Physics Most Common Questions

As the trolley moves across the rough surface, it slows down and stops. Explain, in terms of the work done, the energy change that takes place as the trolley slows down.

work done against / due to / because of friction or kinetic energy (of trolley) used to do work

kinetic energy decreases (to zero)

thermal energy produced

Why a bus begins to topple?

line of action of centre of mass falls outside the base.

State the principle of the conservation of momentum.

no external forces OR isolated system

sum of momenta / (total) momentum remains constant

State the principle of conservation of energy.

In a closed/ isolated system, total energy of the system is conserved/ constant. initial energy = final energy

What is centre of mass?

(the point) where (all) the mass can be considered to be concentrated

State two properties of an object that may be changed by the action of forces.

shape

size / volume / length / density / any linear dimension

direction (of motion) / speed / velocity / momentum / kinetic energy / acceleration

State the two conditions which must be true for an object to be in equilibrium.

no resultant force OR forces are balanced OR all forces in opposite directions are equal OR forces cancel

no resultant {moment / torque / turning effect} OR (sum of) clockwise moment(s) = (sum of) anticlockwise moment(s)

State Hooke’s law

extension is directly proportional to load (if elastic limit is not exceeded)

Explain what is meant by the term limit of proportionality of the spring.

up to the limit of proportionality, extension proportional to load

What type of energy is stored in springs?

Strain/ elastic potential energy.

Describe and explain the motion of an object released from top of an fluid, from when it is released until it reaches terminal velocity.

(initial) acceleration caused by weight / force of gravity

drag / resistance force increases (with speed)

acceleration decreases as resistance force increase

when weight(downward force) = resistance(upward force), (finally / at terminal velocity) no acceleration / constant speed

(finally / at terminal velocity) no resultant force

Explain why opening the parachute cannot reduce the speed of the skydiver to zero.

at zero speed there is no air resistance

weight / downwards force is (still) acting or there is (now) a resultant force (downwards at zero speed)
OR

forces balance at a speed greater than zero

speed cannot decrease / no deceleration once forces balance

Why not 100% of GPE is converted to KE when an object falls?

(work done against) friction / drag / resistance or thermal energy generated

Describe the motion of molecules of the gas.

Molecules move randomly/ in random directions Molecules have high speed
Molecules collide with each other/ with walls. Molecules change direction in collision Molecules move in straight line between collision Molecules collide with the walls (of cylinder)

Describe the motion of molecules of the solid.

molecules vibrate

State two ways in which the molecular structure of liquid is different from the molecular structure of solid.

Liquid molecules not in fixed position
Liquid molecules have random/irregular arrangement Liquid molecules are (slightly) further apart

State two ways in which the molecular structure of solids is different from the molecular structure of liquids.

in solid, particles are arranged in regular lattice and just vibrate

in liquid, particles are arranged randomly, slide past each other, move.

Describe, in terms of molecules and their motion, how a liquid differs from a gas.

molecules close(r)
molecules move only small distances (between collisions) or do not move freely molecules move in clusters or slide past each other

State two ways in which the molecular structure of gas is different from the molecular structure of liquid.

(gas) molecules further apart greater PE
move singly/ in straight lines faster

Why solids have fixed shapes? Explain in terms of forces between molecules and arrangement of molecules.

Solids – molecules in lattice arrangement

solids – strong forces between molecules

Why liquids adapt to the shape of their container? Explain in terms of forces between molecules and arrangement of molecules.

liquids – molecules not fixed in place OR molecules have an irregular arrangement OR molecules (slightly) further apart (on average) than in solids OR spaces between the molecules

liquids – (average) forces too weak to keep molecules in a definite pattern OR forces just enough to hold molecules in the bulk of the liquid

Why gas fills their container? Explain in terms of forces between molecules and arrangement of molecules.

gases – molecules far apart

gases – weak / no forces between molecules (except during collisions)

Explain the melting of a solid in terms of molecules and energy.

molecules must be separated OR (intermolecular) bonds must be broken / overcome

work done (against bonds) OR energy is required / needed NOT increase of KE / speed

Explain, in terms of forces between molecules, why gases expand more than liquids when they have the same rise in temperature. Assume that the pressure remains constant.

average distance between molecules greater (in gas) (attractive) forces between molecules lower or zero in gas

