Download Model Answers for CE Physics Questions

Standard Questions and Answers for NSS Physics
HEAT
Q: What is the meaning of heat energy?
A: Heat is a form of energy which is transferred from one body to another as a result of temperature difference.
Q: The specific heat capacity of water is very high. Explain this significance.
A: 1) Water takes in a lot of energy with only a small temperature rise, so it is ideal for use as a cooling agent, e.g. in the cooling
system of motor cars.
2) A high proportion of body fluid in many living organisms (including human beings) is water. This results in a slow response
to changes in surrounding temperature.
3) The daily and annual temperature differences in coastal areas are smaller than inland.
Q: When water is boiling, heat is transferred to water but its temperature remains unchanged. Explain why.
A: Water absorbs latent heat for changing state. The molecular potential energy is increased while the molecular kinetic energy
remains unchanged.
Q: For the experiment to find the specific latent heat of water,
(i) what is the purpose of using the control experiment?
(ii) what are the major sources of errors?
A: (i) The control is used to find the mass of ice melted by the warmth of the surrounding air.
(ii) Some steam condenses on the top part of the heater and drips back to the cup.
The boiling is very vigorous and a few drops of water may have 'jumped' out of the cup.
Some heat is lost to the surroundings.
Q: For the experiment to find the specific latent heat of water, what is the major source of error?
A: Some parts of the heater are not in contact with the ice pieces, so some heat is lost to the surroundings.
Q: You are given a boiling tube half-filled with a solid. Describe, with the aid of a diagram, how you melt the
substance and state how you obtain the cooling curve of the substance.
A: Heat the substance in a water bath with a Bunsen burner until the substance melts.
To obtain the curving curve, first remove the boiling tube from the beaker and suspend a thermometer in the
melted substance.
Then, record the temperature for every certain length of time, e.g. 1-minute interval.
Using the data, plot a graph of temperature against time. This is the cooling curve of the substance.
Q: Explain why evaporation would cause cooling.
A: The average kinetic energy of molecules is a measure of temperature. During evaporation, the faster moving molecules near
the liquid surface escape, leaving the relatively slower molecules in the liquid. As a result, the average kinetic energy is
decreased, and hence the temperature is lower.
Q: Nam some features of a vacuum flask that help to keep the content warm.
A: 1) The cork prevents the hot air to escape, so heat loss by convection is reduced.
2) The vacuum inside prevents heat loss by conduction and convection.
3) The silvered wall reflects the radiation (infrared) back into the flask and so heat loss by radiation is reduced.
4) The cork is made of insulating material (like plastic), so heat loss by conduction is reduced.
Q: What are the precautions of the experiment that aims at verifying Boyle’s law using a syringe
A: 1. Do not hold the syringe too long (since the gas in the syringe may be warmed by the hand)
2. Change the volume of the air slowly (since pushing in the gas to reduce the volume is compressing the gas, this would
increase the temperature of the gas).
3. The rubber tubing should be as short as possible (since the volume of the gas in the rubber tubing is not measured).
Q: Smoke particles in a smoke cell are seen to be in constant random motion. Explain this kind of Brownian motion.
A: A smoke particle is continually being bombarded on all sides by the air molecules around it. The random collisions produce
unbalanced forces which cause the smoke particles to move in zigzag paths.
Q: Explain the three gas laws microscopically using kinetic theory.
A: Boyle's law ---- If the volume of a gas is reduced without any change in temperature, the molecules will hit the container
walls more frequently. This produces a larger force. The area of the walls has also been reduced. So an increase in force
combined with a decrease in area leads to an increase in pressure.
P-T relation ---- When the temperature of a fixed volume of gas increases, the molecules move faster and hit the container
walls more frequently and more violently. This increases the gas pressure.
V-T relation ---- When the temperature of a gas rises, the kinetic energy and hence the speeds of the molecules increase. If
the volume remains constant, the pressure will increase as the molecules will hit the walls more frequently and more violently.
If the pressure is to remain unchanged, the volume must increase to spread the increased force over a larger area.
Q: What are the assumptions in the kinetic theory of ideal gas?
A: 1. The gas consists of molecules which move randomly and collide elastically with the walls.
