Year 12 Physics Simplified Recall Answers

A complete set of 56 vocabulary flashcards covering key definitions, laws, models, and concepts from Year 12 Physics.

Normal force

The contact force exerted by a surface, acting perpendicular ($90^{\circ}$) to that surface.

Uniform circular motion

Motion where an object moves in a circle at constant speed, its velocity continually changes direction, and the net force acts towards the centre of the circle.

Average speed

Total distance travelled divided by total time taken; it is a scalar quantity, measured in $\text{m\,s}^{-1}$.

Period (circular motion)

The time taken for one complete revolution, denoted by $T$ and measured in seconds ($\text{s}$).

Centripetal force

The resultant force directed towards the centre of a circular path that keeps an object moving in that path, measured in newtons ($\text{N}$).

Centripetal acceleration

The acceleration of an object moving in a circle, directed towards the centre, measured in $\text{m\,s}^{-2}$.

Newton’s Law of Universal Gravitation

States that any two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres: $F = \frac{G m_1 m_2}{r^2}$.

Gravitational field

A region of space where a mass experiences a gravitational force.

Kepler’s Laws of Planetary Motion

Three laws of planetary motion: 1. Planets orbit the Sun in ellipses, with the Sun at one focus; 2. A line from a planet to the Sun sweeps out equal areas in equal times; 3. The square of orbital period is proportional to the cube of the orbit's mean radius ($T^2 \propto r^3$).

Coulomb’s Law

States that the electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of their separation: $F = \frac{1}{4\pi \varepsilon_0} \frac{|q_1 q_2|}{r^2}$ in a vacuum, where like charges repel and opposite charges attract.

Electric field

A region of space in which an electric charge experiences an electric force.

Electric field strength

The electric force per unit positive test charge at a point: $E = \frac{F}{q}$, with the unit $\text{N\,C}^{-1}$.

Electrical potential energy

The energy a charge has because of its position in an electric field, measured in joules ($\text{J}$).

Magnetic field

A region of space where a magnetic force can be experienced.

Magnetic flux

A measure of the magnetic field passing through an area, denoted by $\Phi$, with the unit weber ($\text{Wb}$).

Magnetic flux density

The magnetic flux per unit area perpendicular to the field, denoted by $B$, with the unit tesla ($\text{T}$), equivalent to $\text{Wb\,m}^{-2}$.

Electromagnetic induction

The production of an electromotive force (EMF) when magnetic flux linkage changes.

Electromotive force (EMF)

Energy transferred per unit charge, or the potential difference that drives current, measured in volts ($\text{V}$).

Faraday’s Law

States that the induced EMF is proportional to the rate of change of magnetic flux linkage: $\varepsilon = -N \frac{\Delta \Phi}{\Delta t}$ (for average EMF).

Lenz’s Law

States that the induced current flows in a direction that produces a magnetic field opposing the change in magnetic flux that caused it.

Induced EMF across a moving conductor

The potential difference produced as a conductor moves through a magnetic field, causing charges to experience a magnetic force and separate; if part of a closed circuit, an induced current flows.

Transformer

A device where alternating current in the primary coil creates a changing magnetic flux in a shared iron core, linking the secondary coil and inducing an alternating EMF ($V/V = N/N$ for an ideal transformer).

Electromagnetic radiation

Changing electric and magnetic fields that sustain each other and travel as a transverse wave, oscillating in phase, perpendicular to each other, and perpendicular to the direction of travel.

Muon decay

Evidence for special relativity where atmospheric muons reach the ground because in Earth’s frame their moving clocks are time-dilated (living about $23\,\mu\text{s}$ rather than $2.2\,\mu\text{s}$), while in the muon’s frame the atmospheric distance is length-contracted.

Frame of reference

A coordinate system used to describe position and motion.

Inertial reference frame

A non-accelerating reference frame in which Newton’s laws of motion hold.

Postulates of Special Relativity

1. The laws of physics are the same in all inertial reference frames; 2. The speed of light in a vacuum is the same for all inertial observers, regardless of the motion of the source or observer.

Simultaneity

The condition where two events occur at the same time in a specified reference frame; observers in relative motion may disagree about whether separated events are simultaneous.

