MATSEC AM 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 resultant force on a body equals the rate of change of its momentum; for constant mass this reduces to F = ma, with force in newtons, mass in kilograms and acceleration in m s⁻².

Young modulus

The ratio of tensile stress to tensile strain in the linear region of a stress–strain graph: E = (F/A)/(Δl/l). Units are pascals; a steeper graph means a stiffer material.

Elastic potential energy

Energy stored in a stretched or compressed spring obeying Hooke's law: ½Fx or ½kx², equal to the area under the force–extension graph up to the extension used.

Centripetal acceleration

For uniform circular motion the acceleration points to the centre with magnitude v²/r or rω²; the required force is mv²/r and does no work on the object.

Conservation of momentum

In a closed system with no external resultant force, total momentum before an interaction equals total momentum after. It holds for both elastic and inelastic collisions.

Principle of moments

A body in rotational equilibrium has zero resultant moment: the sum of clockwise moments about any pivot equals the sum of anticlockwise moments about that same pivot.

Resistivity formula

Resistance of a uniform conductor is R = ρl/A, where ρ is resistivity in ohm metres, l the length and A the cross-sectional area; doubling length doubles resistance.

Internal resistance

A real cell has internal resistance r, so terminal p.d. V = ε − Ir. As current drawn increases, terminal voltage falls; lost volts equal Ir.

Capacitor discharge

Charge decays exponentially: Q = Q₀e^(−t/RC). The time constant RC is the time for charge to fall to about 37% of its initial value.

Faraday's law of induction

The induced EMF in a circuit equals the negative rate of change of magnetic flux linkage: ε = −N dΦ/dt. Faster flux change or more turns gives a larger EMF.

Lenz's law

The direction of an induced current always opposes the change producing it, a consequence of energy conservation; it explains magnetic braking and the opposing force on a moving magnet.

Force on a moving charge

A charge q moving with speed v at angle θ to a magnetic field B experiences F = Bqv sinθ, directed perpendicular to both v and B by Fleming's left-hand rule.

Potential divider

Two resistors in series across a supply share voltage in proportion to resistance: V_out = V_in × R₂/(R₁ + R₂). Used for sensing and variable output.

Wave equation

For any wave, speed equals frequency times wavelength: v = fλ. Frequency is set by the source and does not change on refraction; speed and wavelength do.

Stationary wave condition

A string fixed at both ends supports stationary waves only when length L = nλ/2, giving harmonics f = nv/(2L). Nodes are points of zero displacement.

Diffraction grating equation

Bright maxima occur at angles satisfying d sinθ = nλ, where d is the slit spacing and n the order. Larger wavelength or smaller spacing gives wider spacing of maxima.

Coherence

Two sources are coherent if they have a constant phase difference and the same frequency; only then is a stable interference pattern observed. Laser light is highly coherent.

Doppler effect

The observed frequency differs from the emitted frequency when source and observer approach or recede. Approach raises observed frequency; recession lowers it, with shift proportional to relative speed.

Intensity and amplitude

Intensity is power per unit area and is proportional to the square of the amplitude. For a point source in three dimensions, intensity falls as 1/r².

Snell's law

At a boundary, n₁ sinθ₁ = n₂ sinθ₂, where angles are measured from the normal. Light bends towards the normal when entering a denser medium.

Critical angle

The angle of incidence in the denser medium for which the refracted ray grazes the boundary: sinθ_c = n₂/n₁. Beyond it, total internal reflection occurs.

Thin lens equation

1/f = 1/u + 1/v with sign conventions applied consistently; magnification m = v/u. A converging lens forms a real image when the object is beyond the focal point.

Specific latent heat

Energy needed to change the state of unit mass at constant temperature: E = mL. No temperature change occurs during melting or boiling because energy breaks bonds.

Kinetic theory pressure

For an ideal gas pV = ⅓Nm⟨c²⟩, where N is the number of molecules and ⟨c²⟩ the mean square speed. Mean kinetic energy per molecule is (3/2)kT.

First law of thermodynamics

ΔU = Q − W, where Q is heat added to the system and W is work done by the system. For an isothermal process ΔU = 0, so Q = W.

Carnot efficiency

The maximum possible efficiency of a heat engine operating between hot and cold reservoirs at absolute temperatures: η = 1 − T_c/T_h. No real engine can exceed this.

Photoelectric equation

hf = φ + KE_max, where φ is the work function. Below the threshold frequency f₀ = φ/h no emission occurs, whatever the intensity.

de Broglie wavelength

Every particle has an associated wavelength λ = h/p = h/(mv). It is significant only for very small masses, as in electron diffraction experiments.

Radioactive decay law

The number of undecayed nuclei falls as N = N₀e^(−λt), with activity A = λN and half-life t½ = ln2/λ. Decay is random and spontaneous.

Half-life

The time for half the nuclei in a sample to decay, or for activity to halve. After n half-lives the remaining fraction is (½)ⁿ, independent of initial quantity.

Combining uncertainties

For sums and differences add absolute uncertainties; for products, quotients and powers add percentage uncertainties. Multiplying by a constant multiplies the absolute uncertainty by that constant.

Mass–energy equivalence

Mass and energy are equivalent through E = mc². In nuclear reactions the mass defect appears as released energy, for example about 200 MeV per uranium fission.

Stopping potential

The reverse p.d. that just prevents the photocurrent in a photoelectric experiment: eV_s = KE_max. It depends on frequency, not on light intensity.