Waves, Electricity & Atomic Physics

A wave has a frequency of 50 Hz and a wavelength of 4 m. What is its speed?

• A54 m/s
• B12.5 m/s
• C0.08 m/s
• D200 m/s✓

Answer: Wave speed is found by multiplying frequency by wavelength, so 50 Hz times 4 m gives 200 m/s. Adding the two values instead of multiplying them gives the meaningless 54 m/s. Dividing wavelength by frequency or frequency by wavelength the wrong way round gives 0.08 m/s or 12.5 m/s, neither of which is the wave speed.

Sound travelling through air causes air particles to vibrate parallel to the direction the wave travels. What type of wave is this?

• AA wave with no medium
• BA transverse wave
• CAn electromagnetic wave
• DA longitudinal wave✓

Answer: In a longitudinal wave the particles vibrate back and forth in the same direction the wave energy travels, which is how sound moves through air. A transverse wave, like a water ripple, has particles vibrating at right angles to the direction of travel instead. Sound is not electromagnetic and it cannot travel without a medium, unlike light.

In a water wave, what does the amplitude measure?

• AThe maximum displacement of a particle from its undisturbed position✓
• BThe number of complete waves passing a point each second
• CThe time taken for one complete wave cycle
• DThe straight-line distance measured between two neighbouring wave peaks

Answer: Amplitude is the maximum distance a particle moves away from its rest position as the wave passes. The distance between adjacent peaks is the wavelength, not the amplitude. The number of waves passing a point per second is the frequency, and the time for one full cycle is the period.

A wave has a frequency of 20 Hz. What is its period?

• A0.05 seconds✓
• B20 seconds
• C0.2 seconds
• D5 seconds

Answer: The period of a wave is the time for one complete cycle, found by dividing 1 by the frequency, so 1 divided by 20 gives 0.05 seconds. Confusing the period with the frequency value itself gives 20 seconds. Misplacing the decimal point gives 0.2 seconds, and dividing 100 by 20 gives 5 seconds, neither of which is correct.

A cork floats on a pond and ripples pass beneath it, making the cork bob up and down without drifting towards the shore. What does this show about the ripples?

• AThe pond water flows continuously in the direction the ripples are moving
• BThe cork sinks slightly further each time a ripple passes beneath it
• CThe wave transfers energy through the water without moving the water itself far from its starting point✓
• DThe cork moves steadily along with each ripple and gradually travels a long distance away from where it originally started floating

Answer: A wave transfers energy from place to place, but the particles of the medium only oscillate around a fixed position rather than travelling with the wave, which is why the cork just bobs up and down. If the cork drifted away, that would wrongly suggest the water itself was flowing outward. The water does not flow continuously outward and the cork does not gradually sink.

Placed in order of increasing wavelength, which is the correct sequence for these four regions of the electromagnetic spectrum?

• AVisible light, ultraviolet, X-rays, gamma rays
• BGamma rays, ultraviolet, X-rays, visible light
• CGamma rays, X-rays, ultraviolet, visible light✓
• DX-rays, gamma rays, ultraviolet, visible light

Answer: Across the electromagnetic spectrum, gamma rays have the shortest wavelength, followed by X-rays, then ultraviolet, then visible light, so this order runs from shortest to longest wavelength correctly. Swapping gamma rays and X-rays, or swapping ultraviolet and X-rays, breaks that increasing sequence. Listing visible light first and gamma rays last gives the order of decreasing, not increasing, wavelength.

How does the speed of X-rays in a vacuum compare with the speed of radio waves in a vacuum?

• ARadio waves travel faster due to their wavelength
• BX-rays travel faster only in a vacuum, not air
• CThey travel at exactly the same speed✓
• DX-rays travel faster owing to a higher frequency

Answer: All electromagnetic waves, including X-rays and radio waves, travel at the same speed through a vacuum regardless of their frequency or wavelength. A higher frequency does not make X-rays travel faster, and a longer wavelength does not make radio waves travel faster. The equal-speed rule applies specifically in a vacuum, not as a special case for X-rays alone.

