Waves, Electricity & Matter

A ripple spreads across the surface of a pond. In this transverse wave, how do the water particles move compared with the direction the wave travels?

• AThey travel outward with the wave and do not return to their starting position
• BThey vibrate back and forth, along the same line as the direction the wave travels
• CThey stay completely still while only the wave's energy moves forward across the pond
• DThey vibrate up and down, at right angles to the direction the wave travels✓

Answer: Transverse waves are waves in which the particles vibrate at right angles to the direction the wave travels, and water waves are a good everyday example. In a longitudinal wave, such as sound, particles instead vibrate back and forth along the same line as the wave. The water particles themselves do not travel away with the wave; only the wave's energy moves outward while the particles return to roughly their starting height.

Sound travels through air as a longitudinal wave. Which statement correctly describes the particle vibration in this type of wave?

• AThe particles remain fixed in place while sound energy passes straight through them
• BThe particles move in circles around a fixed central point as the wave passes
• CThe particles vibrate up and down at right angles to the direction of travel
• DThe particles vibrate back and forth in the same direction as the wave travels✓

Answer: Longitudinal waves are waves in which the particles vibrate back and forth in the same direction the wave travels, and sound is the clearest everyday example. This is different from a transverse wave such as a water wave, where particles vibrate at right angles to the direction of travel instead. The particles are not fixed in place and do not move in circles; they oscillate along the line of travel as the sound passes through.

An astronaut on the surface of the Moon cannot hear a hammer striking a rock nearby, even though they can see it clearly. Why not?

• ASound waves need a medium such as air to travel through, and the Moon has no atmosphere✓
• BLight waves travel faster than sound waves, so the sound has not yet arrived
• CThe astronaut's helmet blocks all vibrations before they can reach the ears
• DSound cannot be produced at all in the very low gravity found on the Moon

Answer: Sound waves are vibrations that need a medium, such as air, water or a solid, to travel through, because they rely on particles bumping into their neighbours. The Moon has no atmosphere, so there are no particles to carry the vibration from the hammer to the astronaut's ears. Light does not need a medium and reaches the astronaut's eyes easily, which is why the impact can be seen but not heard.

A tuning fork vibrates 440 times every second when struck. What is its frequency, and in which unit is frequency measured?

• A440 watts, because in physics frequency is measured using the watt as its standard unit
• B440 newtons, because in physics frequency is measured using the newton as its standard unit
• C440 hertz, because frequency is measured in hertz (Hz), the number of vibrations per second✓
• D440 metres, because in physics frequency is measured using the metre as its standard unit

Answer: Frequency is the number of vibrations, or complete waves, produced each second, and it is measured in hertz (Hz). A tuning fork vibrating 440 times a second therefore has a frequency of 440 Hz. Newtons measure force, metres measure distance, and watts measure power, so none of these units describe how often something vibrates.

A hiker shouts towards a distant cliff face and hears their own voice repeated a moment later. What causes this effect?

• AThe sound wave refracts around the cliff face and arrives twice from different angles
• BThe hiker's own voice is amplified by vibrations in the surrounding rock and air
• CThe sound wave reflects off the cliff face and travels back to the hiker's ears✓
• DThe cliff face absorbs the sound and then re-emits it as a new, identical wave

Answer: An echo is produced when a sound wave reflects off a hard, flat surface such as a cliff face and travels back to the listener. The reflected wave arrives slightly later than the original sound because it has travelled a longer distance there and back. Sound waves do not refract around large solid obstacles in this way, and a cliff does not absorb and then recreate an identical new sound wave.

Sound generally travels fastest through solids, slower through liquids, and slowest through gases such as air. What explains this pattern?

• ASolids absorb far less sound energy than liquids or gases do as the wave travels
• BParticles are actually packed closest together in gases, so vibrations pass between them much more quickly
• CParticles are packed closest together in solids, so vibrations pass between them more quickly✓
• DGases contain more empty space for the sound wave itself to travel through freely

Answer: Sound travels fastest through solids, more slowly through liquids, and slowest through gases such as air, because particles closer together pass vibrations to their neighbours more quickly. Gas particles are spread far apart with lots of empty space between them, which slows the transfer of vibration down rather than speeding it up. Solids do not simply absorb less sound energy than gases; the difference in speed comes from how tightly the particles are packed.