Explain, in terms of the momentum of the molecules, how gas molecules exert pressure.

molecules collide with walls
momentum (of a molecule) changes / reverses

force is exerted because force is rate of change of momentum or force is change in momentum per unit time (or per second) or F = mu - mv /t.

force spread out over area / surface. p = F / A

The gas bubble rises to the surface. The volume of the gas bubble increases as it rises higher in the water. Explain why the volume of the bubble increases.

pressure in a liquid increases with depth OR pressure decreases (as bubble rises)

pressure (of gas) is inversely proportional to volume OR internal pressure greater than external pressure (momentarily) OR (air) molecules do not have to hit surface of bubble as frequently (to stop the bubble collapsing) OR the bubble is not as strongly compressed

Explain why the volume of the air in the balloon decreases as the temperature decreases.

molecules move more slowly / have less kinetic energy

pressure (inside balloon) decreases or pressure is directly proportional to temperature or p T

volume is directly proportional to temperature or V T molecular collisions less frequent
molecular collisions less violent / hard / exert smaller impulse water / external pressure compresses balloon or

water pressure greater (and balloon compressed)

Explain what happens to the pressure of a constant volume of air when the temperature of the air increases. Use ideas of momentum of molecules in your explanation.

(temperature of air increases) so molecules move faster / their KE increases molecules collide with walls of container and change momentum

greater change of momentum when temperature is higher OR collisions more frequent OR harder collisions OR force = rate of change of momentum

(higher force and hence) higher pressure

What happens to pressure if the volume decreases

pressure decrease
density decrease/ fewer atoms per unit volume
rate of collisions decrease/ fewer collisions per unit volume

Explain, in terms of the behaviour of the molecules, why the pressure has increase.

(molecules) collide with walls more often

OR more collisions with walls per second or per unit time OR greater force per unit area

What is a moment of a force

What is meant by the moment of a force about a pivot. (moment of a force) is the turning effect (about a point / pivot)

Explain, in terms of the atomic lattice and electrons, how thermal energy is transferred through the aluminium.

atoms (touching the hotplate) / lattice vibrate (faster) more

atoms pass on energy / vibration to neighbouring atoms / to other atoms by collision

atoms pass on energy to electrons

electrons hit distant atoms or electrons move (through lattice)

Describe, in terms of molecules, how thermal energy is transferred from the heated sand through the base of the plastic pot.

(thermal energy is transferred by) conduction

sand molecules gain KE OR vibrate more OR hit (other) molecules (when heated)
OR

Energy is transferred to molecules of plastic pot in contact with sand (through collisions)
OR

Energy OR (lattice) vibrations transferred to neighbouring molecules

Explain how electrons transfer thermal energy through the metal of the can

atoms / ions vibrate
(vibrating) atoms / ions hit electrons

electrons propelled / travelling through metal lattice/ moving through metal lattice

electrons hit (distant) atoms and transfer energy to distant atoms free electrons / delocalised electrons mentioned

Explain, in terms of the water molecules, what happens to the thermal energy supplied to the water as it boils.

energy used to breaks bonds. molecules escape from the liquid (as a vapour) (thermal energy) does work against intermolecular forces
molecules separated/ moved apart and gain potential energy.

Eventually, the water reaches boiling point. Thermal energy from the hotplate is still being transferred to the water. Explain, in terms of molecules, the effect of this thermal energy on the water.

molecules escape from the liquid (as a vapour) bonds broken / (attractive) forces overcome

molecules gain potential energy or work done (to separate molecules / break bonds / overcome forces)

Describe what happens to molecules of the liquid as its temperature begins to increase.

molecules (in liquid) move faster or gain energy

molecules move apart (on average)

molecules (in liquid) move faster or gain energy

molecules move apart (on average)

molecules (already very) close / touching

(repulsive) forces (very) large

Explain, in terms of molecules, why a liquid expands when heated.

molecules gain kinetic energy and move faster

molecules push others away and move further apart

As temperature increases why does ice start to melt

k.e. of molecules / (thermal) energy absorbed (from water / surroundings) or energy absorbed from (cooling) water

supplies latent heat or energy used to overcome intermolecular forces / to break bonds

When the temperature of gases increased explain, in terms of molecules, what happens to the pressure of gas

(Pressure of the gas) increases
Molecules gain KE and move faster
Molecules collide with/ hit walls more often/ frequently
More collisions with wall per unit time (e.g. per second)
Molecules collide with/ hit walls with greater force/ impulse/ harder.