Why is prolonged exposure to ultraviolet radiation from the sun a health hazard?

• AIt cooks skin tissue by increasing the vibration of water molecules
• BIt ionises gas high in the atmosphere, disrupting radio signals
• CIt causes mutations only in plant cells, never in human cells
• DIt can damage skin cells and increase the risk of skin cancer✓

Answer: Ultraviolet radiation can penetrate skin cells and damage the DNA inside them, which increases the risk of skin cancer developing over time. Heating tissue by increasing molecular vibration is a property of infrared radiation, not ultraviolet. Ultraviolet radiation harms human cells directly, so it is not limited to affecting only plants.

Why do hospital radiographers stand behind a screen when taking X-ray images of patients?

• AX-rays heat the surrounding air enough to cause burns at a distance
• BX-rays reflect off the patient and could burn the radiographer's skin instantly
• CX-rays are magnetic and could interfere with nearby electronic equipment
• DX-rays and gamma rays can damage or mutate cells in living tissue✓

Answer: X-rays and gamma rays carry enough energy to damage or mutate the cells of living tissue, so radiographers limit their own repeated exposure by standing behind a screen. X-rays do not reflect off skin and cause instant burns, and they are not magnetic. They also do not heat the surrounding air enough to burn someone at a distance.

Which statement correctly matches an electromagnetic wave to a use that fits its properties?

• AUltraviolet waves are used to carry television broadcast signals over long distances
• BX-rays are used to carry mobile phone signals because they pass easily through buildings
• CMicrowaves are used to carry mobile phone signals and to heat food in microwave ovens✓
• DGamma rays are used to heat food in ovens because they transfer energy directly to water

Answer: Microwaves are well suited to carrying mobile phone signals and to heating food, since they are absorbed by water molecules in a way that generates heat. X-rays are used for imaging, not for carrying phone signals, and gamma rays are used in medicine and sterilising, not cooking. Television broadcasts use radio waves rather than ultraviolet radiation.

As you move from radio waves to gamma rays across the electromagnetic spectrum, what happens to frequency and energy?

• ABoth frequency and energy decrease while wavelength increases
• BFrequency increases but energy decreases as wavelength decreases
• CBoth frequency and energy increase while wavelength decreases✓
• DFrequency stays constant but energy increases as wavelength decreases

Answer: Moving from radio waves towards gamma rays, wavelength gets shorter while both frequency and the energy carried by the wave increase together. Frequency and energy rise together, so a rising frequency paired with falling energy is not possible. Frequency does not stay constant across the spectrum, since each region is defined by a different frequency range.

A charge of 12 coulombs flows past a point in a wire in 4 seconds. What is the current?

• A16 A
• B48 A
• C8 A
• D3 A✓

Answer: Current equals charge divided by time, so 12 coulombs divided by 4 seconds gives 3 A. Multiplying the two values instead of dividing gives 48 A, and adding them gives 16 A, neither of which is current. Subtracting the time from the charge instead of dividing gives 8 A, which is also not the correct calculation.

A component has a resistance of 15 ohms and a current of 2 A flows through it. What is the potential difference across it?

• A17 V
• B30 V✓
• C7.5 V
• D13 V

Answer: Potential difference equals current multiplied by resistance, so 2 A times 15 ohms gives 30 V. Dividing resistance by current instead of multiplying gives 7.5 V. Adding or subtracting the current and resistance values gives 17 V or 13 V, but neither of those operations calculates potential difference.

Two resistors, 6 ohms and 9 ohms, are connected in series. What is the total resistance of the circuit?

• A54.0 ohms
• B15 ohms✓
• C3.6 ohms
• D1.5 ohms

Answer: In a series circuit the total resistance is simply the sum of the individual resistances, so 6 ohms plus 9 ohms gives 15 ohms. The value 3.6 ohms is actually the resistance the two resistors would have if connected in parallel instead, not in series. Multiplying the resistances gives 54.0 ohms, and dividing one by the other gives 1.5 ohms, neither of which is the series total.

