Cambridge IAL Physics flashcards

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

SI unit of force

The newton (N) is the force that gives a 1 kg mass an acceleration of 1 m s^-2. In base units, 1 N = 1 kg m s^-2.

Principle of conservation of momentum

For a system with no resultant external force, the total momentum before an interaction equals the total momentum after it. Momentum is a vector, so directions must be assigned signs.

Centripetal acceleration formula

For circular motion at speed v and radius r, the centripetal acceleration is a = v²/r, directed toward the centre. It changes direction, not speed, of the object.

Conditions for two-source interference

Coherent sources maintain a constant phase difference. Constructive interference needs path difference nλ; destructive interference needs path difference (n + ½)λ.

Diffraction grating equation

For a grating of slit spacing d, maxima appear at angles given by d sinθ = nλ, where n is the order. Larger n gives a larger diffraction angle.

Photoelectric effect key result

Electron emission depends on frequency, not intensity. Maximum kinetic energy is hf − φ, where φ is the work function, and the threshold frequency satisfies hf₀ = φ.

Ohm's law and resistance

Resistance R = V/I. Ohm's law states that current is proportional to potential difference for a conductor at constant temperature; the I–V graph is then a straight line.

Kirchhoff's circuit laws

The sum of currents entering a junction equals the sum leaving it (conservation of charge). Around any closed loop, the sum of emfs equals the sum of potential differences.

Ideal gas equation

pV = nRT = NkT, where n is moles, N is molecules, R is the molar gas constant and k is the Boltzmann constant. It assumes negligible molecular volume and no intermolecular forces.

First law of thermodynamics

The internal energy change of a system equals the heat added to it plus the work done on it: ΔU = q + w. Sign conventions must be stated consistently.

Radioactive decay law

Activity A = λN, where λ is the decay constant. Activity and undecayed nuclei both fall exponentially: N = N₀e^(−λt), with half-life t½ = ln2/λ.

Magnetic force on a moving charge

A charge q moving at velocity v in a magnetic field B experiences force F = Bqv sinθ, perpendicular to both v and B. No work is done, so speed stays constant.

Simple harmonic motion condition

SHM requires acceleration proportional and opposite to displacement: a = −ω²x. The period is T = 2π/ω, independent of amplitude for ideal SHM.

De Broglie wavelength

Particles show wave behaviour with wavelength λ = h/p, where p is momentum. Electron diffraction through atomic spacing provides experimental confirmation of this relation.

PET scanner working principle

A positron-emitting tracer produces positrons that annihilate with electrons, emitting two 511 keV gamma photons at about 180° apart. Coincident detection locates the source.

Particle accelerator bending radius

A charged particle in a magnetic field follows a circular path with radius r = p/(Bq). Stronger fields or lower momentum give tighter curves, setting the ring size.

Superconductor applications

Below a critical temperature, resistance falls to zero and magnetic flux is expelled. This supports high-field electromagnets for MRI and reduces energy loss in power transmission.

Critical angle for total internal reflection

Light travelling from a denser to a less dense medium is totally internally reflected when the incident angle exceeds C, where sin C = n₂/n₁. This underpins optical fibres.

LED emission condition

An LED emits photons when electrons and holes recombine across the band gap, giving photon energy ≈ eV, where V is the forward voltage. Colour depends on the semiconductor material.

Nuclear fission moderator role

A moderator such as water or graphite slows fast neutrons to thermal energies, increasing the probability of further fission of uranium-235. Control rods absorb surplus neutrons.

Wind turbine power scaling

Available power in the wind is proportional to the swept area and to the cube of wind speed. Doubling wind speed increases the available power by a factor of about eight.

Ultrasound pulse-echo ranging

A piezoelectric transducer sends a pulse and detects the reflection. Depth is found from distance = speed × time / 2, using the speed of sound in tissue.

X-ray attenuation and absorption

X-ray beam intensity decreases with material thickness and density. Higher atomic number and density absorb more, giving contrast between bone and soft tissue in imaging.

Combining percentage uncertainties

When quantities are multiplied or divided, add their percentage uncertainties. When a quantity is raised to a power n, multiply its percentage uncertainty by n.

Systematic versus random error

Systematic error shifts every reading in the same direction, for example a zero error, and is not reduced by averaging. Random error scatters readings and is reduced by repeats.

Uncertainty from a graph gradient

Draw the steepest and shallowest straight lines that remain consistent with the plotted error bars. Half the difference between their gradients gives the uncertainty.

Linearising a relationship

Rearrange a relationship into the form y = mx + c, then plot the appropriate variables. A straight-line graph confirms the relationship and the gradient gives the constant.

Reading uncertainty of a ruler

A metre rule with 1 mm divisions gives a single reading uncertainty of ±0.5 mm. A length found from two readings carries ±1 mm, and percentage uncertainty falls for larger lengths.

Precision and accuracy distinction

Precision is the closeness of repeated readings to each other; accuracy is closeness to the true value. Repeated readings improve precision but cannot remove a systematic error.

Significant figures in results

Quote a final result to the same number of significant figures as the least precise measurement used, usually two or three. Keep extra figures during intermediate calculation.

Testing a relationship experimentally

Compare the experimental value with the accepted value using percentage difference. If the difference is within the combined percentage uncertainty, the results are consistent with the theory.