy • smoothing/reduce ripple (on direct voltages/currents) • to block d.c. • timing/time delay (circuits) • in oscillator (circuits) • in tuning (circuits) • to prevent arcing/sparks |
7. Magnetic Field
| w18 qp41 Q8 (a) s19 qp41 Q8 (a)(i) |
| Explain what is meant by a magnetic field. |
| region where there is a force experienced by a current-carrying conductor/moving charge/(permanent) magnet pole |
| s19 qp41 Q8 (c) |
| The currents in wires P and Q are different in magnitude. State and explain whether the forces per unit length on the two wires will be different. |
| correct application of Newton’s 3rd law to the forces or F/L is proportional to the product of the two currents so same magnitude |
| w18 qp42 Q8 (a) |
| Define magnetic flux density. |
| force per unit current force per unit length (of wire) current normal to (magnetic) field full sample version: …. is the force (exerted by the magnetic field on the current-carrying wire) per unit length per unit current when the direction of current is normal to the magnetic field. |
| s19 qp41 Q8 (b)(i) |
| State Lenz’s Law |
| direction of induced e.m.f./current (tends to) oppose the change causing it |
| s16 qp41 Q10 (b) |
| Two hollow tubes of equal length hang vertically as shown One tube is made of plastic and the other of alumininum. Two small similar bar magnets A and B are held above the tubes and then released simultaneously The magnets do not touch the sides of the tubes. Explain why magnet B takes much longer than magnet A to fall through the tube. |
| 5 points ver 1: energy perspective as magnet falls, tube cuts magnetic flux e.m.f./(eddy) currents induced in metal/aluminium (tube) (eddy) current heating of tube with energy taken from falling magnet so magnet B has acceleration < g (or magnet B has smaller acceleration/reaches terminal speed) ver 2: force perspective as magnet falls, tube cuts magnetic flux e.m.f./(eddy) currents induced in metal/aluminium (tube) (eddy) currents produce magnetic field According to Lenz’s Law, the induced magnetic field will oppose motion of magnet so magnet B has acceleration < g (or magnet B has smaller acceleration/reaches terminal speed) |
| m19 qp42 Q8 (a) |
| When a direct current is switched on in the wire, the reading on the balance is seen to decrease State and explain the direction of: (i) the force acting on the wire (ii) the current in the wire. |
| (i) 3 points Newton’s third law or equal and opposite forces force on magnet is upwards so, so force on wire downwards (ii) 2 points using (Fleming’s) left-hand rule current flows from B to A |
| w18 qp42 Q8 (b)(i) |
| Explain why, when the current is switched on, the current in the sections PQ and RS of the wire does not affect the balance of the wire. |
| forces (on PQ and RS) are horizontal (hence they create) no moment about the pivot or forces (on PQ and RS) are equal and opposite (hence there is) no net force (on the two sections) |
| w18 qp41 Q8 (b)(ii) |
| There is a force acting on the particle, causing it to accelerate. Explain why the speed of the particle on leaving the magnetic field is still v |
| force (on particle) is normal to velocity/direction of motion/direction of speed the magnetic force on the particle serves as centripetal force, hence only change the direction of the particle; the magnetic force is always perpendicular to the velocity, hence there is NO work done on the partcile by the magnetic field. Hence the kinetic energy of the particle is not changed. |
| s16 qp41 Q10(b) |
| The magnets do not touch the sides of the tubes. Explain why magnet B takes much longer than magnet A to fall through the tube. |
| as magnet falls, tube cuts magnetic flux e.m.f./(eddy) currents induced in metal/aluminium (tube) (eddy) current heating of tube with energy taken from falling magnet / (eddy) currents produce magnetic field that opposes motion of magnet so magnet B has acceleration < g magnet B has smaller acceleration/reaches terminal speed |
| m19 qp42 Q10(c) |
| Use laws of electromagnetic induction to explain why, when the switch is closed, the current increases gradually to its maximum value. |
| increasing current causes increasing flux increasing flux induces e.m.f. in coil (induced) e.m.f. opposes growth of current 所以实际电流等于通电电流-感应电流；最终稳定之后感应电流为0。此时实际电流达到最大值 |
| w16 qp43 Q9(a) w16 qp42 Q11(a) w18 qp41 Q9 (a) m19 qp42 Q10 (b) s19 qp42 Q10(a) w19 qp42 Q11(a) |
| State Faraday’s law of electromagnetic induction |