Two resistors are connected in parallel across the same battery. How does the total resistance of the parallel combination compare with the resistance of the smallest individual resistor?

• AIt is always larger than the resistance of the largest individual resistor
• BIt is always equal to the resistance of the smallest individual resistor
• CIt is always smaller than the resistance of the smallest individual resistor✓
• DIt is always equal to the sum of both resistances

Answer: Adding a parallel branch gives current extra routes to flow, so the combined resistance of a parallel network is always lower than that of its smallest individual resistor. It is not simply equal to the smallest resistor's value, and it is never larger than the largest resistor either. Adding the resistances together is the rule for a series circuit, not a parallel one.

What are the typical frequency and voltage of the UK domestic mains electricity supply?

• A60 Hz alternating current at around 230 V
• B50 Hz alternating current at around 110 V
• C50 Hz direct current at around 230 V
• D50 Hz alternating current at around 230 V✓

Answer: UK mains electricity is supplied as alternating current at a frequency of 50 Hz and a voltage of around 230 V. A frequency of 60 Hz is used in some other countries, such as the United States, not the UK. UK mains is alternating rather than direct current, and its typical voltage is around 230 V rather than 110 V.

In a UK three-pin plug, what is the main function of the earth wire?

• AIt carries the alternating current that powers the appliance safely throughout everyday normal use
• BIt completes the normal operating circuit by returning current to the supply
• CIt provides a low-resistance path to protect against electric shock if a fault occurs✓
• DIt carries current at a higher voltage than the live wire to prevent overload

Answer: The earth wire is a safety feature that gives a low-resistance route for current to flow if a fault causes the casing of an appliance to become live, reducing the risk of electric shock. Carrying the current that powers the appliance is the job of the live wire, and completing the normal circuit back to the supply is the job of the neutral wire. The earth wire does not carry current during normal operation at all.

An appliance operates at 230 V and draws a current of 4 A. What is its power rating?

• A234 W
• B460 W
• C920 W✓
• D57.5 W

Answer: Electrical power equals potential difference multiplied by current, so 230 V times 4 A gives 920 W. Dividing voltage by current instead of multiplying gives 57.5 W, and adding the two values gives 234 W, neither of which calculates power correctly. Halving the correct answer by mistake gives 460 W, which is not the true power rating.

As the temperature of a thermistor increases, what happens to its resistance?

• AIts resistance rises then falls
• BIts resistance decreases✓
• CIts resistance stays constant
• DIts resistance increases

Answer: A thermistor is designed so that its resistance falls as its temperature rises, which is why thermistors are used as temperature sensors in circuits. This behaviour is the opposite of a filament lamp, whose resistance rises as it heats up, so assuming the resistance increases describes the wrong component. A thermistor's resistance does not stay constant, and it does not switch direction partway through heating.

A bar magnet's north pole is brought close to the north pole of a second bar magnet. What happens?

• AThe two poles attract weakly at first
• BThe two poles repel each other✓
• CThe two poles have no effect on each other
• DThe two poles attract each other strongly

Answer: Like magnetic poles always repel one another, so bringing two north poles together pushes the magnets apart. Two north poles never attract, regardless of how strong the magnets are, and they do not simply have no effect on each other. There is no brief attraction stage either, since like poles repel as soon as their fields interact.

What is the key difference between a permanent magnet and an induced magnet?

• AA permanent magnet loses its magnetism the instant it is removed from a field, unlike an induced magnet
• BA permanent magnet can only attract other magnets, while an induced magnet can only repel them
• CA permanent magnet only becomes magnetic when placed near another magnet, while an induced magnet always keeps its own permanent field that never fades
• DA permanent magnet produces its own magnetic field, while an induced magnet only becomes magnetic when placed in another magnetic field✓

Answer: A permanent magnet keeps a magnetic field of its own all the time, whereas an induced magnet, such as an iron nail near a magnet, only becomes magnetic while it is inside another magnetic field. Reversing which magnet has its own field describes the situation backwards. Neither type of magnet is limited to only attracting or only repelling, and it is the induced magnet, not the permanent one, that loses its magnetism almost immediately once removed from the field.

