HKDSE Physics flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Newton's second law

The net force on a body equals the product of its mass and acceleration, F = ma. Force and acceleration are vectors in the same direction, so resolve components first.

Conservation of momentum

In a system with no net external force, total momentum before an interaction equals total momentum after. It applies to elastic and inelastic collisions and to explosions.

Centripetal force

The resultant force directed toward the centre of a circular path, of magnitude mv²/r. It is provided by tension, gravity, friction or another real force, not an extra force.

Principle of moments

For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments. Each moment is force times perpendicular distance.

Projectile motion

Motion under gravity alone with constant horizontal velocity and vertical acceleration g. Horizontal and vertical components are treated independently, linked only by time.

Elastic collision

A collision in which both momentum and kinetic energy are conserved. In inelastic collisions momentum is still conserved but some kinetic energy converts to other forms.

Specific latent heat

The energy needed to change the state of one kilogram of a substance without changing its temperature, given by energy = mL. For melting it is latent heat of fusion.

Specific heat capacity

The energy required to raise the temperature of one kilogram of a substance by one kelvin, measured in J kg⁻¹ K⁻¹. Energy transferred equals mcΔT.

Ideal gas law

The relation pV = nRT for an ideal gas, where T is absolute temperature in kelvin and R is the molar gas constant. It combines Boyle's, Charles's and the pressure laws.

Internal energy of a gas

The total kinetic and potential energy of the molecules of a gas. For an ideal gas it depends only on temperature, since intermolecular potential energy is taken as zero.

Conduction

Transfer of thermal energy through a material by molecular vibration and by movement of free electrons, without bulk movement of the material. Metals conduct well due to free electrons.

Convection

Transfer of thermal energy by the bulk movement of a heated fluid. Warmer, less dense fluid rises and cooler fluid sinks, setting up convection currents. It cannot occur in solids.

Evaporation

The escape of the most energetic molecules from a liquid surface at any temperature. It cools the remaining liquid and is faster with larger surface area, higher temperature and air movement.

Wave equation

The relation v = fλ linking wave speed, frequency and wavelength. When a wave crosses into another medium, frequency stays constant while speed and wavelength change.

Snell's law

For refraction at a boundary, n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. Light entering a denser medium bends toward the normal.

Critical angle

The angle of incidence in the denser medium for which the refracted ray grazes along the boundary at 90 degrees. It satisfies sin C = 1/n.

Total internal reflection

Occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle. No light refracts out; all is reflected back.

Longitudinal wave

A wave in which particle displacement is parallel to the direction of energy transfer, producing compressions and rarefactions. Sound in air is a longitudinal wave.

Stationary wave node

A point of zero displacement in a stationary wave, where destructive interference always occurs. Adjacent nodes are separated by half a wavelength.

Thin lens equation

The relation 1/f = 1/u + 1/v for a thin lens using the real-is-positive convention. Magnification equals image distance divided by object distance, m = v/u.

Ohm's law

For an ohmic conductor at constant temperature, current is directly proportional to potential difference, V = IR. Resistance is measured in ohms.

Resistors in parallel

The reciprocal of the combined resistance equals the sum of reciprocals of individual resistances. The potential difference across each branch is the same and current divides.

Electrical power

The rate at which electrical energy is transferred, given by P = VI = I²R = V²/R. Energy consumed equals power multiplied by time, billed in kilowatt-hours.

Kirchhoff's loop law

Around any closed circuit loop, the algebraic sum of potential differences is zero. It expresses conservation of energy for charges moving round the circuit.

Coulomb's law

The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation. It is attractive for opposite charges.

Force on a current-carrying wire

A straight wire of length L carrying current I in a magnetic field B experiences force F = BIL sin θ, where θ is the angle between wire and field.

Electromagnetic induction

A changing magnetic flux through a circuit induces an e.m.f. Faraday's law states the e.m.f. equals the rate of change of flux linkage.

Alpha particle

A helium nucleus of two protons and two neutrons, emitted in alpha decay. It is strongly ionising, has short range in air and is stopped by paper.

Beta-minus decay

A neutron in the nucleus converts to a proton, emitting an electron and an antineutrino. The atomic number increases by one while the mass number stays unchanged.

Half-life

The time for half the undecayed nuclei in a sample to decay. After n half-lives the remaining fraction is (1/2)ⁿ, independent of the initial amount.

Radioactive decay law

The number of undecayed nuclei decreases exponentially: N = N₀e^(−λt), where λ is the decay constant. Activity equals λN and also decays exponentially.

Mass defect

The difference between the total mass of separate nucleons and the mass of the assembled nucleus. It corresponds to the binding energy by E = mc².

Nuclear fission

A heavy nucleus splits into lighter nuclei after absorbing a neutron, releasing energy and further neutrons. This can sustain a chain reaction in a reactor.