When the temperature of gases decreased explain, in terms of molecules, what happens to the pressure of gas

molecules lose KE and move more slowly
fewer collisions per unit time OR less hard collisions with walls/ balloon lower pressure

OR BETTER:

(Pressure of the gas) decrease
Molecules lose KE and move slowly
Molecules collide with/ hit walls less often/ frequently
Fewer collisions with wall per unit time (e.g. per second) Molecules collide with/ hit walls with less force/ impulse/ harder.

When volume is decreased explain, in terms of the behaviour of molecules, what happens to the pressure of gas.

(Pressure of the gas) increases. Molecules close together

Molecules collide with walls more often/ frequently OR more collisions with walls per second or per unit time.

Greater force per unit area.

When volume is increase explain, in terms of the behaviour of molecules, what happens to the pressure of gas.

lower pressure
fewer collision with wall/ balloon per second or per unit time.
larger surface area of walls OR atoms further apart OR atoms travel further

OR BETTER:

(Pressure of the gas) decrease. Molecules further apart

Molecules collide with walls less often/ frequently OR fewer collisions with walls per second or per unit time.

Greater force per unit area.

Explain, in terms of molecules, why gases are easiest to compress.

More room for molecules OR molecules fit into gaps OR there are gaps between molecules.

No/weak repulsive forces between molecules OR (repulsive) forces between molecules smaller OR pressure on walls, smaller OR only small pores/ pressure required.

OR BETTER:

Gas molecules are further apart so there is larger intermolecular space between molecules.

No/ weak repulsive forces between molecules.

Describe and explain the motion of smoke particles under microscope and account for this motion in terms of the air molecules.

Smoke particles show random/ haphazard/ unpredictable/ movement Smoke particles show sudden change of direction/ zig-zag motion
Smoke particles appear/ disappear from view OR go out of/ come into focus.

IF THEY SAY TO ONLY EXPLAIN THE CAUSE OF THE MOTION OF SMOKE PARTICLE LEAVE THE FIRST THREE POINTS FROM ABOVE.

Air molecules collide with smoke particles from all direction/ unevenly cause random motion of particles

Air molecules move randomly.
Air molecules faster
Air molecules smaller lighter/ have smaller mass

State and explain how the size of attractive forces acting between the molecules of gas compares with the size of attractive forces between the molecules of a solid.

(Attractive) forces (much) smaller between gas molecules

Gas molecules (much) further apart.

State and explain, in terms of the forces between the molecules, how the compressibility of a gas differs from that of a liquid.

gases compressible OR liquids incompressible

no/ weaker repulsive forces between molecules OR vice versa for liquid molecules.

Explain in terms of molecules why it is possible to compress gas, but not liquid.

Gas molecules (very) far apart from OR empty space between gas molecules. Molecules of liquid (very) close together compact OR are touching (each other).

Explain, in terms of molecules, why a liquid expands more than a solid when heated.

forces between liquid molecules weak(er than in solids)

less energy / work done to separate molecules or greater separation for same work done / same increase in energy

State ways in which evaporation differs from boiling.

at surface/ not within liquid

at any temperature/ not at boiling point (evaporation) causes cooling
boiling requires a heat source
bubbles rising

State ways in which boiling differs from evaporation.

boiling throughout liquid (evaporation at surface)

boiling at one temperature (evaporation at any/ all temperature/ below boiling point).

boiling produces bubbles (evaporation does not). boiling not affected by draught/ area (evaporation is)

Explain, in terms of the behaviour of molecules, the process of evaporation.

some molecules at the surface gain more KE, so have more energy than others

heat energy used to work done against/ overcome attractive forces between molecules.

most energetic molecules escape from the surface of the liquid

Explain, in terms of the behaviour of molecules, how evaporation process helps the body to cool down.

evaporation OR heat/ (thermal) energy needed for evaporation leaves sweat cooler

faster molecules/ higher energy molecules escape

heat flows from body to warm the sweat (so body cools)

Explain, in terms of molecules, how sweating helps to cool your body on a hot day

(thermal) energy in the skin / body transferred to (molecules of) sweat

These molecules (have enough KE to) escape from the skin / become water vapour

Leaving behind molecules with lower energy

Which leaves the skin / body at a lower temperature

Explain, in terms of molecules, why temperature decreases during evaporation.