A straight wire carries an electric current. How does the magnetic field around the wire change if the current through it is increased?

• AThe magnetic field becomes stronger✓
• BThe magnetic field becomes noticeably weaker
• CThe magnetic field disappears completely
• DThe magnetic field reverses direction

Answer: The strength of the magnetic field around a current-carrying wire increases as the current through the wire increases. It does not become weaker or vanish altogether as the current rises. Simply increasing the size of the current does not reverse the direction of the field, since direction depends on the direction of current flow, not its size.

A current-carrying wire is placed in a magnetic field so that the wire is perpendicular to the field lines. What happens to the wire?

• AIt experiences a force due to the motor effect✓
• BIt becomes permanently magnetised by the field
• CIt induces an electric current in the magnetic field itself
• DIt experiences no force unless the current is alternating

Answer: When a current-carrying wire sits perpendicular to a magnetic field, it experiences a force known as the motor effect, which is the basic principle behind electric motors. The wire does not induce a current in the field itself, since it is the wire carrying current, not the field. The force acts whether the current is direct or alternating, and the wire does not become a permanent magnet as a result.

A wire of length 0.5 m carries a current of 4 A in a magnetic field of flux density 0.2 T, perpendicular to the wire. What is the force on the wire?

• A4 N
• B0.8 N
• C4.7 N
• D0.4 N✓

Answer: The force on a current-carrying wire is found by multiplying magnetic flux density, current and length, so 0.2 T times 4 A times 0.5 m gives 0.4 N. Leaving out the length term and only multiplying flux density by current gives 0.8 N. Misplacing a decimal point gives 4 N, and adding all three values instead of multiplying them gives 4.7 N.

A block of material has a mass of 750 g and a volume of 250 cm3. What is its density?

• A3 g/cm3✓
• B500 g/cm3
• C100 g/cm3
• D0.3 g/cm3

Answer: Density is calculated by dividing mass by volume, so 750 g divided by 250 cm3 gives 3 g/cm3. Dividing volume by mass instead of mass by volume gives roughly 0.3 g/cm3, which inverts the calculation. Misplacing a decimal point gives 100 g/cm3, and subtracting the volume from the mass gives 500 g/cm3, but neither of these operations calculates density.

A 50 g ice cube melts completely into liquid water inside a closed container that loses no material to the surroundings. What is the mass of the resulting water?

• A45 g
• B50 g✓
• C55 g
• D0 g

Answer: Melting is a change of state, and mass is conserved during a change of state as long as no material escapes, so the water has exactly the same mass as the ice did. A small loss to 45 g or a gain to 55 g would both wrongly suggest mass was not conserved. The mass certainly does not drop to 0 g, since the water still exists inside the sealed container.

How much energy is needed to raise the temperature of 3 kg of water by 8 degrees C, given the specific heat capacity of water is 4200 J/kg degrees C?

• A100800 J✓
• B1008000 J
• C12600 J
• D4200 J

Answer: Energy transferred is found by multiplying mass, specific heat capacity and temperature change, so 3 kg times 4200 J/kg degrees C times 8 degrees C gives 100800 J. Leaving out the temperature change and only multiplying mass by specific heat capacity gives 12600 J. Using the specific heat capacity value alone gives 4200 J, and misplacing a decimal point gives 1008000 J.

A pure substance is heating at its melting point, and its temperature stays constant even though energy is still being transferred to it. What is happening to that energy?