| the induced e.m.f. is (proportional to) the rate of change of magnetic flux (linkage) |
8. Alternating Current
| s16 qp41 Q10(a) |
| An alternating current is passed through the coil.The heating effect of the current in the coil is negligible. Explain why the temperature of the core rises. |
| (non-uniform) magnetic flux in core is changing induces (different) e.m.f. in (different parts of) the core (eddy) currents form in the core which give rise to heating |
| w16 qp42 Q11 (b) |
| An alternating current is passed through an air-cored solenoid.An iron core is inserted into the solenoid and then held stationary within the solenoid. The current in the solenoid is now smaller. Explain why the root-mean-square (r.m.s.) value of the current in the solenoid is reduced as a result of inserting the core. |
| ver 1 for same current iron core gives large(r) (rates of change of) flux (linkage) e.m.f induced in solenoid is greater (for same current) induced e.m.f. opposes applied e.m.f. so current smaller/acts to reducecurrent ver 2 same supply so same induced e.m.f. balancing it (rate of change of) flux linkage is same smaller current for same flux when core present |
| w16 qp42 Q11 (c) |
| Practical transformers are very efficient. However, there are some power losses. State two sources of power loss within a transformer. |
| heating due to eddy currents in core (heating due to current in) resistance of coils hysteresis losses/losses due to changing magnetic field in core |
| s19 qp42 Q10 (b) |
| Explain why, when there is an alternating current in the primary coil, there is a current in the load resistor. |
| current in primary coil gives rise to magnetic flux changing (magnetic) flux in core links with secondary coil induced e.m.f. (in secondary coil) causes current in load/resistor |
9. Qutaum Physics
| w18 qp41 Q11(a) w18 qp42 Q11(a) m19 qp42 Q11(a) sq9 qp42 Q11(a) |
| State what is meant by a photon. |
| discrete amount/quantum/packet of energy of electromagnetic radiation |
| w16 qp43 Q10(a) |
| Explain what is meant by the photoelectric effect. |
| electromagnetic radiation/photons incident on a surface (of a metal) will causes emission of electrons (from the surface). 不要忘记前提是达到threshold frequency，或者辐射的频率足够高。 3分题目需要陈述 |
| w19 qp41 Q11(a) |
| With reference to the photoelectric effect, state what is meant by work function energy. |
| energy (of photon) required to remove electron from a surface / reference to minimum energy / reference to zero kinetic energy 一般提到minimum energy required to move an electron from the surface of a metal 就足够了。注意必须是从表面移出电子所需要的能量；回顾potential well的概念。 |
| m16 qp42 Q11(a) |
| With reference to the photoelectric effect, state what is meant by the threshold frequency |
| minimum frequency of electromagnetic photons for electrons to be emitted from surface of the metal / frequency of photons causing emission of electron(s) from surface with zero kinetic energy |
| m16 qp42 Q11(b) ii |
| Explain why, for any particular wavelength of electromagnetic radiation, most of the electrons are emitted with kinetic energies less than the maximum value E_max |
| maximum kinetic energy when electron emitted from surface energy is required to bring an electron to the surface |
| s19 qp41 Q11 (a) |
| State three pieces of evidence provided by the photoelectric effect for a particulate nature of electromagnetic radiation |
| Any three points from: • (max) energy of emitted electrons depends on frequency • (max) energy of emitted electrons does NOT depend on intensity • rate of emission of electrons depends on intensity (at constant frequency) • existence of threshold frequency below which no emission of electrons • instantaneous emission of electrons • increasing the frequency at constant intensity decreases the rate of emission of electrons (此点我没有提及过，但是可以推导出来，根据 I = n * hf ) |
| w16 qp42 Q12 (a) |
| State an effect, one in each case, that provides evidence for (1) the wave nature of a particle (2) the particulate nature of electromagnetic radiation |
| electron diffraction/electron microscope photoelectric effect/Compton scattering |
| w16 qp42 Q12 (c)(i) |
| State what is meant by the de Broglie wavelength |
| wavelength associated with a particle that is moving/has momentum/has speed/has velocity \[ \lambda_\text{de Broglie} = \frac{h}{\vec{p}} \] |
| w18 qp42 Q11 (b) |
| Describe the appearance of a visible line emission spectrum, as seen using a diffraction grating. |
| mostly dark/dark background discrete (separated) coloured lines |