2. The volume of the molecules is negligible compared with the volume occupied by the gas.
3. The time during a collision is negligible compared with the time between collisions.
4. The intermolecular forces between the molecules are negligible, except during collisions.
P.1
Q: What is the microscopic explanation of gas pressure?
A: A gas is made up of a huge number of tiny particles. They are in constant random motion and collide with each other and the
container walls continually. The force exerted by molecules hitting the container walls produces pressure.
MECHANICS
Q: Explain the principle of a seat-belt.
A: The force acting on a passenger during the collision is given by force = change in momentum / time [F = (mv - mu)/t]. Using a
seat belt would increase the deceleration time. For a given change of momentum, the force is reduced if the time of impact is
prolonged
Q: Explain why the bumper of a car should not be made too hard.
A: If the car is made of very strong material, it will be brought to rest in a short time during a collision. Since force = change in
momentum / time [ F = (mv - mu) / t ], the final momentum is zero, a change in momentum in a short time results in a large
force which may cause serious injuries. So softer material increases the collision time, and reduces the force during collision.
Q: Both a lift and a man inside the lift fall freely. The man experiences “weightlessness”. Explain why.
A: Both the lift and the man fall freely, they would fall with the same acceleration g, and the normal reaction from the lift on the
man will be zero. There is no reaction force between the man and the lift. The man thus appears weightless.
(Strictly speaking, weightlessness is not a very good term because the weight, being the force of gravity, is not zero. A better
term is that the object's apparent weight is zero. Only when the object is in a place very far away from the earth and other
astronomical planets, the object will then have true weightlessness.)
Q: What does the area under a force-time graph in a collision represent?
A: The area under a force-time graph represents the change in momentum of the object or the impulse acting on the object.
Q: What are the changes in momentum and kinetic energy in an elastic collision and an inelastic collision?
A: The total momentum is conserved in all types of collisions no matter it is elastic or inelastic. However, the total kinetic energy
of the colliding objects is decreased in inelastic collision while the kinetic energy is conserved (i.e. unchanged) for an elastic
collision.
Q: A pellet moves towards a lump of plasticine which is fixed to the ground. After collision, the pellet becomes embedded in the
plasticine. In this collision, the total momentum of the pellet and the plasticine is not conserved. Does it violates the
conservation of momentum? Explain briefly.
A: The total momentum of the pellet and the plasticine is not conserved because there is an extermal force acting on the plasticine
by the ground during the collision. In fact, the total momentum of the “pellet+plasticine+earth” is conserved if the momentum
of the ground (or the earth) is included. The movement of the earth is not noticeable because of its huge mass.
Q: A student performs an experiment to test the formula of centripetal force. He
whirls a rubber bung attached to one end of a string which passes through a glass
tube with smooth openings, and has a weight W hanging at its other end. What are
the possible errors in this experiment?
A: 1) The main source of error is the frictional force between the glass tube and the
string.
2) It is difficult to maintain a constant angular velocity.
3) The rubber bung is not whirled in a horizontal circle.
Q: What is meant by a geostationary satellite?
A: A satellite is said to be a geostationary satellite or in a parking orbit if its period of rotation is the same as that of the earth.
Since a geostationary satellite has a period of 24 hours and remain in the same position above the earth, it is apparently
stationary.
WAVES
Q: State, with the aid of a diagram, a method to measure the focal
length of a convex lens.
A: The focal length of a convex lens can be found by capturing the image
of a very distant object, e.g. a building, on a screen. The distance from
the lens to the image is measured. This distance is approximately equal
to the focal length of the lens.
Q: What are the conditions for total internal reflection to take place?
A: For total internal reflection to take place, the following two conditions must be both satisfied:
1) The light ray travels from an optically denser medium to a less dense medium.
2) The angle of incidence is greater than the critical angle.
P.2
Q: What are the advantages of using optical fibre cables over copper cables in transmitting signals?
A: 1) An optical fibre is much thinner and lighter.
2) Each optical fibre can carry a lot of signals simultaneously as compared with copper cables.
3) The signals do not weaken very much inside an optical fibre cable even over long distances.
4) The cost is cheaper.
Q: What is meant by coherent sources?