• AIt is increasing the kinetic energy of the particles, causing the temperature to rise
• BIt is being stored entirely as an increase in the particles' speed of vibration
• CIt is increasing the internal energy by breaking bonds between particles as the substance changes state✓
• DIt is being released back into the surroundings as the substance cools

Answer: At a melting point, the energy supplied goes into breaking the bonds holding particles in fixed positions, increasing internal energy without raising the temperature until melting is complete. If the energy were raising the particles' kinetic energy, the temperature would rise, which contradicts the constant temperature described. The substance is being heated, not cooled, so energy is not being released to the surroundings, and it is not simply making particles vibrate faster.

A fixed mass of gas is heated inside a sealed, rigid container of constant volume. What happens to its pressure, and why?

• AThe pressure decreases because the faster-moving particles spread further apart to fill all the available space inside the container
• BThe pressure increases only because many more gas particles are somehow created inside the container as its temperature rises
• CThe pressure increases because the particles move faster and collide with the container walls more frequently and forcefully✓
• DThe pressure stays exactly the same because the volume of the sealed rigid container has not changed at all throughout the entire heating process

Answer: Heating a gas at constant volume gives its particles more kinetic energy, so they move faster and strike the container walls more often and with greater force, raising the pressure. The particles cannot move further apart in a rigid container, since its volume is fixed. Pressure is not fixed just because volume is constant, since temperature still affects it, and no new gas particles are created by heating.

Which statement correctly describes the structure of an atom according to the nuclear model?

• AA large, positively charged nucleus contains almost all the atom's mass, with no electrons present
• BA small, negatively charged nucleus contains almost all the atom's mass, surrounded by positively charged electrons
• CA small, positively charged nucleus contains very little of the atom's mass, surrounded by negatively charged electrons
• DA small, positively charged nucleus contains almost all the atom's mass, surrounded by negatively charged electrons✓

Answer: In the nuclear model, an atom has a tiny, positively charged nucleus that holds almost all of its mass, with negatively charged electrons arranged around it at a much greater distance. The nucleus is small compared with the whole atom, not large, and electrons are always present in a neutral atom. Swapping the charges the wrong way round, or claiming the nucleus holds very little mass, both misdescribe the nuclear model.

Two atoms of the same element are isotopes of each other. What is true about them?

• AThey have the same number of protons but different numbers of neutrons✓
• BThey have different numbers of both protons and neutrons
• CThey have the same number of neutrons but different numbers of protons
• DThey have the same number of protons and neutrons but different numbers of electrons

Answer: Isotopes of an element always share the same number of protons, since that number defines the element, but they differ in their number of neutrons. Having a different number of protons would actually make them atoms of different elements, not isotopes of the same one. Isotopes are defined by their neutron count, not by having different numbers of electrons while every other particle count matches.

Which type of nuclear radiation is stopped by a few centimetres of air or a thin sheet of paper?

• ABeta radiation
• BNeutron radiation
• CGamma radiation
• DAlpha radiation✓

Answer: Alpha radiation has the weakest penetrating power of the common types, so it is absorbed by only a few centimetres of air or a sheet of paper. Beta radiation is more penetrating and needs a sheet of aluminium to stop it, while gamma radiation needs thick lead or concrete. Neutron radiation is also far more penetrating than alpha and is not stopped by paper.

A radioactive sample has an initial activity of 800 counts per minute. Its half-life is 3 hours. What will its activity be after 9 hours?

• A200 counts per minute
• B100 counts per minute✓
• C400 counts per minute
• D0 counts per minute

Answer: Nine hours is exactly three half-lives, and each half-life halves the activity, so 800 falls to 400, then 200, then 100 counts per minute. Stopping after only one half-life gives 400, and stopping after two gives 200, both too early for the full nine hours. The activity never reaches zero, since radioactive decay only ever halves the remaining activity rather than eliminating it completely.

A worker is exposed to gamma rays from a sealed radioactive source but does not touch or inhale any radioactive material. What has happened to the worker?

• AThe worker has been contaminated but not irradiated
• BThe worker has been irradiated but not contaminated✓
• CThe worker has been both irradiated and contaminated
• DThe worker has been neither irradiated nor contaminated

Answer: Being exposed to radiation from an external, sealed source without any radioactive material getting onto or into the body is called irradiation, and it stops as soon as the worker moves away from the source. Contamination means radioactive particles have actually been transferred onto or into a person, which has not happened here since the source stayed sealed. Since irradiation clearly did occur, describing the worker as unaffected or as contaminated instead would both be incorrect.

