Physics Definitions

Comprehensive vocabulary list covering definitions from Quantities and Measurements to Nuclear Physics based on the lecture notes.

Homogeneity of Equations

For an equation to be homogeneous, every term on both sides of the equal sign of the equation should have the same units.

Systematic Errors

Errors that result in all readings or measurements being always smaller or always larger than the true value by a fixed amount.

Random Errors

Errors that result in readings or measurements being scattered about a mean value.

Accuracy

The degree of closeness of average value of the measurements to the true value, which is affected by systematic error.

Precision

The degree of agreement between repeated measurements of the same quantity, which is affected by random error.

Scalar

A quantity that has magnitude only.

Vector

A quantity that has both magnitude and direction.

Distance

The length of path followed by an object.

Displacement

The distance moved in a specified direction from a reference point.

Speed

The rate of change of distance travelled with respect to time.

Average Speed

The distance travelled over total time taken.

Velocity

The rate of change of displacement with respect to time.

Average Velocity

The change in displacement over time taken.

Acceleration

The rate of change of velocity with respect to time.

Average Acceleration

The change in velocity over time taken.

Projectile Motion/Parabolic Motion

Motion where an object has constant velocity in one direction and constant vertical acceleration in the direction perpendicular to that constant velocity.

Newton’s 1st Law of motion

A body continues in its state of rest or uniform motion in a straight line, unless a resultant external force acts on it.

Inertia

The reluctance of a body to start moving or to change its motion once it has started, measured by the mass of the body.

Weight

A measure of the gravitational force that the Earth exerts on an object.

Equilibrium

The 2 conditions of equilibrium are: The resultant force in all directions is zero. The resultant torque about any axes is zero

Newton’s 3rd Law of motion

If a body A exerts a force on body B, then body B exerts an equal but opposite force of the same type on body A.

Momentum

The product of a body's mass $m$ and its velocity $v$, acting in the same direction as the velocity.

Newton’s 2nd Law of motion

The rate of change of momentum of a body is directly proportional to the resultant force acting on it and occurs in the direction of the force.

Impulse

The product of a force $F$ acting on an object and the time $\triangle t$ for which the force acts.

Conservation of momentum

The total momentum of a system remains constant, provided that the net external force acting on the system is zero.

Elastic Collision

A collision in which both total momentum and total kinetic energy are conserved.

Inelastic Collision

A collision in which total momentum is conserved but total kinetic energy is not conserved.

Perfectly Inelastic Collision

A collision where total momentum is conserved, but total kinetic energy is not conserved and particles stick together so their final velocities are the same.

Head-on Collision

A collision in which the centre of mass of the objects are collinear before and after the collision.

Hooke’s Law

The extension of a spring is proportional to the applied load if the limit of proportionality is not exceeded.

Density

The mass per unit volume of a substance.

Pressure

The force per unit area, where the force acts at right angles to the area.

Upthrust

The vertical upward force exerted on a body by a fluid when submerged, due to the pressure at the bottom being larger than at the top.

Archimedes Principle

States that the upthrust on a submerged object is equal to the weight of fluid displaced by the object.

Principle of Moments

For rotational equilibrium, the sum of clockwise moments about any point must equal the sum of anticlockwise moments about the same point.

Moment

The product of the force and the perpendicular distance from the point to the line of action of the force.

Couple

A pair of equal and opposite parallel forces whose lines of action do not coincide.

Torque of a couple

The product of one force and the perpendicular distance between the two forces.

Centre of Gravity

The point at which the weight of the body appears to act.

Work Done

The product of the force $F$ and the displacement $S$ in the direction of the force.

Joule

The work done by a force of $1\,N$ on an object when it is displaced by $1\,m$ in the direction of the force.

Principle of Conservation of Energy

The total energy of an isolated system is constant; it can be transformed or transferred but cannot be created or destroyed.

Power

The rate of work done or energy conversion with respect to time.

Field

A region of space in which a body experiences a force associated with the field.

Angular Displacement

The angle an object makes with respect to a reference line.

Radian

The angle subtended by an arc length equal to the radius of the arc.

Angular Velocity

The rate of change of angular displacement with respect to time.

Period (Circular Motion)

The time taken to make one complete revolution.

Frequency (Circular Motion)

The number of complete revolutions made per unit time.

Newton’s Law of Gravitation

States that two point masses attract with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Gravitational Field Strength

The gravitational force experienced per unit mass at a point in space.

Gravitational Potential Energy

The work done by an external agent to bring a small test point mass from infinity to a point in the field without acceleration.

Gravitational Potential

The work done per unit mass by an external agent to bring a small test point mass from infinity to a point in the field without acceleration.

Geostationary Satellite

A satellite at a fixed position relative to Earth's surface, with a period of $24\,hrs$, rotating eastwards above the Equator.