A: Two sources having constant phase difference and the same frequency are said to be coherent sources.
Q: Waves bend during refraction as well as diffraction. What is the difference between refraction and diffraction?
A: When light travels from one medium to another, its speed changes and as a result, refraction occurs. Thus refraction is due to a
change of speed and so two media are required for refraction to occur. The bending of waves around corners is called
diffraction. So diffraction occurs when light hits a barrier.
Q: The vibration of a stretched string gives rise to sound wave. What are the similarities and differences between the wave in
string and the sound wave in air?
A: The similarities and the differences are listed as follows:
WAVES IN STRING
SOUND WAVES IN AIR
Stationary wave
Travelling wave
Transverse wave
Longitudinal wave
Speed determined by tension and Speed determined by temperature
mass per unit length
of air
They have the same frequency.
Q: How is stationary wave formed?
A: Stationary waves result from the superposition of two progressive waves of equal amplitude and frequency travelling with the
same speed in opposite directions.
Q: What are the differences between progressive wave and stationary wave?
A: The differences are listed as follows:
Progressive wave
The waveform moves.
All points along the wave are vibrating with the same
amplitude.
The phase difference between any two points within a
wavelength varies from 0o to 360o.
Energy is transmitted in the direction of travel.
Stationary wave
The waveform does not move.
The amplitude of vibration varies from a maximum at the
antinodes to zero at nodes.
All points between two adjacent nodes are vibrating in phase.
Points on either side of a node are vibrating 180o out of phase.
No energy is transmitted.
Q: What are the necessary conditions for producing an observable interference pattern?
A: The two wave sources must
1. have the same frequency
2. have constant phase difference
3. be separated by a distance not too great compared with the wavelength
4. have the same amplitude or nearly the same amplitude
Q: Explain why alternate bright and dark fringes can be observed on a screen in a double slit experiment.
A: Light waves from the two slits are two coherent sources because they come from a single source and are in phase and of the
same frequency. At places where the light waves arrive in phase (corresponding to path difference = m  where m is an
integer), constructive interference takes place and a bright fringe is formed. At places where the light waves arrive exactly out
of phase, destructive interference takes place (corresponding to path difference = (m+1/2)), a dark fringe is formed. Since the
path difference changes continuously, alternate bright and dark fringes are formed.
Q: Explain why two identical light sources will never produce an interference pattern. Also explain why Young double slit
experiment can overcome the problem.
A: The emission of light from ordinary light source is random. There is no definite and stable phase relationship between the light
waves coming from the sources, so the light from two separate sources is incoherent, i.e. the phase difference changes from
time to time. As a result, the interference is sometimes constructive and sometimes destructive. Thus, no stable interference
pattern is observed.
However, in Young’s double slit experiment, the two light sources are derived from a single source (or a single wavefront), and
since every point on a wavefront is in phase, the phase difference is zero (zero is a constant). The two sources are thus
coherent, so a stable interference pattern is observed.
Q: What would happen if white light is used in double slit interference?
A: If white light is used, coloured fringes will be observed with violet fringe closer to the centre and red fringe away from the
centre, but the central fringe is white. The explanation is that each colour produces its own fringe and since the fringe position
is given by mD/a, so red light ( = 7 x 10-7 m) has the longest wavelength and produces fringes further away from the centre.
Violet light has the shortest wavelength ( = 4 x 10-7 m) and so produces bright fringes closer to the center. The reason for a
central white fringe is that each colour light produces its own zero order (m = 0) fringe at the centre, so when the different
colour light overlap, the combined effect is white light.
P.3
Q: In double slit interference, the light energy is zero at dark fringes. Where does this energy go to?
A: Energy is redistributed in the pattern. Energy in the dark region is transferred to the bright region.
Q: Explain why loud sound and soft sound can be heard when one moves across the two
loudspeakers.
A: This series of loud and soft sounds is the result of interference when the sound waves
emitted by the two loudspeakers meet. Where the sound waves meet in phase, constructive
interference occurs and a loud sound is heard; where the sound waves meet out of phase,
destructive interference occurs and a soft sound is heard.
Q: The reception of TV is sometimes affected when an aeroplane flies overhead. Explain this phenomenon.
