Define nucleon number .

The total number of protons and neutrons in the nucleus. Also called the mass number ( A ).

Define proton number .

The total number of protons in the nucleus. Also called the atomic number ( Z ).

An atom of the same element with an equal number of protons but a different number of neutrons .

The number of neutrons in a nucleus is found from: number of neutrons = .......... – proton number

The number of neutrons in a nucleus is found from: number of neutrons = nucleon number – proton number

True or False? The nucleon number tells you the number of neutrons in a nucleus.

False. The nucleon number is the total number of protons and neutrons . The number of neutrons = nucleon number – proton number.

Deuterium and tritium are both isotopes of hydrogen with proton number 1. How many neutrons does each have?

Deuterium has one neutron (nucleon number 2). Tritium has two neutrons (nucleon number 3).

In a neutral atom, how does the number of protons compare with the number of electrons?

They are equal — the number of protons = number of electrons.

Describe the setup of the alpha particle scattering experiment.

Alpha particles are fired at a thin gold foil , with a detector on the other side to count how many particles are deflected at different angles.

In alpha scattering, what did most particles passing straight through the foil suggest?

The atom is mostly empty space .

In alpha scattering, a small number of particles were deflected through angles greater than 90°. What did this suggest?

The nucleus is very small and dense , containing most of the atom's mass and charge .

Alpha particles deflected through small angles suggested a small, concentrated .......... at the centre of the atom

Alpha particles deflected through small angles suggested a small, concentrated charge at the centre of the atom

True or False? The plum pudding model predicted that alpha particles would be deflected straight back by the gold foil.

False. The plum pudding model predicted the particles would pass straight through . The unexpected backscattering disproved it and led to the nuclear model.

Define the plum pudding model .

A model in which the atom is a sphere of positive charge with negatively charged electrons embedded in it, making it neutral overall.

What did Bohr add to Rutherford's model of the atom?

Electrons occupy shells ( energy levels ) at particular distances from the nucleus.

Which particle did James Chadwick discover in 1932?

The neutron , which completed the model of the atom we know today.

Define thermionic emission .

The release of electrons from the surface of a heated metal , when conduction electrons gain enough thermal energy to escape.

How does thermionic emission differ from the photoelectric effect?

In thermionic emission electrons gain energy from thermal (heat) energy , whereas in the photoelectric effect they absorb energy from incident photons .

Once electrons are released from a metal surface, they can be accelerated by .......... fields

Once electrons are released from a metal surface, they can be accelerated by electric or magnetic fields

In an electron gun, from which electrode are electrons emitted, and towards which are they accelerated?

Emitted from the negative cathode and accelerated towards the positive anode .

True or False? Thermionic emission requires light to release electrons.

False. Electrons are released by heating the metal (thermal energy), not by light. Releasing electrons using light is the photoelectric effect .

Define a linear accelerator (LINAC) .

A particle accelerator that uses electric fields to accelerate ions to high speeds in straight lines .

In a LINAC, why must each tube be built successively longer?

The AC supply has a fixed frequency , so the polarity switches at a constant rate . As the ions speed up , longer tubes ensure they spend the same time accelerating in each one.

A particle accelerator that uses magnetic and electric fields to accelerate ions from a central point along a spiral path.

A cyclotron uses two hollow semicircular electrodes called ..........

A cyclotron uses two hollow semicircular electrodes called dees

True or False? A LINAC uses magnetic fields to accelerate ions in a straight line.

False. A LINAC uses only electric fields . It is the cyclotron that uses both electric and magnetic fields.

Give two medical uses of a cyclotron.

Producing medical isotopes (tracers) and creating high-energy beams for radiotherapy .

What are the two key principles by which particle detectors operate?

Ionisation and deflection (scattering) by applied electric fields.

How does ionisation allow a particle detector to count particles?

A charged particle ionises atoms in the medium, freeing electrons. These are accelerated by an electric field and discharged to form pulses of current , which are counted as individual particles.

Why does a charged particle moving perpendicular to a uniform magnetic field travel in a circular path?

The magnetic force is always perpendicular to the particle's velocity , so it acts as a centripetal force directed towards the centre of the circle.

The magnetic force on the particle provides the .......... force needed for circular motion

The magnetic force on the particle provides the centripetal force needed for circular motion

How does the radius of the path depend on the particle's momentum?

The radius is directly proportional to momentum ( r ∝ p ), so particles with greater momentum move in larger circles .

What happens to the radius if the magnetic field strength is increased?

The radius decreases — r ∝ 1/ B , so a stronger field makes the particle move in a smaller circle .

True or False? A particle with greater charge moves in a larger circle.