Unlike sound, light can travel through the vacuum of space, which is how sunlight reaches Earth. What does this tell us about light?

• ALight needs an extremely thin medium that even a vacuum still contains
• BLight is not really a wave at all, unlike sound, which is a wave
• CLight travels only through solids and cannot pass through liquids or gases
• DLight does not need a medium such as air or water in order to travel✓

Answer: Light is an electromagnetic wave and, unlike sound, does not need a medium such as air or water to travel through. This is why sunlight can cross the empty vacuum of space to reach the Earth. Light is still a wave, and it travels through solids, liquids and gases as well as through a vacuum, so it is not restricted to only some of these.

Light travels at about 300,000 kilometres per second. How long does light take to travel a distance of 900,000 kilometres?

• A300 seconds
• B9 seconds
• C0.3 seconds
• D3 seconds✓

Answer: Light travels at about 300,000 kilometres per second, one of the fastest speeds known. To find the time taken for a distance, the distance is divided by the speed, so 900,000 km divided by 300,000 km per second gives 3 seconds. The other options do not match this calculation and would imply a much faster or much slower speed for light.

A pinhole camera forms an image of a candle flame on a screen inside a box. Why does the image appear upside down?

• AThe pinhole itself flips the image the same way a mirror reverses left and right
• BThe screen material absorbs the upright part of the image and only shows the reflection
• CLight travels in straight lines, so rays from the top of the flame cross over and land at the bottom of the image✓
• DLight bends sharply as it passes through the tiny pinhole, inverting the whole image

Answer: Light travels in straight lines, so rays from the top of an object pass through the pinhole and land near the bottom of the screen, while rays from the bottom land near the top, flipping the image upside down. The pinhole itself does not act like a mirror, and it does not bend light sharply the way a lens would. The screen material simply displays the pattern of light that reaches it rather than absorbing part of the image.

The lens at the front of the human eye is convex in shape. What is its main function?

• AIt absorbs most colours of light and only allows white light through to the retina
• BIt reflects light rays back out of the eye to prevent damage to the retina
• CIt refracts and focuses light rays so they form a clear image on the retina✓
• DIt amplifies the light signal before converting it directly into a nerve impulse

Answer: The convex lens at the front of the eye refracts light rays and bends them so that they converge to form a clear, focused image on the retina at the back of the eye. The lens does not reflect light back out of the eye, and it does not selectively absorb colours or amplify the light signal itself. Turning light into a nerve impulse is instead the job of light-sensitive cells on the retina.

A smooth mirror produces a clear, sharp reflection, while a sheet of white paper reflects light but produces no clear image. What is the difference?

• AThe mirror gives specular reflection from a smooth surface, while the paper gives diffuse scattering from a rough surface✓
• BThe mirror absorbs almost all of the light that reaches it, while the paper reflects nearly all of the light back out again
• CThe mirror gives diffuse scattering from its smooth surface, while the paper instead gives specular reflection from a rough surface
• DThe mirror transmits nearly all of the light straight through its surface, while the paper reflects that light straight back outward

Answer: A smooth surface such as a mirror produces specular reflection, where parallel light rays stay parallel after reflecting and form a clear image. A rough surface such as paper produces diffuse scattering, where the rays reflect at many different angles and no clear image forms. Neither surface absorbs almost all the light or transmits it straight through, since both are reflecting most of the light that reaches them.

When white light passes through a glass prism, it spreads out into a spectrum of colours. What causes this to happen?

• AThe glass absorbs every colour of light except for the ones that appear in the final spectrum produced
• BEach colour of light has a different frequency and is refracted by a slightly different amount✓
• CThe prism itself adds colour to the light, since white light on its own contains no colour at all
• DEach colour of light actually has the same frequency, but each one reflects off the glass by a different amount

Answer: White light is made up of many colours, each with a slightly different frequency, and a glass prism refracts each colour by a slightly different amount as it enters and leaves the glass. This spreads the colours out into a visible spectrum. The prism does not add colour to the light or remove any colours by absorption; it simply separates the colours that were already present in the white light.