A: The TV waves travelling directly to the aerial interferes with the signal reflected from the aeroplane. The interference is
seldom completely destructive because the reflected signal is usually much weaker.
ELECTRICITY AND MAGNETISM
Q: Charges accumulated in oil tankers and aircraft can be dangerous. Explain how the charges can be removed in oil tankers
and aircraft.
A: Oil tankers often have metal chains which touch the ground. It carries static charges produced from the vehicle body to the
ground. Aircraft would be charged by friction because of moving in air. Tyres for aircrafts are made of conductive rubber so
that any charge developed on the aircraft body during flights is conducted to the ground on landing.
Q: Explain why wool, cotton and artificial fibres become dirty easily and explain how this problem can be reduced in factories
producing these products.
A: Wool, cotton and artificial fibres become charged easily, especially in dry weather. After charged, they would attract small,
light things, such as dust and become dirty. This problem can be solved by keeping the air in factories humid since it is harder
to charge objects in humid air. (The moisture is more conducting)
Q: Explain why filament lamps break most easily at the moment when they are switched on.
A: The resistance of a conductor increases with temperature rise. When filament lamps are first switched on, the temperature is
low, hence the resistance is low, thus the current can be very large and breaks the filament. However, when the lamp is
switched on for a while, its temperature rises, the filament resistance becomes higher and limits the current. The filament will
not be broken easily.
Q: What is the function of the earth wire?
A: Earth wire is a safety device which connects the metal body of an electric appliance to the earth (ground). If a fault develops,
resulting in the live wire touching the metal body, a large current flows to the earth since there is no (or very low) resistance in
the conducting path to the earth. If there were no earth wire, the metal body would become high potential (live) and anyone
who happened to touch it could receive a fatal electric shock.
Q: What is the function of a fuse?
A: A fuse melts when the current exceeds a specified value. If a fault, e.g. a short circuit or overloading, develops and a large
current flows, the fuse melts and breaks the circuit before the cable becomes overheat and catches fire.
Q: Explain why the switch and the fuse should be installed in the live wire.
A: The switch and the fuse are fitted in the live wire. They would work equally well in the neutral wire, but then the appliance
and the cable would still be at high potential when the switch was turned off or the fuse blown. This would be dangerous if a
fault developed (e.g. the cable is broken accidentally, or the live wire touches the metal body).
Q: What is the function of the commutator in electric motor?
A: The commutator is used to reverse the direction of current for every half cycle so that the force acting on the coil reverses
every half cycle. As a result, the direction of torque (or moment) and hence the direction of rotation of the coil remains
unchanged.
Q: Why is semiconductor used in Hall probe?
A: As semiconductors have a smaller charge carrier density n than metals, the Hall voltage VH (= B I / n q t) for semiconductor is
greater than that for metal for the same current and field and is easier to measure.
Q: What is the principle of a Hall probe?
A: A Hall probe consists of a thin slice of semiconductor with a low density of charge carriers. A d.c. supply passes a steady
current through the slice. When a magnetic field is applied perpendicularly to the current, forces will act on the charge-carriers,
causing them to move to one side of the slice, no matter they are positive or negative. The accumulation of charges will set up
an electric field. The deflection of charge carriers ceases when the electric and magnetic forces balance. The Hall voltage
across the opposite ends is measured by a millivoltmeter. If a known magnetic field B’ produces a Hall voltage V’, then a
measured Hall voltage V corresponds to an unknown magnetic field B given by B = (V/V’)B’.
Q: Explain why, with an example, Lenz’ law is another expression of conservation of
energy.
A: When a magnet is moving towards a solenoid, the induced current produces a north pole
on the left, thereby opposing the motion of the magnet. Thus the kinetic energy of the
magnet is decreased. The lost in KE becomes electrical energy. Thus Lenz's law
demonstrates the conservation of energy.
P.4
Q: Explain why a bicycle dynamo generates alternating current.
A: Because of the soft iron core, the magnetic field lines are linked to the coil and
are continuously cut by the coil as the magnet rotates. As the north pole moves
towards the soft iron C-core (hence the coil), induced current flows in one
direction. After a moment, the north pole moves away from the C-core (hence
the coil), induced current flows in opposite direction. Thus an a.c. is produced.
