JCE Physics flashcards

32 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 an object equals its mass times acceleration, ΣF = ma. The acceleration is in the same direction as the net force and inversely proportional to mass.

Static versus kinetic friction

Static friction takes any value up to μsN and prevents sliding. Once sliding starts, kinetic friction equals μkN and acts opposite to the relative motion.

Work–energy theorem

The net work done on an object equals its change in kinetic energy, W_net = ½mv² − ½mu². Forces perpendicular to the displacement do no work.

Conservation of mechanical energy

When only conservative forces act, KE + PE stays constant. If friction or applied forces do work, the change in mechanical energy equals that non-conservative work.

Elastic collision

A collision in which total kinetic energy is conserved as well as momentum. In one dimension with equal masses, the bodies exchange velocities when one is initially at rest.

Centripetal acceleration

For motion in a circle, acceleration points to the centre with magnitude v²/r or ω²r. It changes the direction of velocity, not the speed.

Projectile motion

With negligible air resistance, horizontal velocity is constant and vertical motion has acceleration g downward. Time of flight depends only on the vertical component.

Orbital speed

The speed needed for a circular orbit of radius r is v = √(GM/r). It does not depend on the satellite's mass, only on the central mass and radius.

Ideal gas law

pV = nRT relates pressure, volume, amount in moles and absolute temperature. Temperature must be in kelvin; R is the universal gas constant.

First law of thermodynamics

ΔU = Q − W, where W is work done by the gas. Heat added raises internal energy unless the gas does equal work on its surroundings.

Carnot efficiency

The maximum efficiency of a heat engine between reservoirs is 1 − T_C/T_H, with temperatures in kelvin. Real engines always fall below this limit.

Latent heat

Energy absorbed or released during a phase change at constant temperature, Q = mL. The specific latent heat differs for melting and vaporisation.

Wave speed relation

For any periodic wave, speed equals frequency times wavelength, v = fλ. Passing into a new medium changes speed and wavelength but not frequency.

Total internal reflection

Occurs when light in a denser medium strikes a boundary beyond the critical angle θc, where sinθc = n₂/n₁. No light refracts out of the medium.

Doppler effect

The observed frequency rises when source and observer approach and falls when they recede. For sound the shift depends on motion relative to the medium.

Electric field of a point charge

E = kq/r², directed radially away from positive charge and toward negative charge. The field strength falls with the square of distance.

Capacitor energy

The energy stored in a capacitor is U = ½CV² = ½QV = Q²/2C. It is released when the capacitor discharges through a circuit.

Resistance formula

R = ρL/A: resistance rises with length and resistivity and falls with cross-sectional area. Resistivity is a property of the material, not the shape.

Series versus parallel circuits

In series, current is the same and voltages add. In parallel, voltage is the same and currents add, so equivalent resistance is lower than any branch.

Kirchhoff's loop rule

Around any closed circuit loop, the sum of potential differences is zero. This expresses conservation of energy for charge carriers moving through the circuit.

Magnetic force on moving charge

F = qvB sinθ, perpendicular to both velocity and field. A charge moving parallel to the field, or at rest, experiences no magnetic force.

Faraday's law of induction

An induced emf equals the negative rate of change of magnetic flux, ε = −dΦ/dt. Faster flux change or more turns gives a larger emf.

Lenz's law

The induced current flows in the direction that opposes the change in magnetic flux that produced it. It is a consequence of energy conservation.

Transformer equation

For an ideal transformer, V_s/V_p = N_s/N_p and power is conserved, so current steps down when voltage steps up. It requires alternating current.

Photoelectric equation

Maximum photoelectron kinetic energy is hf − φ, where φ is the work function. Below the threshold frequency no electrons are emitted regardless of intensity.

de Broglie wavelength

Every particle has wavelength λ = h/p, where p is momentum. It is significant for electrons and negligible for everyday macroscopic objects.

Atomic line spectra

Atoms emit photons only at energies matching differences between discrete levels. The resulting lines identify the element and are unique to it.

Radioactive half-life

The time for half of the nuclei in a sample to decay. After n half-lives, the fraction remaining is (1/2)^n, and activity falls the same way.

Mass defect and binding energy

A nucleus is lighter than its separated nucleons by the mass defect. Multiplying it by c² gives the binding energy holding the nucleus together.

Snell's law

n₁sinθ₁ = n₂sinθ₂ at a boundary between media. Light bends toward the normal when entering a medium with a higher refractive index.

Momentum conservation

In an isolated system, total momentum before and after an interaction is equal. This holds for all collisions, elastic or inelastic, and for explosions.

Power in a resistor

Electrical power dissipated is P = IV = I²R = V²/R. It appears as heat, which is why resistors have a power rating.