False. r ∝ 1/ q , so a particle with greater charge moves in a smaller circle.

Define pair creation (as seen in particle tracks).

When an uncharged particle produces a particle–antiparticle pair , seen as two tracks appearing from a single point and curving in opposite directions with equal radius

What property of a particle does the curvature of its track reveal?

The particle's momentum A smaller radius means a smaller momentum; a larger radius means a larger momentum, since r = p / BQ

The radius of a charged particle's circular track is directly proportional to its .......... , since r = p / BQ

The radius of a charged particle's circular track is directly proportional to its momentum , since r = p / BQ

What does the direction of a track's curvature indicate, and which rule determines it?

The sign of the particle's charge Fleming's left hand rule is used to work out whether the charge is positive or negative

Why does the radius of a particle's track decrease as it moves through a detector?

The particle ionises other particles in its path, losing kinetic energy This reduces its velocity and hence its momentum , so the radius (r ∝ p) decreases

True or False? A tightly curved track (small radius) means the particle has a large momentum

False. Since r = p / BQ, a small radius corresponds to a small momentum . A larger momentum gives a larger-radius, straighter track

A particle found in the nucleus — a proton or a neutron

Electrons can probe very close to a nucleon without interacting because they do not experience the ..........

Electrons can probe very close to a nucleon without interacting because they do not experience the strong nuclear force

Why can high-energy electrons build a better picture of nucleon size than alpha particles ?

Electrons do not experience the strong nuclear force , so they get extremely close to the nucleon without interacting Alpha particles are made of protons and neutrons, so they do feel the strong force

How does accelerating electrons to even higher energies allow the internal structure of a nucleon to be resolved?

Higher velocity gives a smaller de Broglie wavelength, since λ ∝ 1/v A small enough wavelength can resolve individual quarks inside the nucleon

True or False? A larger de Broglie wavelength resolves finer detail

False. A smaller de Broglie wavelength is needed to resolve smaller detail. Since λ ∝ 1/v, electrons must be accelerated to higher velocity and energy

Define annihilation .

When a particle meets its equivalent antiparticle , both are destroyed and their mass is converted into energy in the form of two gamma-ray photons

Define pair production .

When a photon interacts with a nucleus and its energy is used to create a particle–antiparticle pair Energy is converted into matter — the opposite of annihilation

Why can a single photon not produce a particle–antiparticle pair on its own?

A nearby nucleus is needed so that both energy and momentum are conserved A lone photon would break the conservation laws

When an electron and positron annihilate, their mass is converted into two photons emitted in ..........

When an electron and positron annihilate, their mass is converted into two photons emitted in opposite directions

True or False? The equation E = hf can be used for all particles

False. E = hf applies only to photons , not to all particles

Define the electronvolt (eV) .

A unit of energy equal to the energy transferred to an electron accelerated through a potential difference of 1 V 1 eV = 1.6 × 10 -19 J

How do you convert between electronvolts and joules ?

Multiply eV by 1.6 × 10 -19 to get joules Divide joules by 1.6 × 10 -19 to get eV

A LevelPhysicsEdexcelFlashcards8. Nuclear & Particle PhysicsExploring the Structure of Matter

Exploring the Structure of Matter (Edexcel A Level Physics): Flashcards

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FrontNucleon & Proton Number
Define nucleon number.

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Cards in this collection (68)

