Matter and radiation

AQA Physics for Section 1 - Particles & Radiation

Describe the structure of an atom.

Small nucleus containing protons & neutrons, with electrons orbiting. Most mass is in the nucleus.

Why are electrons held in an atom?

Electrostatic attraction between negatively charged electrons and positively charged protons.

What is an ion? How are ions formed?

A charged atom, formed when electrons are gained or lost.

What are isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons.

What is specific charge?

Charge per unit mass.
S.C. = ^Q/_m

What is the strong nuclear force? Why is it needed?

A short-ranged attractive force between nucleons that holds nucleons together and overcomes electrostatic repulsion between protons.

What are the key properties of the strong nuclear force?

• Very short range (3-4 fm)

• Attractive at 0.5 → 3-4 fm

• Repulsive at < 0.5 fm

• Acts equally between any 2 nucleons

How do the nuclear force and electrostatic force differ?

• Strong nuclear: Short-ranged, mainly attractive, holds nucleons together

• Electrostatic: Infinite range, repulsive between protons

What is radioactive decay?

The spontaneous and random emission of radiation from an unstable nucleus.

What is emitted in alpha decay? How does it change the nucleus?

• 2 protons & 2 neutrons emitted

• A - 4, Z - 2 | New element formed

What happens in beta-minus decay? How does it change the nucleus?

• A neutron decays into a proton, emitting an electron and an antineutrino

• A = A, Z + 1

When does beta-minus decay occur?

When a nucleus has too many neutrons.

What is gamma radiation?

High-energy electromagnetic radiation emitted by an unstable nucleus.

Alpha particle properties?

• +2 charge

• high mass

• highly ionising

• least penetrative → stopped by air / paper

Beta particle properties?

• -1 charge

• low mass

• moderately ionising

• medium penetrative → stopped by thin aluminium

Gamma radiation properties?

• 0 charge

• 0 mass

• weakly ionising

• highly penetrating → stopped by thick lead / concrete

What is a hypothesis?

An untested idea or theory.

Why was the neutrino hypothesis proposed?

To explain the missing energy / momentum in beta decay.

Name the EM spectrum (increasing wavelength). What speed do they travel at in a vacuum?

• Radio → Micro → Infrared → Visible → Ultraviolet → X-Ray → Gamma

• 3.00 × 10⁸ ms⁻¹

What is an electromagnetic wave made of?

Oscillating electric and magnetic fields at right angles to each other and to the direction of travel.

What does it mean for EM waves to be ‘in phase’?

Peaks and troughs of both electric and magnetic fields occur at the same time.

What is a photon?

A packet (quantum) of electromagnetic energy.

How are electromagnetic waves emitted?

When charged particles lose energy (electron transition / deceleration / change in direction)

What is a laser beam made of?

Photons of the same frequency

How is laser power related to photons?

E = nhf

P = En