Mass and energy (A-level only) Flashcards – AQA A-level Physics (7408)

What equation relates a system’s energy change to its mass change?

$\Delta E = \Delta m c^2$, where $c$ is the speed of light in a vacuum.

Does mass–energy equivalence apply only to nuclear reactions?

No. It applies to all energy changes.

How does a system’s mass change when it gains or loses energy?

It gains mass when it gains energy and loses mass when it loses energy.

Why can a small mass change correspond to a substantial energy change?

Because $c^2$ is very large.

What is the nuclear binding energy of a nucleus?

The energy required to separate it completely into its constituent protons and neutrons.

How much energy is released when free nucleons form a nucleus?

An amount equal to the nucleus’s binding energy.

What is the mass defect of a nucleus?

The difference between the sum of the masses of its separate nucleons and the mass of the bound nucleus.

What equation gives the binding energy of a nucleus with $Z$ protons and $N$ neutrons, in terms of the proton, neutron and nuclear masses?

$E_\mathrm{b} = (Zm_\mathrm{p} + Nm_\mathrm{n} - m_\mathrm{nucleus})c^2$

How is the atomic mass unit, $\mathrm{u}$, defined?

As one twelfth of the mass of a carbon-12 atom.

How do you convert a mass defect in $\mathrm{u}$ to binding energy in $\mathrm{MeV}$?

Multiply the mass defect by $931.5$.

What must be considered when using atomic and nuclear masses in a binding-energy calculation?

Use the masses consistently, including electron masses when relevant.

What is nuclear fission?

The splitting of a heavy nucleus into smaller nuclei.

How can nuclear fission be initiated in a suitable nucleus?

By the nucleus absorbing a neutron.

What does nuclear fission typically release?

Energy and additional neutrons.

When can nuclear fission produce a chain reaction?

When neutrons released by fission cause further fissions.

What does the energy released in nuclear fission reflect?

The difference between the total mass before and after the reaction.

What is nuclear fusion?

The joining of light nuclei to form a heavier nucleus.

What obstacle must positively charged nuclei overcome to get close enough for fusion?

They repel one another.

How can very high temperatures help nuclei fuse?

They can provide particles with the energy needed to approach closely enough.

Why does fusion of suitable light nuclei release energy?

The total mass of the products is less than the total mass of the reacting nuclei.

What equation gives the energy released in fission or fusion from the initial and final total masses?

$Q = (m_\mathrm{initial} - m_\mathrm{final})c^2$

In a nuclear reaction with a positive mass difference, what carries the released energy?

The reaction products and any emitted radiation.

How do you convert a nuclear reaction’s mass difference in atomic mass units to energy in MeV?

Multiply the mass difference by 931.5.

Which masses must be included when calculating the mass difference for a nuclear reaction?

The masses of all reactants and all products.

What consistency rule applies to the types of masses used in a nuclear reaction energy calculation?

Use atomic masses or nuclear masses consistently.

What is the average binding energy per nucleon of a nucleus?

Its total binding energy divided by its nucleon number.

How does average binding energy per nucleon vary with nucleon number?

It rises steeply for light nuclei, reaches a broad maximum around iron, then falls gradually for heavier nuclei.

Which nuclear process can increase the average binding energy per nucleon of suitable nuclei at each end of the graph?

Fusion for light nuclei; fission for heavy nuclei.

How is an increase in total nuclear binding energy related to energy release?

The increase in total binding energy corresponds to energy released.

How can knowledge of nuclear processes and their energy release inform societal decisions?

It provides scientific evidence for decisions about nuclear energy.