Define nucleon number.
The total number of protons and neutrons in the nucleus. Also called the mass number (A).
Define proton number.
The total number of protons in the nucleus. Also called the atomic number (Z).
Define isotope.
An atom of the same element with an equal number of protons but a different number of neutrons.
The number of neutrons in a nucleus is found from: number of neutrons = .......... – proton number
The number of neutrons in a nucleus is found from: number of neutrons = nucleon number – proton number
True or False?
The nucleon number tells you the number of neutrons in a nucleus.
False.
The nucleon number is the total number of protons and neutrons. The number of neutrons = nucleon number – proton number.
Deuterium and tritium are both isotopes of hydrogen with proton number 1. How many neutrons does each have?
Deuterium has one neutron (nucleon number 2). Tritium has two neutrons (nucleon number 3).
In a neutral atom, how does the number of protons compare with the number of electrons?
They are equal — the number of protons = number of electrons.
Describe the setup of the alpha particle scattering experiment.
Alpha particles are fired at a thin gold foil, with a detector on the other side to count how many particles are deflected at different angles.
In alpha scattering, what did most particles passing straight through the foil suggest?
The atom is mostly empty space.
In alpha scattering, a small number of particles were deflected through angles greater than 90°. What did this suggest?
The nucleus is very small and dense, containing most of the atom's mass and charge.
Alpha particles deflected through small angles suggested a small, concentrated .......... at the centre of the atom
Alpha particles deflected through small angles suggested a small, concentrated charge at the centre of the atom
True or False?
The plum pudding model predicted that alpha particles would be deflected straight back by the gold foil.
False.
The plum pudding model predicted the particles would pass straight through. The unexpected backscattering disproved it and led to the nuclear model.
Define the plum pudding model.
A model in which the atom is a sphere of positive charge with negatively charged electrons embedded in it, making it neutral overall.
What did Bohr add to Rutherford's model of the atom?
Electrons occupy shells (energy levels) at particular distances from the nucleus.
Which particle did James Chadwick discover in 1932?
The neutron, which completed the model of the atom we know today.
Define thermionic emission.
The release of electrons from the surface of a heated metal, when conduction electrons gain enough thermal energy to escape.
How does thermionic emission differ from the photoelectric effect?
In thermionic emission electrons gain energy from thermal (heat) energy, whereas in the photoelectric effect they absorb energy from incident photons.
Once electrons are released from a metal surface, they can be accelerated by .......... fields
Once electrons are released from a metal surface, they can be accelerated by electric or magnetic fields
In an electron gun, from which electrode are electrons emitted, and towards which are they accelerated?
Emitted from the negative cathode and accelerated towards the positive anode.
An electron is accelerated from rest through a potential difference V. Which equation relates its final kinetic energy to V?
$\frac{1}{2} m v^{2} = e V$
The kinetic energy gained equals the work done by the field, eV.
True or False?
Thermionic emission requires light to release electrons.
False.
Electrons are released by heating the metal (thermal energy), not by light. Releasing electrons using light is the photoelectric effect.
Define a linear accelerator (LINAC).
A particle accelerator that uses electric fields to accelerate ions to high speeds in straight lines.
In a LINAC, why must each tube be built successively longer?
The AC supply has a fixed frequency, so the polarity switches at a constant rate. As the ions speed up, longer tubes ensure they spend the same time accelerating in each one.
Define a cyclotron.
A particle accelerator that uses magnetic and electric fields to accelerate ions from a central point along a spiral path.
A cyclotron uses two hollow semicircular electrodes called ..........
A cyclotron uses two hollow semicircular electrodes called dees
True or False?
A LINAC uses magnetic fields to accelerate ions in a straight line.
False.
A LINAC uses only electric fields. It is the cyclotron that uses both electric and magnetic fields.
Give two medical uses of a cyclotron.
Producing medical isotopes (tracers) and creating high-energy beams for radiotherapy.
What are the two key principles by which particle detectors operate?
Ionisation and deflection (scattering) by applied electric fields.
How does ionisation allow a particle detector to count particles?
A charged particle ionises atoms in the medium, freeing electrons. These are accelerated by an electric field and discharged to form pulses of current, which are counted as individual particles.
Why does a charged particle moving perpendicular to a uniform magnetic field travel in a circular path?
The magnetic force is always perpendicular to the particle's velocity, so it acts as a centripetal force directed towards the centre of the circle.
State the equation for the radius of a charged particle's circular path in a magnetic field.
$r = \frac{m v}{B q} = \frac{p}{B q}$
where p = momentum, B = magnetic flux density and q = charge.
The magnetic force on the particle provides the .......... force needed for circular motion
The magnetic force on the particle provides the centripetal force needed for circular motion
How does the radius of the path depend on the particle's momentum?
The radius is directly proportional to momentum (r ∝ p), so particles with greater momentum move in larger circles.
What happens to the radius if the magnetic field strength is increased?
The radius decreases — r ∝ 1/B, so a stronger field makes the particle move in a smaller circle.
True or False?
A particle with greater charge moves in a larger circle.
False.
r ∝ 1/q, so a particle with greater charge moves in a smaller circle.
Define pair creation (as seen in particle tracks).