Q: State the energy loss in a transformer and the methods of improvement.
A: 1. Resistance of coils ------ Coils have resistance and so they are heated up by
currents flowing through them.
Method of improvement: Use thicker copper wires to reduce resistance.
2. Eddy currents ------ As the core is made of soft iron and is in a changing magnetic field, eddy currents are induced in it,
and as a result, the core heats up.
Method of improvement: Use laminated core to reduce eddy currents.
3. Magnetisation and demagnetisation of the core ------ The a.c. continually magnetises and demagnetises the core. This
requires extra energy.
Method of improvement: Use soft iron to make the core because soft iron is a material which can be easily magnetised and
demagnetised.
Q: Explain why high voltage is used to transmit power.
A: Using high voltage to transmit power can reduce power loss in the cables since if high voltage is used, the current in cables is
reduced (since P = IV), so the power loss in cables (I2R) is also reduced.
Q: Explain why a.c. is used instead of d.c. when power is transmitted.
A: It is because an a.c. voltage can be stepped up or down easily without much power loss using a transformer.
Q: In the set-up shown, the magnet is oscillating up and down. It is found that an a.c. is
produced. Explain why.
A: When the magnet oscillates up and down, there is a change of magnetic field around the
solenoid and a current is induced in the solenoid. By Lenz’s law, when the magnet moves
towards the solenoid, induced current flows in one direction (from Y to X through the solenoid).
When the magnet moves away from the solenoid, the induced current flows in opposite
direction (from X to Y through the solenoid). So an a.c. is produced.
RADIOACTIVITY AND NUCLEAR ENERGY
Q: Explain the track of alpha radiation in cloud chamber.
A: Alpha particles form straight and thick tracks. Alpha particles are heavy and so their track is not easily affected due to
collisions by other particles, as a result, the track is straight. Alpha particles have high ionizing power, therefore, a lot of ion
pairs are formed, so the track is thick.
Q: What is the origin of background radiation?
A: Background radiation is due to cosmic radiation coming from the outer space and by small amounts of radioactive substances
present in the rocks and soil, and radon gas in the atmosphere.
Q: What are the results of Rutherford scattering experiment and how can the results be explained using Rutherford’s new
model of atom?
A: The result is as follows: Nearly all the alpha particles passed through the gold foil. However, a small number of alpha particles
were deflected through various angles and a few even bounced back! The explanation is that in the new atomic model, most of
an atom is empty space and all the positive charge and most of the mass are concentrated in a tiny core or nucleus. When
positively charged alpha particles were fired at the gold foil, most of them passed straight through the empty space of the gold
atoms. The few which happened to come close to the positively charged nucleus were acted on by a strong electrostatic force
of repulsion and as a result, were deflected or even bounced back
Q: Why is fusion reaction currently not practical?
A: Fusion reaction is currently very difficult to achieve because of the strong electrical repulsion between the hydrogen nuclei
when they are close to each other. This can only be done by heating the gas to a high temperature, say 10 7 K. It is difficult to
get such a high temperature and no physical container can withstand such a high temperature.
Q: A GM counter is placed in front of a radioactive source emitting beta radiation. Explain
(i) why the readings obtained are not identical, and
(ii) what would happen when 5 mm aluminium sheet is then placed between the counter and the source.
A: (i) It is because radioactive decay is a random process.
(ii) After the insertion of the sheet, the reading has a significant drop. It is because the 5 mm aluminium stops (or absorbs) the
beta radiation.
Q: What is the physical meaning of decay constant? Explain briefly.
A: Decay constant is constant probability of decay per unit time. (Since k  
P.5
dN / N
and dN/N is the probability of decay.)
dt
Q: What conditions are necessary for a uranium fission reaction to occur.
A: Two conditions are necessary.
(1) The mass of the uranium must be greater than the critical mass.
(2) The concentration of uranium-235 must be high enough so that a chain reaction can sustain.
Q: Explain why it is good to use fusion as a source of energy.
A: (1) The raw material is hydrogen or deuterium which can be obtained by electrolysis of water, which is cheap and in plentiful
supply.