Electric current is a flow of charge around a circuit. In which unit is current measured?

• AVolts (V)
• BAmperes (A)✓
• COhms (Ω)
• DWatts (W)

Answer: Electric current is a flow of electric charge around a circuit, and it is measured in amperes (amps). Volts measure potential difference, ohms measure resistance, and watts measure power, so none of these describe the flow of charge itself. A higher current means more charge is flowing past a point in the circuit each second.

Three identical bulbs are connected in series in a single loop with a battery. How does the current compare at different points around this loop?

• AThe current splits evenly between the three bulbs, and then it recombines again after passing the last one
• BThe current is smallest close to the battery and then increases again after passing each bulb in turn
• CThe current is largest close to the battery and then gradually falls as it reaches each bulb in turn
• DThe current is the same at every point in the loop, since there is only one path for charge to flow✓

Answer: In a series circuit there is only one loop for charge to flow around, so the current is the same at every point in that loop, including before and after each bulb. The current is not used up as it passes through a bulb, so it cannot be larger near the battery and smaller further away, or the reverse. Current also does not split between components that are connected in series rather than in parallel.

Two bulbs are connected in parallel branches across the same battery. How does the current from the battery relate to the current in each branch?

• AThe current from the battery is used up in the first branch it reaches
• BThe current from the battery flows into one branch at a time, alternating between them
• CThe current from the battery splits between the branches, so each branch usually carries less current than the total✓
• DThe current from the battery stays whole in each branch, since parallel branches do not share charge

Answer: In a parallel circuit, the current from the battery divides between the separate branches, so each branch usually carries only part of the total current, and the currents recombine after the branches meet again. The current does not stay whole in every branch, since that would mean more charge leaving the battery than entering it. It also does not alternate between branches or get used up in the first branch.

The potential difference across a battery drives current around a circuit. In which unit is potential difference measured?

• AOhms
• BVolts✓
• CJoules
• DAmperes

Answer: Potential difference is the driving force that pushes current around a circuit, and it is measured in volts. Amperes measure current, ohms measure resistance, and joules measure energy, so none of these are the unit for potential difference. A battery with a higher potential difference generally drives a larger current through a given circuit.

A component has a potential difference of 12 volts across it when a current of 4 amperes flows through it. Using resistance = potential difference divided by current, what is its resistance?

• A48 ohms
• B3 ohms✓
• C16 ohms
• D8 ohms

Answer: Resistance can be calculated using resistance = potential difference divided by current. Dividing 12 volts by 4 amperes gives a resistance of 3 ohms. The other values do not match this calculation and would come from adding, multiplying or misreading the numbers instead of dividing them correctly.

A copper wire allows current to flow through it easily, while the plastic coating around it does not. What does this tell us about their resistance?

• ACopper has no resistance at all, while plastic has a fixed, unchanging resistance
• BCopper has a very high resistance, while plastic has a very low resistance
• CCopper and plastic both have exactly the same resistance to current
• DCopper has a very low resistance, while plastic has a very high resistance✓

Answer: Conductors, such as copper, have a very low resistance and allow current to flow through them easily, while insulators, such as plastic, have a very high resistance and block the flow of current. This is why cables use copper wire wrapped in a plastic coating. Neither material has zero resistance or exactly the same resistance as the other, since their properties are very different.

Rubbing a balloon on a woollen jumper makes the balloon stick to a wall afterwards. What causes the balloon to become charged?

• AElectrons are transferred between the balloon and the jumper, leaving the balloon with an overall charge✓
• BProtons are transferred between the balloon and the jumper, which leaves the balloon with an overall charge
• CThe wall simply attracts the balloon directly, without any charge ever being created by the rubbing at all
• DThe rubbing action itself creates brand new electrons that did not exist before the balloon was ever charged

Answer: When two different materials are rubbed together, electrons can transfer from one material to the other, leaving one object with extra negative charge and the other with a matching positive charge. Rubbing the balloon on the jumper transfers electrons onto the balloon, giving it an overall charge that lets it stick to a wall. Protons are held tightly within the nucleus of an atom and are not transferred by rubbing, and no new electrons are created in the process.