Time dilation

The effect where a clock moving relative to an observer is measured to run slower than a clock at rest in that observer’s frame.

Mass-energy equivalence

The principle that mass and energy are equivalent, expressed by $E = m c^2$, where $c$ is the speed of light in a vacuum.

Speed of light limit

The principle that no object with mass can travel at the speed of light because the energy required to accelerate it further increases without limit, requiring infinite energy.

Twin paradox

A relativistic scenario where a travelling twin returns younger than the stay-at-home Earth twin because the traveller experiences less elapsed proper time and changes reference frames.

Ladder-in-the-barn paradox

A paradox where a fast-moving ladder is length-contracted to fit inside a barn in the barn frame, while the barn is contracted in the ladder frame; resolved because observers disagree about simultaneity.

Young’s double-slit experiment

An experiment supporting the wave model of light where two coherent light waves overlap to form an interference pattern of bright bands (constructive interference) and dark bands (destructive interference).

Black-body radiation

The electromagnetic radiation emitted by an ideal object that absorbs all incident radiation, with a spectrum that depends on the object’s temperature.

Ultraviolet catastrophe

The failure of classical theory that predicted an infinite intensity at short wavelengths for black-body radiation, which Planck resolved by proposing that energy is emitted or absorbed in discrete packets, or quanta.

Photon

A discrete quantum (packet) of electromagnetic radiation, with energy given by $E = h f$.

Photoelectric effect

The ejection of electrons from a material when light shines on it; an electron absorbs a photon and is emitted if the photon energy is at least the work function, with one photon transferring its energy to one electron.

Threshold frequency

The minimum light frequency, denoted by $f_0$, that can eject electrons from a material.

Planck’s constant

The constant relating photon energy to frequency in $E = h f$, denoted by $h$ ($h \approx 6.63 \times 10^{34}\,\text{J\,s}$ as recorded in the notes).

Work function

The minimum energy needed to remove an electron from a material, given by $W = h f_0$ and measured in joules ($\text{J}$).

Wave-particle duality of light

The property where light exhibits wave-like behaviour (e.g. Young's double-slit interference) and particle-like behaviour (e.g. the photoelectric effect, black-body radiation, and the Compton effect) depending on how it is measured.

Rutherford’s atomic model

An atomic model featuring a tiny, dense, positively charged nucleus with electrons around it, which could not explain why orbiting electrons do not lose energy and spiral into the nucleus, or the discrete line spectra.

Bohr model

An atomic model where electrons occupy allowed stationary orbits with specific energies and do not radiate energy while in them, absorbing or emitting photons only when changing energy levels.

Hydrogen line spectrum (Bohr model explanation)

Spectral lines produced when hydrogen's electron moves between specific energy levels, absorbing a photon to move higher and emitting a photon of energy $\Delta E = h f$ when dropping lower.

Elementary particle

A particle that has no known smaller constituents.

Antiparticle

A particle with the same mass as its corresponding particle, but opposite electric charge and opposite values of other relevant additive quantum numbers.

Quarks

The six elementary particles (up, down, charm, strange, top, and bottom) that experience the strong, weak, and (if electrically charged) electromagnetic forces.

Baryon

A hadron composed of three quarks (or three antiquarks for an antibaryon).

Meson

A hadron composed of one quark and one antiquark.

Leptons

The six elementary particles: electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino; they experience the weak force, and charged leptons experience the electromagnetic force.

Gauge bosons

Force-mediating particles: gluons (strong nuclear force), $W$, $W$, and $Z$ bosons (weak nuclear force), and photons (electromagnetic force).

Lepton number

The number of leptons minus the number of antileptons.

Baryon number

The number of baryons minus antibaryons, with quarks assigned $+\frac{1}{3}$ and antiquarks assigned $-\frac{1}{3}$.

Conservation of lepton and baryon numbers

The rule that in particle interactions, total lepton number and total baryon number are conserved, meaning their totals before and after the interaction are equal.

Symmetry in particle interactions

Charge-reversal ($\text{C}$), time-reversal ($\text{T}$), and crossing (or spatial-reflection/parity) symmetries that describe interactions, imply conservation rules, and provide tests of particle-physics models through observed violations.