Physics · 28 pages
Physics explainers
Clear, accurate explainers for the core concepts of physics — energy, force, momentum, gravity, electricity, waves and the quantum world — written for the general reader and grounded in SI and internationally agreed definitions. Each page leads with a concise answer and links across to the wider CASRAI science explainers and standards dictionary.
Browse the topic
All 28 physics explainers pages
Nuclear fission vs nuclear fusion
Nuclear fission splits a heavy atomic nucleus, such as uranium-235, into two lighter nuclei, releasing energy and neutrons. Nuclear fusion joins two light nuclei, such as hydrogen isotopes, into a heavier nucleus, releasing even more energy per unit mass. Both convert a tiny amount of mass into energy following E = mc². Fission powers today’s nuclear plants; fusion powers the Sun and stars.
ComparisonPotential vs kinetic energy
Potential energy is stored energy that an object has because of its position, shape or state — such as a raised weight or a stretched spring. Kinetic energy is the energy an object has because of its motion. The two are interchangeable: as a falling object speeds up, potential energy converts into kinetic energy. Both are measured in joules and are forms of mechanical energy.
ComparisonMass vs weight
Mass is the amount of matter in an object, measured in kilograms, and it does not change with location. Weight is the force that gravity exerts on that mass, measured in newtons, and it varies depending on the local strength of gravity. On the Moon an object keeps the same mass but weighs about one-sixth as much, because lunar gravity is weaker than Earth’s.
ComparisonAC vs DC current
Direct current (DC) flows in one constant direction, as from a battery. Alternating current (AC) reverses direction many times per second, following a wave. AC is used for mains electricity because it can be efficiently stepped up and down in voltage by transformers for long-distance transmission. DC is used by batteries, electronics and solar panels, which need a steady one-way flow.
ComparisonSpeed vs velocity
Speed is how fast an object is moving — a scalar quantity with magnitude only. Velocity is speed in a stated direction — a vector quantity with both magnitude and direction. A car at 50 km/h has a speed; at 50 km/h due north it has a velocity. Because velocity includes direction, an object circling at constant speed has a changing velocity.
ComparisonDistance vs displacement
Distance is the total length of the path an object travels — a scalar quantity with magnitude only. Displacement is the straight-line change in position from start to finish, together with its direction — a vector quantity. If you walk 3 km around a park and end where you began, your distance is 3 km but your displacement is zero. Displacement can never exceed distance.
ComparisonScalar vs vector
A scalar is a quantity with magnitude (size) only, such as mass, temperature, time, speed or energy. A vector has both magnitude and direction, such as displacement, velocity, acceleration and force. The key test is whether direction is part of the quantity: if it is, it is a vector; if not, a scalar. Vectors add geometrically, not just numerically.
DefinitionAtom
An atom is the smallest unit of a chemical element that retains that element’s identity. It has a tiny, dense central nucleus containing positively charged protons and neutral neutrons, surrounded by a cloud of negatively charged electrons. The number of protons, called the atomic number, defines which element the atom is. Atoms are the building blocks of all ordinary matter and join together to form molecules.
DefinitionAcceleration
Acceleration is the rate at which an object’s velocity changes with time. Because velocity includes direction, an object accelerates whenever it speeds up, slows down or changes direction. It is a vector quantity measured in metres per second squared (m/s²). A car pulling away, braking, or turning a corner is accelerating. The SI unit reflects a change of velocity (m/s) happening each second.
DefinitionThermodynamics
Thermodynamics is the branch of physics concerned with heat, energy, work and temperature, and how energy is transferred and transformed. It is governed by four laws: energy is conserved, total entropy (disorder) tends to increase, you cannot reach absolute zero, and systems in contact reach thermal equilibrium. These laws explain why engines, refrigerators and living things behave as they do.
DefinitionTheory of relativity
The theory of relativity is Einstein’s description of space, time and gravity, in two parts. Special relativity (1905) shows that the speed of light is constant for all observers, so time and space are relative to motion. General relativity (1915) explains gravity not as a force but as the curving of spacetime by mass and energy. Both have been confirmed by precise experiments.
DefinitionFriction
Friction is the force that opposes the relative motion of two surfaces touching each other. It acts along the surfaces, opposite to motion or attempted motion, arising from roughness and molecular attraction between them. Friction lets us walk, grip and brake, but also wastes energy as heat. Its main types are static, kinetic, rolling and fluid friction.
DefinitionSpeed of light
The speed of light in a vacuum, written c, is exactly 299,792,458 metres per second — about 300,000 kilometres per second. It is a fundamental constant of nature: the same for every observer regardless of their motion, and the maximum speed at which matter, energy or information can travel. Since 1983 this fixed value defines the metre.