When an uncharged particle produces a particle–antiparticle pair, seen as two tracks appearing from a single point and curving in opposite directions with equal radius
What property of a particle does the curvature of its track reveal?
The particle's momentum
A smaller radius means a smaller momentum; a larger radius means a larger momentum, since r = p / BQ
The radius of a charged particle's circular track is directly proportional to its .........., since r = p / BQ
The radius of a charged particle's circular track is directly proportional to its momentum, since r = p / BQ
What does the direction of a track's curvature indicate, and which rule determines it?
The sign of the particle's charge
Fleming's left hand rule is used to work out whether the charge is positive or negative
Why does the radius of a particle's track decrease as it moves through a detector?
The particle ionises other particles in its path, losing kinetic energy
This reduces its velocity and hence its momentum, so the radius (r ∝ p) decreases
True or False?
A tightly curved track (small radius) means the particle has a large momentum
False.
Since r = p / BQ, a small radius corresponds to a small momentum. A larger momentum gives a larger-radius, straighter track
Define nucleon.
A particle found in the nucleus — a proton or a neutron
To resolve the diameter of a nucleon, how must an electron's de Broglie wavelength compare to it?
It must be comparable to (or smaller than) the size of the nucleon
Given by $λ = \frac{h}{m v} \approx nucleon diameter$
Electrons can probe very close to a nucleon without interacting because they do not experience the ..........
Electrons can probe very close to a nucleon without interacting because they do not experience the strong nuclear force
Why can high-energy electrons build a better picture of nucleon size than alpha particles?
Electrons do not experience the strong nuclear force, so they get extremely close to the nucleon without interacting
Alpha particles are made of protons and neutrons, so they do feel the strong force
How does accelerating electrons to even higher energies allow the internal structure of a nucleon to be resolved?
Higher velocity gives a smaller de Broglie wavelength, since λ ∝ 1/v
A small enough wavelength can resolve individual quarks inside the nucleon
True or False?
A larger de Broglie wavelength resolves finer detail
False.
A smaller de Broglie wavelength is needed to resolve smaller detail. Since λ ∝ 1/v, electrons must be accelerated to higher velocity and energy
Define annihilation.
When a particle meets its equivalent antiparticle, both are destroyed and their mass is converted into energy in the form of two gamma-ray photons
Define pair production.
When a photon interacts with a nucleus and its energy is used to create a particle–antiparticle pair
Energy is converted into matter — the opposite of annihilation
Why can a single photon not produce a particle–antiparticle pair on its own?
A nearby nucleus is needed so that both energy and momentum are conserved
A lone photon would break the conservation laws
When an electron and positron annihilate, their mass is converted into two photons emitted in ..........
When an electron and positron annihilate, their mass is converted into two photons emitted in opposite directions
In annihilation, what is the energy carried away by each of the two photons equal to?
$E_{p h o t o n} = h f = \frac{h c}{λ} = c^{2} Δ m$
where Δm is the rest mass of one particle
What is the minimum energy a single photon must carry to create a particle–antiparticle pair?
At least twice the rest-mass energy of the particle:
$2 Δ E = 2 c^{2} Δ m$
True or False?
The equation E = hf can be used for all particles
False.
E = hf applies only to photons, not to all particles
Define the electronvolt (eV).
A unit of energy equal to the energy transferred to an electron accelerated through a potential difference of 1 V
1 eV = 1.6 × 10^-19 J
How do you convert between electronvolts and joules?
Multiply eV by 1.6 × 10^-19 to get joules
Divide joules by 1.6 × 10^-19 to get eV
One electronvolt is equal to .......... joules
One electronvolt is equal to 1.6 × 10^-19 joules
Why can mass be expressed in units of MeV/c² or GeV/c²?
From Einstein's relation ΔE = c²Δm, rearranged to Δm = ΔE / c²
Dividing an energy unit by c² gives a mass unit, which is convenient in particle physics
What is 1 GeV in joules?
1 GeV = 1 × 10⁹ eV = 1.6 × 10^-10 J
What is 1 GeV/c² in kilograms?
1 GeV/c² = 1.78 × 10^-27 kg
Roughly the rest mass of a proton
True or False?
The electronvolt is a unit of voltage
False.
The electronvolt is a unit of energy, not voltage — it is the energy gained by an electron accelerated through 1 V
Define time dilation.
Clocks run slower for moving objects — a fast-moving particle's lifetime is stretched, so it survives much longer than when at rest
Define length contraction.
Moving rulers are shorter than stationary ones in the direction of motion — the distance a fast particle must travel appears contracted
Muons have a lifetime of only about 2 μs, too short to reach sea level. Why are they still detected there in large numbers?
They travel at relativistic speeds (about 0.98c), so time dilation stretches their lifetime far beyond 2 μs
This lets them survive the journey to the ground
How does time dilation make short-lived particles easier to detect?
Their lifetime is dilated, so they survive longer and leave longer tracks in particle detectors
Relativistic effects become significant only when particles move at velocities greater than about .......... the speed of light
Relativistic effects become significant only when particles move at velocities greater than about 90% the speed of light
What is the evidence that length contraction occurs for fast-moving particles?
Many exotic particles with very short lifetimes are detected after travelling appreciable distances through detectors
Without length contraction they would decay before escaping
True or False?
A fast-moving unstable particle decays after its normal (rest) lifetime as measured in the laboratory
False.
In the laboratory frame the particle's lifetime is dilated — it survives far longer than its rest lifetime, which is why muons reach sea level

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