Two bar magnets are brought close together. What happens when their north poles face each other, compared with when a north pole faces a south pole?

• ALike poles repel each other, while a north pole and a south pole attract each other✓
• BBoth arrangements always repel, since identical magnets can never attract each other
• CBoth arrangements always attract, since magnets only ever pull objects towards them
• DLike poles attract each other, while a north pole and a south pole repel each other

Answer: Like magnetic poles, such as two north poles, repel each other, while unlike poles, such as a north pole and a south pole, attract each other. This is a basic rule of how magnets interact. Magnets do not always attract or always repel regardless of which poles are facing each other; the outcome depends entirely on which poles are brought together.

An electromagnet used in a scrapyard crane can pick up scrap metal and then release it on command. How is this possible?

• AThe magnetic field is instead produced by the metal itself, so it switches itself on and off automatically
• BThe magnetic field is instead produced by friction between the crane and the metal being lifted at the time
• CThe crane instead uses a permanent magnet that is physically rotated in order to turn the field on and off
• DThe magnetic field is produced by an electric current, so it can be switched on and off with the current✓

Answer: An electromagnet produces its magnetic field using an electric current flowing through a coil of wire, so the field can be switched on by turning the current on and switched off by turning it off again. This lets a scrapyard crane pick up metal and then release it on command. The magnetic field is not produced by the metal itself or by friction, and the crane does not rely on physically rotating a permanent magnet.

A 50 g ice cube is left to melt completely into liquid water inside a sealed container. What is the mass of the water once melting is finished?

• A50 g, because melting is a physical change and mass is conserved✓
• B0 g, because melting destroys the original solid mass completely
• C100 g, because melting causes the particles to absorb extra mass from the air
• D25 g, because half the mass escapes as water vapour during melting

Answer: Melting is a physical change, and physical changes conserve mass because no atoms are created, destroyed or removed from the system. A sealed container also prevents any water vapour from escaping, so the mass of liquid water equals the original mass of the ice, 50 g. Melting does not cause matter to escape as vapour, absorb extra mass from the air, or destroy any of the original mass.

A gas fills any container it is placed in and can be compressed easily, unlike a solid, which keeps a fixed shape. How does the particle model explain this?

• AGas particles are identical to solid particles but simply weigh less than solid particles do
• BGas particles are far apart and move freely in all directions, unlike the closely packed particles in a solid✓
• CGas particles are far apart but completely fixed in position, unlike the moving particles in a solid
• DGas particles are closely packed and move freely in all directions, unlike the widely spaced particles in a solid

Answer: In a gas, the particles are far apart and move quickly and freely in all directions, which is why a gas spreads out to fill any container and can be compressed. In a solid, the particles are packed closely together and only vibrate around fixed positions. Gas particles are not fixed in place like a solid's particles, and the difference between gases and solids is about particle spacing and movement rather than particle weight.

Ice floats on top of liquid water, even though most solids sink in their own liquid form. What does this reveal about ice?

• AIce is less dense than liquid water because its particles form an open structure as they freeze✓
• BIce contains trapped air bubbles that are only added to it after it has already finished freezing completely
• CIce is actually more dense than liquid water, because its particles pack more tightly together as they freeze
• DIce simply weighs less than liquid water overall, regardless of the volume of each one being compared

Answer: Water is unusual because it expands slightly as it freezes, forming an open, spaced-out arrangement of particles that makes ice less dense than liquid water. This is why ice floats rather than sinking. Most other substances contract and become more dense when they freeze. Ice floating is not caused by trapped air bubbles added afterwards or by a change in overall weight rather than density.

Smoke particles viewed under a microscope in a sealed jar of air are seen to jitter about randomly in all directions. What does this demonstrate?