DefinitionElectricity
Electricity is the range of physical phenomena caused by electric charge. It includes static electricity (charge at rest) and electric current (charge in motion, usually electrons flowing through a conductor). A current flows when a voltage, or potential difference, pushes charge around a complete circuit. Measured in amperes, volts and watts, electricity carries energy that can be converted into light, heat, motion and sound.
DefinitionGravity
Gravity is the force of attraction between any two objects that have mass. The greater the masses and the closer they are, the stronger the pull. Gravity holds us on the ground, gives objects their weight, and keeps the Moon and planets in orbit. Einstein’s general relativity reframed it as the curving of spacetime by mass and energy.
DefinitionQuantum physics
Quantum physics, also called quantum mechanics, is the branch of physics that describes nature at the scale of atoms and subatomic particles. At this scale energy comes in discrete packets called quanta, particles such as electrons also behave like waves, and outcomes are described by probabilities rather than certainties. It underpins chemistry, lasers, transistors and modern electronics, and differs sharply from the physics of everyday objects.
DefinitionEnergy
Energy is the capacity to do work or cause change. It comes in many forms — kinetic, potential, thermal, chemical, electrical, nuclear and radiant — and can convert from one to another. The total energy in an isolated system stays constant: energy is never created or destroyed, only transformed. It is measured in joules (J).
DefinitionForce
A force is a push or a pull acting on an object, able to change its speed, direction or shape. It is a vector quantity, with both magnitude and direction, measured in newtons (N). A net force causes a mass to accelerate, with the force equal to mass times acceleration. Forces include gravity, friction, tension, electromagnetism and the contact push between touching surfaces.
DefinitionMomentum
Momentum is a measure of an object’s motion, equal to its mass multiplied by its velocity. It is a vector quantity, pointing in the direction of motion, with SI units of kilogram metres per second (kg·m/s). A heavy or fast object has large momentum and is hard to stop. With no outside force, total momentum is conserved.
DefinitionVelocity
Velocity is the rate of change of an object’s position in a stated direction. It is a vector quantity, combining how fast something moves (its speed) with the direction of motion, measured in metres per second (m/s). Velocity differs from speed, which has no direction. Because direction is included, an object moving at a steady speed around a curve has a continually changing velocity.
DefinitionWork in physics
In physics, work is done when a force moves an object through a distance in the direction of the force. It equals the force multiplied by the distance moved in that direction, and is measured in joules (J). Work transfers energy: doing work on an object changes its energy. No movement means no work is done, however hard you push.
DefinitionPower in physics
In physics, power is the rate at which work is done or energy is transferred. It tells you how quickly energy is used, not how much. Power equals energy transferred divided by the time taken, measured in watts (W), where one watt is one joule per second. A more powerful device does the same work in less time.
DefinitionMagnetism
Magnetism is a physical force of attraction or repulsion that arises from moving electric charges and from magnetic materials such as iron. Every magnet has two poles, north and south; like poles repel and opposite poles attract. Magnets are surrounded by a magnetic field that exerts this force. Magnetism and electricity are two aspects of a single force, electromagnetism, and each can produce the other.
DefinitionSound waves
Sound waves are vibrations that travel through a medium — a gas, liquid or solid — as compressions and rarefactions of its particles. They are longitudinal waves: the particles vibrate back and forth along the wave’s direction of travel. Sound needs a medium, so it cannot cross a vacuum. Its frequency sets the pitch and its amplitude sets the loudness.
DefinitionElectromagnetic spectrum
The electromagnetic spectrum is the full range of electromagnetic waves, arranged by wavelength and frequency. From longest to shortest wavelength, it runs through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All these waves travel at the speed of light in a vacuum and differ only in wavelength and energy. Visible light is just the tiny band our eyes can detect.
DefinitionNuclear energy
Nuclear energy is the energy stored in the nucleus of an atom, held by the strong force that binds protons and neutrons together. It is released when heavy nuclei split (fission) or light nuclei join (fusion), converting a tiny amount of mass into energy following E = mc². Nuclear power stations use controlled fission to generate electricity, producing large amounts of energy from very little fuel.
DefinitionInertia
Inertia is the tendency of an object to resist changes to its motion: an object at rest stays at rest, and a moving object keeps moving at constant velocity, unless a net force acts on it. Inertia depends only on mass — the greater the mass, the greater the inertia. This idea is the basis of Newton’s first law of motion.
DefinitionWavelength
Wavelength is the distance between two consecutive corresponding points on a wave, such as crest to crest or compression to compression. It is the length of one full cycle, given the symbol λ (lambda) and measured in metres. Wavelength is inversely related to frequency: the higher the frequency, the shorter the wavelength. For waves of a given type, longer wavelengths carry lower energy.
