Magnetism and magnetic fields Study Guide

Electromagnetic (EM) waves are different from sound waves or water waves because they do not require a material medium to travel. They are transverse waves composed of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation. This dual field structure allows them to carry energy through a vacuum, such as space, as well as through various materials.

When an EM wave is generated, for example, by the vibration of electrons in an atom (like in a light bulb) or by changes in a magnetic field (like in a radio transmitter), it carries energy away from the source. This energy is stored in the electric and magnetic fields. As the wave travels, these fields propagate through space, transferring the energy. When an EM wave encounters an absorber—a material or object—its energy can be transferred. This transfer occurs when the oscillating electric and magnetic fields interact with the charged particles (electrons and protons) within the absorber. These interactions cause the charged particles in the absorber to vibrate or accelerate, increasing their kinetic energy or converting the wave's energy into other forms, such as thermal energy (heat), electrical energy, or chemical energy.

For example, when sunlight (visible EM waves) hits a dark surface, the electrons in the surface absorb the energy, increasing their vibrations and leading to a rise in temperature. In a microwave oven, microwaves cause water molecules in food to vibrate rapidly, generating heat. In a radio receiver, radio waves induce an alternating current in an antenna, which is then processed to produce sound. Thus, the fundamental mechanism of energy transfer by EM waves involves the interaction of their oscillating electric and magnetic fields with the charged constituents of a material, leading to the absorption and conversion of the wave's energy.

Radioactive decay is a process where an unstable atomic nucleus loses energy by emitting radiation. This process changes the nucleus into a different nucleus (or an isotope of the same element, in the case of gamma decay sometimes). We can represent these changes using nuclear equations, which are similar to chemical equations but describe changes within the nucleus rather than the outer electron shells. For a nuclear equation to be balanced, two quantities must be conserved: the mass number (total number of protons and neutrons) and the atomic number (total number of protons). The mass number is written at the top left of the element symbol, and the atomic number is written at the bottom left. When a nucleus undergoes alpha decay, it emits an alpha particle (a helium nucleus, represented as ⁴₂He or ⁴₂α). This means the parent nucleus's mass number decreases by 4 and its atomic number decreases by 2. In beta-minus decay, a neutron in the nucleus changes into a proton, emitting a beta-minus particle (an electron, represented as ⁰₋₁e or ⁰₋₁β). The mass number remains unchanged, but the atomic number increases by 1. For gamma decay, the nucleus releases gamma rays (⁰₀γ), which are electromagnetic radiation, not particles. This type of decay does not change the mass number or atomic number of the nucleus; it simply releases excess energy from an excited nucleus. Understanding these changes allows us to predict the products of radioactive decay or identify the type of decay based on the parent and daughter nuclei.