Physics 20

A physical quantity is written as a number, an SI unit, and - for vectors like velocity or force - a direction.

Fundamental units (metre, kilogram, second) are defined directly; derived units (m/s, N, J) are built by combining fundamental ones.

Every measured quantity has limited accuracy, so we keep only the significant figures the instrument justifies.

Scientific notation (e.g. 3.00 x 10^8 m/s) and order-of-magnitude estimates make very large and very small quantities manageable.

Experimental data is organized into a table and plotted so a pattern (a straight line, a curve) becomes visible.

A wave transfers energy through a medium as a disturbance, while the medium itself is not carried along - only the disturbance moves.

Waves move at a constant speed in a uniform medium; a change in medium (or its condition) usually changes the wave's speed.

Waves can be transverse (particles move across the direction of travel) or longitudinal (along it), and we describe them with terms like crest, trough, compression, and rarefaction.

When a wave slows entering a new medium its frequency stays the same, so its wavelength shortens - and it bends (refracts); at a barrier it reflects, angle of incidence equalling angle of reflection.

Period (T, in seconds) is the time for one cycle; frequency (f, in hertz) is cycles per second, and the two are reciprocals: T = 1/f.

Luminous objects make their own light; non-luminous ones only reflect it; materials are transparent, translucent, or opaque to light.

An interface is the boundary between two media; at it light reflects so the angle of incidence equals the angle of reflection, both measured from the normal, and the incident ray, reflected ray, and normal lie in one plane.

Refraction is the bending of light at a boundary between materials with different indices of refraction, caused by the change in light's speed as it crosses.

Light travels in straight lines (rectilinear propagation), which is why an object blocking a source casts a shadow with a dark umbra and a partial penumbra.

Specular (regular) reflection from a smooth surface gives a clear image; diffuse reflection from a rough surface scatters light, though each ray still obeys the same law.

The kinetic molecular theory says matter is made of particles in constant motion; temperature measures the average kinetic energy of those particles, while heat is the thermal energy transferred between objects.

Specific heat capacity c is the quantity of heat needed to raise a unit mass of a substance by one degree, with units of J/(kg*K).

When substances at different temperatures mix, heat flows from hot to cold until they reach the same temperature - the Principle of Heat Exchange, a form of energy conservation measured with a calorimeter.

Temperature can be measured directly; heat cannot. The Kelvin scale starts at absolute zero, and T(K) = T(C) + 273.

The heat for a temperature change is Q = mc*deltaT; water's unusually high specific heat is why a lake heats and cools slowly.