| s16 qp41 Q12(a) |
| In isolated atoms, electron energy levels have discrete values. Suggest why, in a solid, there are energy bands, rather than discrete energy levels. |
| (in a solid electrons in) neighbouring atoms are close together (and influence/interact with each other) this changes their electron energy levels (many atoms in lattice) cause a spread of energy levels into a band |
10. Nuclear Physics
| w19 qp42 Q10(c) |
| Use your answer in (b)(ii) to comment on the possible diameter of a gold nucleus. |
| (diameter must be) less than/equal to 10–13 or 10–14 m 此题考察卢瑟福的金箔实验或者叫alpha粒子散射实验； |
| w18 qp41 Q12(a) w19 qp42 Q12(a)(ii) |
| State what is meant by radioactive decay. State what is meant by a random decay |
| unstable nucleus emission of particles/photons the emission is spontaneous (particles/radiation) are ionising after decay (decay is) unpredictable/cannot be predicted |
| w19 qp42 Q12(a)(ii) |
| State what is meant by decay constant. |
| probability of decay (of a nucleus) per unit time |
| s19 qp42 Q12(c)(ii) |
| Suggest why the total activity of the radioactive source measured at the time calculated in (i) may be greater than 0.40A0Suggest why the total activity of the radioactive source measured at the time calculated in (i) may be reater tn 0.40A |
| daughter product may be radioactive or random nature of decay |
| s16 qp41 Q13(a) |
| State three reasons why the activity of the sample of copper-66 is not equal to the measured count rate. |
| 1. background count rate/radiation 2. multiple possible counts from each decay 3. radiation emitted in all directions 4. dead-time of counter ( I didn’t mention this before) 5. (daughter) product unstable/also emits radiation 6. self-absorption of radiation in sample or absorption in air/detector window |
| w18 qp42 Q12(a) |
| State what is meant by nuclear fusion and nuclear fission |
| fusion: two nuclei combine to form a (single) nucleus fission: a (single) large nucleus divides to form (smaller) nuclei extra point: •fusion is initiated by (very) high temperatures •fission is initiated by neutron bombardment •resulting nuclei in fission are of similar size •(both processes) release energy •binding energy per nucleon increases •total binding energy increases •fission involves release of neutrons |
| s19 qp42 Q12(a) |
| State what is meant by the binding energy of a nucleus. |
| energy required to separate the nucleons (in a nucleus) to infinity or energy released when nucleons come together (to form nucleus) from infinity |
| w18 qp41 Q11(c)(ii) |
| Explain why, after the decay, the nucleus is no longer stationary. |
| momentum is conserved so nucleus must have momentum in opposite direction to gamma photon released. exp: 原子核经过gamma decay之后，释放了gamma photon；光子具有动量；所以之前静止的原子核会往光子运动的反方向运动。 |
| w16 qp43 Q12(a)(i) |
| Define radioactive half-life. |
| time for number of atoms/nuclei oractivity to be reduced to one half of either number of original nuclide/single isotope; or original value/initial activity activity 是速率； number of nuclide 是数量，由于指数型衰减的性质存在，两者对应的半衰期是相同的。 |
| w16 qp42 Q14 (c) |
| comment on the energy of the α-particle such that the reaction can take place. |
| mass of products is greater than mass of Al plus α reaction causes (net) increase in (rest) mass (of the system) α-particle must have at least this amount of kinetic energy |
| s19 qp41 Q12 (b)(ii) |
| The polonium-208 nucleus is initially stationary. The initial kinetic energy of the He nucleus (α-particle) is found to be less than the energy calculated in (i) part 2. Suggest two possible reasons for this difference. |
| kinetic energy (of recoil) of lead (nucleus) energy of γ-ray photon |
11. Medical Physics
| m16 qp42 Q12(a) i s17 qp41 Q10(a) i |
| In an X-ray tube, the hardness of an X-ray beam may be controlled. State what is meant by the hardness of the beam |
| the penetration of the beam greater hardness means greater penetration/shorter wavelength/higher frequency/higher photon energy |
| w16 qp42 Q13 m19 qp42 Q9 w19 qp42 Q7 |
| Outline the principles of computed tomography (CT scanning). |
| ver1: X-ray image of a (single) slice/cross-section (through the patient) taken from different angles/rotating X-ray (beam) computer is used to form/process/build up/store image 2D image (of the slice) repeated for many/different (neighbouring) slices to build up 3D image each line is worth 1 point ver2: X-rays are used section (of object) is scanned scans/images taken at many angles/directions or images of each section are 2-dimensional images of (many) sections are combined (to give) 3-dimensional image of (whole) structure |