• AThe smoke particles are generating their own random movement through internal chemical energy
• BThe smoke particles are reacting chemically with the glass walls of the sealed jar
• CThe smoke particles are being bombarded unevenly by fast-moving, invisible air particles✓
• DThe smoke particles are being bombarded evenly by slow-moving, visible air particles

Answer: Brownian motion is the random, jittery movement of larger particles, such as smoke, caused by being struck unevenly by much smaller, fast-moving, invisible particles of the surrounding air. This random motion provides evidence that air is made of particles that are constantly moving. The smoke particles are not generating their own movement through internal energy, and they are not reacting chemically with the glass container.

A drop of food colouring spreads out through a glass of still water until the colour is evenly mixed, without any stirring. What causes this?

• AParticles instead diffuse from a region of lower concentration to a region of higher concentration
• BGravity pulls the coloured particles evenly downward through the water over time
• CParticles diffuse from a region of higher concentration to a region of lower concentration✓
• DThe water evaporates slightly, concentrating the colour uniformly throughout the glass

Answer: Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, as the particles spread out and mix due to their random motion. This explains why food colouring gradually spreads evenly through water without any stirring. Diffusion does not work in the opposite direction, from lower to higher concentration, and gravity and evaporation are not the cause of this even spreading.

Liquid water evaporating into water vapour can easily be reversed by cooling, but burning a piece of wood cannot be reversed. What is the key difference?

• ABoth evaporation and burning are chemical changes, but only burning releases enough energy to be noticed
• BEvaporation is a chemical change that forms new substances, while burning is a physical change that does not
• CBoth evaporation and burning are physical changes, but only evaporation happens quickly enough to reverse
• DEvaporation is a physical change that does not alter the substance, while burning is a chemical change that forms new substances✓

Answer: Evaporation is a physical change, since the water molecules themselves are not altered and the vapour can condense back into liquid water on cooling. Burning is a chemical change, since the wood reacts with oxygen to form new substances such as ash, carbon dioxide and water vapour, which cannot simply be turned back into wood. The two processes are not both physical or both chemical changes; one is reversible and one is not.

An object with a mass of 8 kg rests on the ground, where the gravitational field strength is 10 N/kg. Using force equals mass times gravitational field strength, what is the object's weight?

• A800 N
• B18 N
• C8 N
• D80 N✓

Answer: Weight can be calculated using force = mass multiplied by gravitational field strength. On Earth, the gravitational field strength is 10 N/kg, so an 8 kg object has a weight of 8 times 10, which is 80 N. The other values do not match this calculation and would come from adding the numbers, misplacing a digit, or forgetting to multiply by the field strength at all.

The Sun looks far bigger and brighter than any other star in the night sky. Why is this?

• AThe Sun is a planet rather than a star, unlike the much smaller points of light seen at night
• BThe Sun is a star like the others, but it appears larger and brighter because it is far closer to Earth✓
• CThe Sun is the largest star in our galaxy, which is why it outshines the others
• DThe Sun burns far hotter than other stars, so it gives out much more light

Answer: The Sun is a star, just like the many other stars visible in the night sky, but it looks far bigger and brighter because it is enormously closer to Earth than any other star. Many other stars in the galaxy are actually larger and produce more light energy than the Sun. The Sun is a genuine star and is not exceptional in size or output compared with stars generally, only in its distance from us.

The UK experiences longer, warmer days in June and shorter, colder days in December. What mainly causes this seasonal change?

• AThe Sun itself produces more energy during June, cooling down again by the time December arrives
• BThe Earth's axis is tilted, so the UK receives more direct sunlight for longer in June than in December✓
• CThe Earth's axis is tilted, so the UK is physically closer to the Sun in June than in December
• DThe Earth is much closer to the Sun overall in June, which raises temperatures across the UK

Answer: The Earth's axis is tilted, so different hemispheres are angled towards or away from the Sun at different times of year, changing how directly sunlight falls on a location and how many hours of daylight it receives. In June the UK is tilted towards the Sun, giving longer, warmer days, and in December it is tilted away, giving shorter, colder days. The Earth's overall distance from the Sun changes only slightly over a year and is not the main cause of the seasons, and the Sun's own energy output does not change from month to month in this way.