A radio station broadcasts at a frequency of 98 MHz. Radio waves travel at 3.0 × 10⁸ m/s.

What is the wavelength of the radio waves?

• A3.1 m✓
• B0.33 m
• C3.1 × 10⁶ m
• D2.9 × 10¹⁶ m

Answer: Wavelength = wave speed ÷ frequency. 98 MHz = 98 × 10⁶ Hz, so wavelength = 3.0 × 10⁸ ÷ 9.8 × 10⁷ = 3.06 m, about 3.1 m. The tempting 3.1 × 10⁶ m ignores the mega prefix; 0.33 m divides frequency by speed, and 2.9 × 10¹⁶ m multiplies speed by frequency.

A water wave has a period of 0.25 s and a wavelength of 1.2 m.

What is the speed of the wave?

• A4.8 m/s✓
• B0.30 m/s
• C0.21 m/s
• D1.45 m/s

Answer: Frequency = 1 ÷ period = 1 ÷ 0.25 = 4.0 Hz. Wave speed = frequency × wavelength = 4.0 × 1.2 = 4.8 m/s. The tempting 0.30 m/s multiplies the period by the wavelength instead of using the frequency, 0.21 m/s divides the period by the wavelength, and 1.45 m/s adds them.

A lamp and a resistor are connected in series to a 9.0 V battery. The current is 0.30 A and the potential difference across the resistor is 6.0 V.

What is the resistance of the lamp?

• A20 Ω
• B30 Ω
• C0.90 Ω
• D10 Ω✓

Answer: In series, the battery's potential difference is shared, so the lamp has 9.0 − 6.0 = 3.0 V across it. The same current flows through both, so the lamp's resistance = V ÷ I = 3.0 ÷ 0.30 = 10 Ω. The tempting 20 Ω is the resistor's resistance, 30 Ω is the total resistance of the circuit, and 0.90 Ω multiplies the lamp's potential difference by the current.

A phone charger supplies a current of 0.50 A at a potential difference of 5.0 V for 2.0 hours.

How much energy does it transfer?

• A300 J
• B3600 J
• C18 000 J✓
• D5.0 J

Answer: Time = 2.0 × 3600 = 7200 s. Charge = current × time = 0.50 × 7200 = 3600 C. Energy = charge × potential difference = 3600 × 5.0 = 18 000 J. The tempting 300 J uses the time in minutes (120), 3600 is the charge in coulombs rather than the energy, and 5.0 J leaves time out altogether.

A thermistor is connected in series with a 1000 Ω resistor and a 6.0 V battery. The thermistor's resistance changes with temperature: 0 °C: 3000 Ω 20 °C: 1200 Ω 40 °C: 500 Ω 60 °C: 250 Ω

What is the potential difference across the 1000 Ω resistor at 40 °C?

• A2.0 V
• B4.0 V✓
• C3.0 V
• D6.0 V

Answer: At 40 °C the total resistance = 500 + 1000 = 1500 Ω, so the current = 6.0 ÷ 1500 = 0.0040 A. The potential difference across the resistor = 0.0040 × 1000 = 4.0 V. The tempting 2.0 V is the potential difference across the thermistor, 3.0 V shares the battery's 6.0 V equally, and 6.0 V ignores the thermistor. In series, the larger resistance takes the larger share.

A current of 2.0 A flows through a heater of resistance 12 Ω.

What is the power of the heater?

• A24 W
• B48 W✓
• C6.0 W
• D288 W

Answer: Power = current² × resistance = 2.0² × 12 = 4.0 × 12 = 48 W. The tempting 24 W forgets to square the current, 6.0 W divides the resistance by the current, and 288 W squares the resistance instead of the current (2.0 × 12²).