Simple Harmonic Motion

Motion about a fixed point where acceleration is directly proportional to displacement and directed opposite to the displacement.

Amplitude

The magnitude of the maximum displacement of a particle from its equilibrium position.

Free Oscillations

Oscillation occurring with no resistive or driving forces, maintaining constant total energy and amplitude.

Light Damping

Oscillations where amplitude decays exponentially with time and the frequency is slightly smaller than the undamped frequency.

Critical Damping

No oscillation occurs and the system returns to the equilibrium position in the shortest time.

Heavy Damping

No oscillation occurs and the system takes a long time to return to its equilibrium position.

Resonance

A phenomenon where driving frequency matches natural frequency, resulting in maximum amplitude and maximum energy transfer.

Progressive Wave

A disturbance that transfers energy from one point to another without the transfer of matter.

Wavelength

The distance between two consecutive points which are in phase, such as two successive crests or troughs.

Wave Intensity

The rate of transfer of energy per unit area normal to the direction of wave propagation.

Transverse Wave

A wave in which particles oscillate perpendicular to the direction of energy transfer.

Longitudinal Wave

A wave in which particles oscillate parallel to the direction of energy transfer.

Polarization

A phenomenon whereby the oscillations of a transverse wave are restricted to a single plane.

Principle of Superposition

When waves meet at a point, the resultant displacement is the vector sum of the displacements of the individual waves.

Stationary Wave

The result of superposing two progressive waves of identical frequency, amplitude, and speed travelling in opposite directions.

Diffraction

The spreading of waves after passing through an aperture or round an obstacle.

Coherence

Waves or sources that have the same frequency and a constant phase difference.

Rayleigh Criterion

Two images are just resolved when the peak intensity of one image lies on the first minimum of the other image.

Thermal Equilibrium

Occurs when two objects in thermal contact are at the same temperature and there is no net flow of heat between them.

Zeroth Law of Thermodynamics

If objects A and B are each in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other.

Ideal Gas

A gas obeying the equation of state $pV = nRT$ at all pressures, volumes, and temperatures.

Mole

The amount of substance that contains a number of particles equal to Avogadro’s constant.

Internal Energy

The sum of random distribution of microscopic kinetic energies and potential energies of all molecules in a system.

First Law of Thermodynamics

The increase in internal energy of a closed system equals the sum of the heat supplied to the system and the work done on the system.

Coulomb’s Law

The electric force between two point charges is proportional to the product of charges and inversely proportional to the square of the distance between them.

Electric Field Strength

The electric force exerted per unit positive charge placed at a point in the field.

Electric Potential

The work done per unit positive charge by an external agent in bringing a small test positive charge from infinity to a point without acceleration.

Capacitance

The ratio of the charge stored to the potential difference.

Potential Difference

The amount of electrical energy per unit charge converted to other forms of energy when charges pass between two points.

Electromotive Force (EMF)

The electrical energy per unit charge converted from other forms to drive charges around a complete circuit.

Ohm’s Law

The current through a metallic conductor is proportional to the potential difference across it under constant physical conditions.

Root-mean-square value

The value of steady direct current or voltage that dissipates energy at the same rate as the mean power dissipated by an a.c. in a resistor.

Magnetic Flux Density

Numerically equal to the magnetic force per unit length of a long straight conductor carrying unit current at right angles to the field.

Tesla

Uniform magnetic flux density where a wire carrying $1\,A$ experiences a force per unit length of $1\,N\,m^{-1}$ when acting normally.

Faraday’s Law

Induced emf is proportional to the rate of change of magnetic flux linkage.

Lenz’s Law

The direction of induced emf is such that it produces an induced magnetic field to oppose the change in magnetic flux linkage.

Photoelectric Effect

Emission of electrons from a metal surface when incident radiation frequency is higher than the threshold frequency.

Photon

A quantum of electromagnetic radiation with energy $E = hf$, where $h$ is Planck's constant.

Heisenberg Uncertainty Principle

States that the product of the uncertainty of measurement of position and momentum of a particle can never be smaller than Planck’s constant $h$.

Nuclide

A particular nucleus with a specified number of protons and neutrons.

Atomic mass unit (u)

Defined as $1/12^{th}$ of the mass of a carbon-12 atom.

Mass defect

The difference between the total mass of the separated nucleons and the mass of the nucleus.

Binding energy

The energy required to separate a nucleus into its constituent protons and neutrons to infinity.

Nuclear Fission

The splitting of a high nucleon number nucleus into two smaller nuclei of approximately equal mass with energy release.

Nuclear Fusion

The building up of a larger nucleus from two low nucleon number nuclei with energy release.

Half-life

The average time taken for the