Magnetism and magnetic fields Revision Notes

What is Light?

Light is a type of energy that propagates as an electromagnetic wave. It does not require a medium to travel and can therefore pass through a vacuum, explaining how sunlight reaches Earth. This characteristic distinguishes electromagnetic waves from mechanical waves, such as sound waves, which need a material medium to transport their energy.

Electromagnetic Waves and their Properties

All electromagnetic waves are transverse waves. This means that the vibrations (oscillations) are perpendicular to the direction in which the wave is travelling and transferring energy. They consist of oscillating electric and magnetic fields that are also perpendicular to each other. A key property is that all electromagnetic waves travel at the same incredibly high speed in a vacuum, approximately 3.0 x 10^8 m/s, often referred to as the speed of light.

The Electromagnetic Spectrum

Visible light is only a tiny part of a much larger collection of electromagnetic waves known as the electromagnetic spectrum. This spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Each part of the spectrum has different uses and properties due to its distinct wavelength and frequency ranges.

Understanding Half-Life (Higher Tier)

Half-life is the time it takes for half of the radioactive nuclei in a sample to decay, or for the activity of the sample to halve. This is a fundamental concept in radioactive decay. It is a constant value for a given isotope and is not affected by external conditions like temperature or pressure. For example, if a substance has a half-life of 10 days, after 10 days, only 50% of the original radioactive material remains. After another 10 days (total 20 days), 25% remains, and so on. Understanding half-life is crucial for calculating the remaining amount of a radioactive isotope over time.

Calculating Fraction Remaining (Higher Tier)

To calculate the fraction of a radioactive isotope remaining after a certain number of half-lives, denoted by 'n', we use the formula (1/2)^n. For instance, after 1 half-life, the fraction remaining is (1/2)^1 = 1/2. After 2 half-lives, it's (1/2)^2 = 1/4. After 3 half-lives, it's (1/2)^3 = 1/8. This formula allows us to quickly determine the proportion of the original radioactive material that has not yet decayed, assuming we know the number of half-lives that have passed.

Calculating Net Decline as a Ratio (Higher Tier)

The net decline during radioactive emission can be expressed as a ratio. If the fraction remaining after 'n' half-lives is (1/2)^n, then the fraction that has decayed is 1 - (1/2)^n. If a question asks for the net decline as a ratio of the decayed amount to the original amount, this would be (1 - (1/2)^n) : 1. Often, it's clearer to express the ratio of the original activity (or amount) to the remaining activity (or amount). This ratio is 1 : (1/2)^n, which simplifies to 2^n : 1. For example, after 3 half-lives, the ratio of original to remaining is 2^3 : 1, which is 8:1. This means the original amount was 8 times larger than the amount remaining.

Example Calculation (Higher Tier)

Consider a radioactive sample that undergoes 4 half-lives. To find the net decline as a ratio of the original activity to the remaining activity: First, calculate the fraction remaining after 4 half-lives: (1/2)^4 = 1/16. This means 1/16th of the original activity remains. The net decline, expressed as a ratio of original activity to remaining activity, is 1 : 1/16. To express this as a whole number ratio, multiply both sides by 16, giving 16 : 1. This indicates that the original activity was 16 times greater than the activity after 4 half-lives. If the question asked for the fraction that has decayed, it would be 1 - 1/16 = 15/16.

Alpha Particles (α)

Alpha particles consist of two protons and two neutrons, identical to a helium nucleus. They are positively charged and relatively heavy. Due to their size and charge, they interact strongly with atoms in the material they pass through, causing significant ionisation. This strong interaction leads to a very limited penetrating power. Alpha particles can typically travel only a few centimetres in air. They are easily stopped by a thin sheet of paper, a few millimetres of skin, or even clothing. Consequently, an alpha source outside the body poses a low external hazard, but if ingested or inhaled, it becomes highly dangerous due to its intense ionising ability within tissues.

Beta Particles (β)

Beta particles are high-energy electrons (or positrons). They are much lighter and smaller than alpha particles and carry an electric charge (negative for electrons, positive for positrons). Beta particles interact less strongly with matter than alpha particles, resulting in greater penetrating power. They can travel several metres in air and are typically stopped by a few millimetres of aluminium. While they can penetrate skin and cause radiation burns if the source is external, their ionising power is less than that of alpha particles. Beta emitters are dangerous both internally and externally, although the internal hazard is generally less than that of alpha emitters.

Gamma Rays (γ)

Gamma rays are high-frequency electromagnetic radiation, without mass or charge. They are emitted from the nucleus of an atom. Because they are not charged particles, they interact very weakly with matter, making them highly penetrating. Gamma rays can travel long distances through air and require thick layers of dense materials such as lead or concrete to significantly reduce their intensity. Due to their high penetrating power, gamma sources present a significant external hazard, as they can pass through the body and cause damage to internal organs. Their ionising power is the lowest of the three common types of radiation.

Summary of Penetration and Shielding

The penetration power of common ionising radiations follows a clear order: alpha < beta < gamma. Alpha particles are stopped by paper or skin. Beta particles are stopped by a few millimetres of aluminium. Gamma rays require thick lead or concrete for effective shielding. This directly relates to their applications and safety precautions. For example, alpha sources are not used in medical imaging requiring penetration, while gamma sources are often used for sterilisation or medical diagnostics due to their ability to pass through materials.

What The Diagram Shows

Light is a physics diagram for Magnetism and magnetic fields. Use it beside the written explanation so the visual supports the exact idea being revised.

What To Explain

The answer should use the diagram to identify the physical relationship, then explain it using the relevant quantity, force, field, wave or energy idea.

Syllabus Link

Private mapping: OCR GCSE combined-science-a-gateway J250, Magnetism and magnetic fields.

Common Trap

Do not describe the diagram only as a picture. State the concept, then explain what the pattern, relationship, structure, or graph means.