(2) The end products of fusion reactions are helium which is inert and non-radioactive. So the problem of waste disposal does
not arise.
Q: Why is temperature as high as 107to 108 K necessary for fusion reaction to take place?
A: Fusion requires two positively charged nuclei come very closely, thus the nuclei must have extremely high kinetic energy to
overcome the mutual electrostatic repulsion. To attain this, a temperature of about 107 to108 K is necessary.
ASTRONOMY AND SPACE SCIENCE
Q: Describe the motion of the celestial sphere and the motion of the stars including the Sun and the moon.
A: All the stars (except the Sun and the solar planets which are so close to the Earth) are attached and fixed on the celestial sphere.
The stars move as the celestial sphere was rotating westwards (from east to west) daily around the fixed Earth at the centre
(opposite to the Earth’s actual rotation).
Q: Describe the motion of the stars if you stargazein towards northern horizon in northern hemisphere. What happens if you
are in southern horizon and view towards southern horizon?
A: All stars in northern hemisphere rotate about the north celestial pole in anticlockwise direction (view towards northern horizon)
while all stars in southern hemisphere rotate about the south celestial pole in clockwise direction (view towards southern
horizon).
Q: Explain briefly the motion of the stars if you are at (a) the north pole, (b) at a place of mid-latitude in northern hemisphere
and (c) Earth’s equator.
A: At the north pole
Here, the latitude is 90o. The north celestial pole coincides with the zenith. The celestial equator coincides with the horizon.
Due to the daily motion, stars are seen to move around the zenith (i.e. the north celestial pole (NCP)) and move horizontally.
Stars never rise or set, so an observer at the north pole can see all the stars in the northern celestial hemisphere at any time but
can never see any stars in the southern celestial sphere.
At a place of mid-latitude
The north celestial pole is now above the northern horizon. Circumpolar stars can be seen at any time. Some stars are not close
to the NCP and so they rise in the east and set in the west and rotate about the NCP in anticlockwise direction. Some stars are
so close to the south celestial pole and never rise above the horizon and can never be seen.
At Earth’s equator:
If you are at equator (latitude = 0o), the NCP coincides with the north point, and the celestial equator passes through the zenith.
Since the celestial sphere rotates about an axis passing through the north point and south point, all celestial bodies rise
vertically and set vertically daily.
Q: What is meant by retrograde motion?
A: Planets normally move eastward near the ecliptic, but sometimes they stop and move westward for a few months. The reverse
motion is called retrograde motion.
Q: Describe briefly the positions of stars (including the Sun and the planets) in the Ptolemaic model and his assumptions.
A: In the Ptolemaic model, the Earth is the centre of the universe (a geocentric model). The Sun, the Moon and the then five
known planets are placed at different distances from the Earth. The assumptions are
(1) A planet moves in epicycle, which in turn revolves around the Earth in deferent.
(2) The centres of the epicycles of Mercury and Venus are fixed on a line joining the Sun and the Earth.
Q: What are the discoveries of Galileo using his handmade telescope? Which is the most convincing for supporting the
Copernican model?
A: Galileo discovered that (1) there are hilly terrains on the Moon, (2) there are black spots (sunspots) on the Sun’s surface, (3)
there are four satellites orbiting the Jupiter and (4) Venus shows a complete cycle of phases, like that of the Moon. Among the
discoveries, the complete cycle of phases of Venus can only be explained by the Copernican model. In the Ptolemaic model,
since the centre of the epicycle of Venus was fixed on the line joining the Sun and the Earth, Venus would always appear as a
crescent as seen from the Earth. However, in the Copernican model, the Venus and the Earth are both revolving about the Sun,
with Venus closer to the Sun, the positions of Venus relative to the Earth are constantly changing, an observer on the Earth thus
sees different proportion of the Venus surface lit by sunlight. Venus then shows a complete cycle of phase.
Q: What is the difference between apparent magnitude and the absolute magnitude of a star?
A: Apparent magnitude is used to describe the apparent brightness of a celestial body as seen from the Earth. It depends on the
luminosity of the star and its distance from the Earth.
