Magnetism and magnetic fields Revision Notes

Contamination: Definition and Hazards

Contamination occurs when a radioactive material, in the form of atoms or particles, gets onto an object, into the air, into food, or inside a living organism. This means the object or organism itself becomes radioactive. The main hazard of contamination is that the radioactive source is in direct contact with cells, leading to continuous internal or external exposure as the radioactive nuclei decay. Internal contamination, such as ingesting or inhaling radioactive particles, is particularly dangerous as it irradiates tissues from within until removed or decayed. Even small amounts can be hazardous due to the proximity to sensitive organs. Proper handling, containment, and decontamination procedures are essential.

Irradiation: Definition and Hazards

Irradiation is the exposure to radiation from an external radioactive source without the radioactive material itself becoming present on or within the exposed object or person. For example, standing near a radioactive source makes you irradiated, but you are not contaminated and do not become radioactive. The hazard from irradiation depends on the type and energy of the radiation, the intensity of the source, the distance from the source, and the duration of exposure. Once the external source is removed or shielded, the irradiation stops. Protective measures against irradiation include increasing distance from the source, using appropriate shielding (e.g., lead or concrete), and limiting the time spent near the source.

Key Differences Between Contamination and Irradiation

The fundamental difference lies in the presence of the radioactive material. In contamination, the radioactive material is physically present on or inside the object/person, making the object/person radioactive. The hazard persists as long as the contaminant is present. In irradiation, the radiation passes through the object/person, but the radioactive material itself is not transferred, and the object/person does not become radioactive. The hazard exists only while exposed to the external source. Contamination requires decontamination, while irradiation requires shielding, distance, and time limitation. Contamination is often harder to remove and poses a greater long-term risk due to internal exposure potential.

Safety Precautions and Risk Mitigation

To mitigate the hazards of contamination, procedures focus on preventing the spread of radioactive materials. This includes wearing protective clothing (gloves, masks), working in controlled environments (fume hoods), and careful waste disposal. Should contamination occur, decontamination involves thorough cleaning or medical intervention. For irradiation, the focus is on reducing exposure to the external source. This is achieved through the 'ALARA' principle: As Low As Reasonably Achievable. This involves maximising the distance from the source, using shielding materials to absorb radiation, and minimising the time of exposure.

Production of Radio Waves (Higher)

Radio waves are generated by oscillating electrical circuits. When electrons within a conductor, such as an aerial in a transmitting antenna, are made to oscillate back and forth (i.e., accelerate) by an alternating current (AC) signal, they radiate energy in the form of electromagnetic waves. The frequency of the emitted radio wave is identical to the frequency of the oscillation of the electrons in the circuit. This principle is fundamental to radio transmission, where electrical signals are converted into radio waves.

Induction of Oscillations by Radio Waves (Higher)

Radio waves can induce oscillations in electrical circuits. When radio waves encounter a suitable conductor, such as a receiving aerial, the oscillating electric and magnetic fields within the wave exert forces on the free electrons in the conductor. This causes these electrons to oscillate at the same frequency as the incoming radio wave. This induced electron oscillation creates an alternating electrical current within the receiving circuit. This current is then detected and amplified to extract information.

Frequency Relationship (Higher)

The frequency of the radio waves produced or induced is directly determined by the frequency of the oscillations in the electrical circuit. If electrons in a circuit oscillate at 100 MHz, the radio waves produced have a frequency of 100 MHz. Similarly, a 50 MHz radio wave will induce electron oscillations at 50 MHz in a receiving circuit. This matching of frequencies is crucial for efficient power transfer and communication.

Electron Shells and Energy Levels

Electrons in an atom are arranged in distinct energy levels or 'shells' at different distances from the nucleus. Electrons in shells closer to the nucleus are held more tightly and have lower energy. Electrons in shells further from the nucleus possess higher energy. Each shell has a limited capacity for electrons. The specific arrangement of electrons in these shells defines an atom's electron configuration and influences its chemical behaviour.

Interaction with Electromagnetic Radiation

The electron arrangement within an atom is not static and can change when the atom absorbs or emits electromagnetic radiation. Absorption of radiation provides energy to an electron, causing it to jump from a lower energy level to a higher one, creating an excited atom. When an electron in a higher energy level falls back to a lower energy level, it releases the excess energy as electromagnetic radiation, often as light. The specific wavelengths emitted are characteristic of the element.

Formation of Ions by Electron Loss

Atoms can become positively charged ions, called cations, by losing one or more of their outer electrons. This process is known as ionisation. The loss of electrons leaves the atom with more positively charged protons than negatively charged electrons, resulting in a net positive charge. For example, a neutral atom with 11 protons and 11 electrons that loses one electron will become an ion with 11 protons and 10 electrons, giving it a +1 charge.

Differential Interaction of EM Waves (Higher)

Substances interact with electromagnetic (EM) waves in four main ways: absorption, transmission, refraction, and reflection. The specific interaction depends critically on both the properties of the substance and the wavelength of the incident EM wave. For example, materials that are transparent to visible light might be opaque to X-rays or microwaves. Understanding these interactions is fundamental to many applications, from designing optical lenses to medical diagnostics using different parts of the EM spectrum.

Absorption of EM Waves (Higher)

Absorption occurs when a material takes in the energy from an EM wave. This energy is typically converted into internal energy, often leading to a temperature increase. Different materials have specific absorption spectra, meaning they absorb certain wavelengths more efficiently than others. For example, the ozone layer absorbs most harmful ultraviolet (UV) radiation from the Sun, protecting life on Earth. Water effectively absorbs microwave radiation, which is why microwave ovens heat food containing water.

Transmission and Refraction of EM Waves (Higher)

Transmission is the process where EM waves pass through a material. Materials can be transparent (transmitting light readily), translucent (scattering light), or opaque (blocking light). The degree of transmission is wavelength-dependent; glass transmits visible light but absorbs most UV light. Refraction is the bending of EM waves as they cross a boundary between two different media, due to a change in their speed. The amount of bending varies with the wavelength for a given material, a phenomenon known as dispersion, which is observed when a prism splits white light into a rainbow.

Reflection of EM Waves (Higher)

Reflection is the bouncing back of EM waves from a surface. The type of reflection (specular or diffuse) depends on the smoothness of the surface. Smooth surfaces cause specular reflection, where light rays reflect at the same angle as they struck the surface (e.g., a mirror). Rough surfaces cause diffuse reflection, scattering light in many directions. The colour of an opaque object is determined by the wavelengths of visible light it reflects; it absorbs all other wavelengths. For instance, a red object reflects primarily red light and absorbs other colours.