Molecules do work against attractive forces as they evaporate.
Faster/ more energetic water molecules (more likely to) escape from surface Slower/ less energetic molecules remain (so temperature is lower).
Average energy of remaining molecules decrease.

In terms of molecules, state and explain how the rate of evaporation of the puddles in is affected by a reduction of wind speed.

Reduces (rate of evaporation)

No/ fewer evaporated molecules blown away/ removed by wind, OR greater humidity/ vapour pressure.

In terms of molecules, state and explain how the rate of evaporation of the puddles in is affected by an increase in water temperature.

Increases (rate of evaporation)

Molecules move faster/ have more energy OR molecules have energy to escape.

Which container has faster rate of cooling wide and shallow OR narrow and deep.

Water in wider container AND has water with larger surface (area).
Rate of evaporation higher/ faster/ quicker OR higher chance of evaporation.

How to reduce evaporation

Cover/ decrease surface area
Reduce temperature
Reduce draught(a current of cool air in a room or other confined space) Increase humidity of air.

How can the rate of evaporation be increased

Increase temperature/ supply more heat/ make hotter

Blow air across surface. Reduce humidity Decrease pressure.

How to increase the range of a thermometer.

capillary tube longer or liquid with lower expansivity or smaller bulb with less volume or increase diameter of tube.

What happen to range if smaller bulb is used, with smaller volume of liquid?

(range) increases and less expansion/increase in volume (of mercury per unit temperature rise)

How to increase the sensitivity of a thermometer.

capillary tube thinner/finer or liquid with higher expansivity or larger bulb with more volume of liquid or smaller bulb with same volume of liquid.

What is a linear scale

same distance moved (by thread) for same temperature change

State and explain the two features of a liquid-in-glass thermometer that are necessary for linearity

statement 1: bore of constant (cross sectional) area
explanation: idea of same movement / change in length of liquid / thread AND
for same increase in volume / expansion (of liquid)

statement: (liquid has) constant thermal expansion
explanation: liquid moves same distance for each °C temperature rise

What is a range

difference between the maximum temperature and minimum temperature it can measure

What is sensitivity

distance moved by the thread per °C OR per unit temperature change

If a larger bulb is used, with more of the same liquid what happens to its sensitivity.

greater sensitivity

volume increase (of liquid in second thermometer) is greater or liquid moves a greater distance (for the same temperature increase)

If a larger bulb is used, with more of the same liquid what happens to its range.

smaller range and either of:
• smaller temperature increase for liquid / meniscus to reach end of tube • expands more / greater sensitivity and tube of same length

Suggest how to reduce the loss of thermal energy from the surface of a block.

lagging / insulation (around block) OR insulate (the block)

raise temperature of block by a smaller amount OR heat for a shorter time OR use lower power heater for same time OR higher power for same temperature rise / shorter time

polish the surface of the block OR wrap the block in shiny material OR paint (shiny) white

reduce initial temperature of block (to below room temperature) OR raise temperature of room

reduce draughts

When temperature increase, explain in terms of molecules, why does the internal energy increase.

kinetic energy (and potential energy) of molecules increase (hence internal energy increases)

State, in terms of molecules, what is meant by an increase in internal energy.

increase in kinetic energy of molecules or increase in potential energy of molecules

What are the advantages of solar cells.

no polluting gases / quiet / low maintenance / can be placed on roofs / clean / cheap to run

What are the disadvantages of solar cells.

intermittent supply / unattractive / takes up space / uses land / d.c. output

What are the disadvantages of hydroelectric power schemes

damage to habitats (for fish) / construction is expensive / droughts / flood risk if dam bursts

Use of infrared radiation

remote controls

(infrared) sensors / alarms specific electrical appliances thermal imaging

State two environmental benefits of the electrically powered bicycle.

less noise OR no noise

less OR no air / gaseous pollution (from the bicycle) OR does not produce acid rain