Absolute magnitude is defined as the apparent magnitude that a celestial body would have if it were at a distance of 10 pc
away from the Earth. It depends only on the amount of light given out by the celestial bodies and does not depend on distance.
P.6
Q: What is meant by 1 parsec?
A: 1 parsec (1 pc) is the distance at which a length of 1 AU subtends an angle of 1 arcsecond. Alternatively,
1 parsec is the distance at which a star has a parallax of 1 arcsecond.
Q: How do we know the existence of dark matter?
A: If most of the mass of the galaxy is concentrated at the centre of the galaxy, Newton’s laws predict that the rotational speed v
of the stars will decrease with distance r from the centre as v  GM / r  1 / r . But the measured rotational speed does
not decrease with r. This implies that a large amount of ‘invisible mass’ is lying in the galaxy.
Q: How do will know that the universe is expanding?
A: The spectra of distant galaxies are all red-shifted (the wavelength is shifted to longer wavelength). This indicates that all
distant galaxies are receding from each other, and so the universe is expanding.
ENERGY
Q: Explain briefly the working principle of a gas discharge lamp such as a fluorescent lamp.
A: Gas discharge lamp ionizes gas by electron emission. The ions and electrons are attracted to the electrodes and collided with
mercury atoms, causing the electron to make an upward transition. After returning to ground state, UV photons are emitted. The
phosphor coating absorbs the UV radiation and produces visible light.
Q: Explain the working principle of an induction cooker.
A: An induction cooker sets up a fast changing magnetic field which induces eddy currents in the metal cooking pot. The eddy
currents produce heating effect.
Q: Suggest ways to reduce the OTTV of the building.
A: The ways include: add low-e coating to the windows, install shading fins, install double-glazed windows, use solar control
window films, use reflective glass or tinted glass
Q: State two advantages and two disadvantages of electric vehicles over internal combustion engine vehicles.
A: Advantages:
1) Electric vehicles has a high end-use energy efficiency as energy is saved because electric vehicle makes use of
regenerative braking system that converts the kinetic energy of the car to electrical energy during braking. This energy is
stored in battery.
2) Produce less air pollutants
Disadvantages:
1) Higher initial cost
2) The travelling distance is shorter because of the limited capacity of batteries.
3) It takes a long time to fully charge an electric vehicle.
4) Charging stations are not enough.
Q: List some advantages of hybrid vehicles
A: Hybrid vehicles have a high end-use energy efficiency due to the regenerative braking system.
When the vehicle is stationary, the combustion engine is turned off to save energy.
Q: Why would the maximum wind power generated by wind turbine not be reached in practice?
A: When calculating the maximum wind power, it is assumed that the wind stops after passing through the blades and transfers all
the kinetic energy to the turbine. This is not practical in real situations.
Q: List some impacts of the extraction and conversion of nuclear power on environment and society.
A: Uranium is extracted from mines. The waste generated from mining may contaminate groundwater and surface water
resources.
Accidents may occur in nuclear reactors leading to leakage of radiation.
Radioactive wastes are highly radioactive. The surrounding has to be vacant. This prevents the land from other uses.
Q: What are the functions of moderator and control rods in nuclear reactor?
A: Pressurized water is used as moderator to slow down the neutrons so as to increase the time of stay of neutrons in the reactor to
increase the change of fission reaction (by hitting uranium-235 atoms). If moderator were not used, the chain reaction would stop.
Control rods are made of boron steel that absorbs neutrons. Raising and lowering the control rods vary the number of neutrons,
thereby controlling the rate of reaction. In case of emergency, the control rods are completely lowered to stop the fission reaction.
Q: State the working principle of hydroelectric power and discuss the energy changes in the process.
A: When the turbine passage opens, water from the upper reservoir flows through the dam at a high speed. Some gravitational PE
of the water is converted to KE of water. The turbine is turned by the falling and fast-moving water. The turbine in turn spins the
shafts of the generator. The generator produces electricity as the turbines rotate. Some kinetic energy of water is converted to
electrical energy.
Q: State two functions of pressurized water in pressurized water reactor.
A: Pressurized water serves two functions:
1) It acts as a moderator to slow down neutrons.
2) It acts as a coolant to absorb the heat from the reactor core and transfer the energy to the steam generator.
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