| m16 qp42 Q12(a) ii |
| State how the hardness of the beam may be decreased. |
| decrease the accelerating voltage / decrease the potential difference between the andoe and cathode |
| s17 qp41 Q10(a) ii |
| State how the hardness of the beam from an X-ray tube is increased. |
| greater accelerating potential difference / greater p.d. between anode and cathode |
| m18 qp42 Q12(a) |
| Suggest two causes of lack of sharpness of an X-ray image |
| • scattering of X-ray beam/no lead grid • lack of collimation of beam/aperture large • anode area large • beam p.d. low/photon energy low/X-ray soft |
| m16 qp42 Q12(b) |
| State one advantage and one disadvantage of producing a CT scan image of a person rather than a standard X-ray image. |
| advantage: image gives depth/image is 3D/final image can be viewed from any angle disadvantage: greater exposure/more risk to health/more expensive or person must remain stationary |
| w17 qp42 Q4(b) |
| Ultrasound frequencies as high as 10 MHz are used in medical diagnosis. Suggest one advantage of the use of high-frequency ultrasound rather than lower-frequency ultrasound. |
| high-frequency brings small wavelength, hence small(er) structures can be resolved/observed/identified |
| w16 qp42 Q5 (a) |
| Suggest why the ultrasound from the transducer is pulsed. |
| transducer/transmitter can be also be used as the receiver (transducer both transmits and receives) receives reflected pulses between the emitted pulses (needs to be pulsed) in order to measure/determine depth(s) (needs to be pulsed) to determine nature of boundaries |
| s16 qp42 Q4(a) (i) w16 qp42 Q5(a) (ii) w18 qp41 Q4(b) (i) s19 qp42 Q4(a) w19 qp42 Q5(a)(i) |
| State what is meant by the specific acoustic impedance of a medium State what is meant by specific acoustic impedance. |
| the product of speed of sound in the medium and density hint: the speed of sound in different medium should be referred to, because 330m/s is only true for air. While ultrasound propogate in bones and tissues product of speed of (ultra)sound and density (of medium) reference to speed of sound in medium |
| s19 qp42 Q4(b) |
| State how the ratio \[ \frac{\text{intensity of transmitted wave}, I_T}{\text{intensity of incident wave}, I_0} \] depends on the relative magnitudes of ZA and ZB. |
| 注意，此题不是考察反射声波强度和入射之比，而是传输的强度与入射强度之比，需要将常用公式变形为 \[ 1 – \left(\frac{Z_A-Z_B}{Z_A+Z_B}\right)^2 \] Any two from: • if ZA ≫ZB then ratio is (nearly) zero or if ZB≫ZA then ratio is (nearly) zero or if ZB and ZA are very different then ratio is (nearly) zero or the greater the difference the lower the ratio • if ZA≈ZB then ratio is (nearly) 1 or if ZA = ZB then ratio is 1 or the smaller the difference the closer the ratio to 1 (not ‘large’) •IT/I0 = 1 – [(ZA – ZB)2/(ZA + ZB)2] |
| w18 qp41 Q4(a) |
| Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. |
| pulses (of ultrasound from generator) reflected at boundaries (between media) time delay (between transmission and receipt) gives information about depth intensity of reflected pulse gives information about nature (of tissues)/type (of tissues)/boundary 以上的四分必须写 Any two from: • (reflected pulses) detected by the (ultrasound) generator • gel used to minimise reflection at skin/maximise transmission into skin • degree of reflection depends upon impedances of two media (at boundary) 以上三个任意选两个 |
| w17 qp42 Q4(a) |
| Explain the principles behind the generation of ultrasound waves for diagnosis in medicine. 注意：此题考察的是超声波的制造，有5分 |
| quartz/piezo-electric and crystal/transducer p.d. across crystal causes it to distort/deform applying alternating p.d.(AC) causes oscillations/vibrations when applied frequency is natural frequency, crystal resonates natural frequency of crystal is in ultrasound range |
13. Astrophysics/ Cosmology
| s24 qp42 Q10(a) |
| State what is meant by the luminosity of a star |
| total power radiated by the star |
| s24 qp42 Q10(a) |
| State what is meant by the standard candle in cosmology |
| A standard candle refers to a star which has known(pre-determined) luminosity. |
| s24 qp42 Q10(b) |
| Explain how a standard candle in a distant galaxy can be used to determine the distance of the galaxy from an observer |
| standard candle has known luminosity radiant flux intensity measured by observer distance can be calculated using F = L/4πd2 \[ F = \frac{L}{4\pi d^2} \] |