Physical Environment

Heat is a form of thermal energy transferred between systems due to temperature differences (SI unit: Joule, practical unit: Calorie, $1\text{ cal} \approx 4.184\text{ J}$). Temperature measures the average kinetic energy of constituent particles, governing heat flow.

Celsius: Lower Fixed Point (LFP) = $0^\circ\text{C}$, Upper Fixed Point (UFP) = $100^\circ\text{C}$ (100 divisions). Fahrenheit: LFP = $32^\circ\text{F}$, UFP = $212^\circ\text{F}$ (180 divisions). Relationship: $\frac{C}{5} = \frac{F-32}{9}$. Clinical thermometer has a constriction (kink) preventing mercury backflow.

Convert $104^\circ\text{F}$ fever to Celsius:
$\frac{C}{5} = \frac{104 - 32}{9} = \frac{72}{9} = 8 \implies C = 40^\circ\text{C}$.

Never sterilize a clinical thermometer in boiling water ($100^\circ\text{C}$)! Its scale ends at $110^\circ\text{F}$ ($43.3^\circ\text{C}$); excessive mercury expansion will shatter the glass tube.

Six’s maximum-minimum thermometer records diurnal temperature extremes for regional weather forecasts.

Latent Heat is thermal energy absorbed or released by unit mass of a substance during a change of state at constant temperature without altering thermometer readings.

Latent heat of fusion of ice: $80\text{ cal/g}$ ($3.36 \times 10^5\text{ J/kg}$). Latent heat of vaporization of water: $537\text{ cal/g}$ ($2.26 \times 10^6\text{ J/kg}$). Steam at $100^\circ\text{C}$ inflicts more severe burns than boiling water at $100^\circ\text{C}$ because each gram of steam releases $537$ additional calories.

Energetic surface molecules evaporate by taking latent heat from the remaining liquid and container, lowering the temperature (clay pots, sweat evaporation, dog panting).

Evaporation is a slow surface phenomenon occurring at any temperature. Boiling is a rapid bulk phenomenon occurring strictly at a fixed boiling point.

Ice at $0^\circ\text{C}$ cools drinks faster than water at $0^\circ\text{C}$ because each gram of ice absorbs $80\text{ cal}$ of latent heat upon melting.

Reflection is the phenomenon where light traveling through a medium bounces back into the same medium upon striking an interface. Smooth surfaces produce regular reflection; rough surfaces produce diffuse reflection.

1. Incident ray, reflected ray, and normal at the point of incidence lie in the same geometric plane.
2. Angle of incidence equals angle of reflection ($\angle i = \angle r$).

Images formed in plane mirrors are virtual, erect, identical in size, situated at equal distance behind the mirror ($u = v$), and laterally inverted (left appears right, right appears left).

Pinhole cameras project inverted real images due to rectilinear propagation of light. Large apertures blur images into overlapping disks.

The laws of reflection apply universally to both regular and diffuse reflections. Diffuse reflection scatters light because local normals point in random directions.

A magnet attracts ferromagnetic materials and aligns north-south when freely suspended. Poles at the ends possess maximum attractive power; the central zone is neutral.

Induction precedes attraction: opposite poles are induced on the near end of unmagnetized iron. Isolated magnetic monopoles do not exist; cutting a magnet produces two complete dipoles.

Earth behaves like a giant bar magnet whose magnetic south pole resides near the geographic north pole, guiding magnetic navigational compass needles.

Heating, hammering, or dropping a magnet disrupts domain alignment, completely destroying magnetism. Store magnets with soft iron keepers.

"Repulsion is the sure test of magnetism" because attraction also occurs between a magnet and unmagnetized iron.

A simple circuit consists of a source (cell), conducting path (wires), switch, and load (bulb). Current flows exclusively through closed loops.

Current flowing through high-resistance nichrome wires generates heat ($H = I^2Rt$). Safety fuses (lead-tin alloy) feature high resistance and low melting point to melt during power surges.

Oersted proved electric currents generate magnetic fields. An electromagnet (soft iron core wound with insulated wire) acts as a powerful temporary magnet used in bells, cranes, and medical tools.

LEDs convert electrical energy directly to light without filaments, saving power and outlasting incandescent bulbs.

Never replace a blown fuse with ordinary copper wire; its high melting point will fail to interrupt surges, risking household fires.