(the bicycle) uses no / less fossil fuel

does not contribute to greenhouse effect OR does not release CO2

What are the consequences of burning coal?

air pollution / harmful gases / acid rain
CO2 / greenhouse gases / contribution to global warming not renewable

damage from mining / drilling or any valid environmental consequence of transport of coal

ound waves from a television are diffracted through doorways. Light waves from a television are not diffracted through doorways. Suggest why light waves and sound waves behave differently in this situation.

wavelength of light is (much) smaller than width of doorway or wavelength of sound

wavelength of sound is similar to width of doorway OR λ width of gap for diffraction to occur OR larger wavelength results in greater diffraction ORA

How compressions differ from rarefactions.

compressions are regions of high pressure
in compressions particles are close together than normal.

What is rarefaction.

region of low pressure where molecules/ particles are further apart.

Why sound waves are described as longitudinal.

particles vibrate/ oscillate OR compressions/ rarefactions move. in the direction of travel of the wave.

What is a transverse wave

particles vibrate at right angle/ perpendicular to wave direction.

State how longitudinal waves differ from transverse wave.

(longitudinal wave) vibration direction parallel to propagation / energy travel direction

transverse wave vibration direction perpendicular to propagation / energy travel direction

consists of rarefactions AND compressions

State how transverse waves differ from longitudinal wave.

particles (in transverse waves) vibrate perpendicular to the direction of travel (of the wave) OR
particles in longitudinal waves vibrate parallel to the direction of travel of the wave

longitudinal waves have compressions and rarefactions transverse waves have troughs and crests

Use of Optical fibres

Fibre passing to site to be examined / treated. Light travels down optic fibres by multiple total internal reflections at the inner surface in to the body.
endoscopy
use to view internal organs using cameras.
light encoded
optic fibre transmits light. Light travels in fiber.
total internal reflection at inner surface or within fibre.

What is total internal reflection

Reflection in a more optically dense material where there is no refracted ray OR all light in a more dense material is reflected./ incident ray in more dense material

angle of incidence greater than critical angle no light refracted
all the light is reflected
reflected with i = r

What is critical angle

The greatest angle of incidence at which refraction can occur

The angle of incidence at which the refracted ray travels along the boundary/ angle of refraction is 90.

The angle of incidence/ above which total internal reflection occurs.

What is principal focus?

Point on the principal axis OR use of term paraxial OR centre line

(point) where incident parallel rays meet after passing through lens OR origin of rays that emerge parallel after passing through lens

Explain, in terms of the behaviour of light rays, what is meant by principal focus for a thin converging lens.

(point) where (parallel) rays (of light) meet (after passing through lens)

point) where parallel rays (of light) meet / are focussed (after passing through lens) or (point) through which rays (of light) that emerge parallel pass (before reaching lens)Explain, in terms of the wavefronts, why the light changes direction as it enters the solid.

Explain, in terms of the wavefronts, why the light changes direction as it enters the solid.

idea of one side of wavefront enters / hits solid first OR wavefront does not all hit the solid all at once;

idea of this side slowed down first OR this side delayed relative to other side

angle of wave(front) changes OR different parts of wavefront delayed by different amounts

What is focal length

distance between principal focus / focal point and optical centre / lens

What is a virtual image

rays do not meet at image / only appear to come from image / do not originate from image / cannot be seen on a screen / light does not pass through the image

Why is an image real

light passes through it OR can be projected/seen on a screen OR refracted rays cross/meet

Describe one medical use of X‐rays.

Treating cancer → absorbed by tumor → tumor killed.
X-ray shadowgraph → absorbed by bones → photograph is produced.

State one reason why it is necessary to take safety precautions when X‐rays are used.

can cause burns / (cell) mutation / cell damage / tumours / cancer / damages DNA etc.

State two conditions required for light to be totally internally reflected.

light (must pass) from medium where it travels slower or to medium where it is faster or from medium with larger refractive index or to medium with smaller refractive index

angle of incidence (must be) greater than the critical angle / sin–1(1 ÷ n)

What is meant by digital?

consists of signal with two possible states - high and low state / voltage

Describe the difference between digital and analogue signals?

digital signal: consists of signal with 2 possible states - high and low states / voltages

analogue signal: consist of signal with continuously varying voltage so can take any value