Appearance
1.2 — The House and the Kitchen
A kitchen is the densest concentration of applied physics in an ordinary building. Heat moves four different ways in it, water changes state twice, chemistry happens on purpose in the pan and by accident in the bin, and the whole room is held together by a set of decisions that were made a century ago and never revisited.
Heat
How does a microwave oven actually heat food?
A device called a magnetron produces radio waves at 2.45 gigahertz — a wavelength of about 12 centimetres — and the metal box reflects them so they bounce around the food from every direction.
A water molecule is electrically lopsided: the oxygen end is slightly negative and the two hydrogen ends slightly positive. Put it in an electric field and it twists to line up. The field inside the oven reverses about five billion times a second, so every water molecule in the food is being wrenched back and forth billions of times per second, colliding with its neighbours as it goes. Those collisions are heat. Fats and sugars respond too, less strongly.
Three things follow directly. The oven heats water, not the plate — a dry ceramic dish stays cool while the food on it steams, and a dish that does get hot is either getting it by conduction from the food or contains water in the glaze. It cannot brown anything, because browning needs a surface above about 150 °C and wet food cannot exceed 100 °C until it dries out. And metal is a problem not because metal is magic but because a thin edge or a sharp point concentrates the induced current until the air beside it breaks down and sparks — a smooth thick metal object is usually inert, a crumpled ball of foil is a firework.
The invention was an accident with an unusually well-documented moment. Percy Spencer, an engineer at Raytheon working on radar magnetrons in 1945, noticed a chocolate bar melting in his pocket while he stood in front of a live set. He tried popcorn next, then an egg, which exploded. Raytheon's first commercial oven, the 1947 Radarange, was nearly two metres tall and water-cooled.
Why does a fridge have a grille at the back that gets warm?
Because a fridge does not make cold. It moves heat from inside to outside, and a fridge is really a heat pump running the wrong way round on purpose.
The cycle has four parts. A compressor squeezes a working fluid — the refrigerant — into a hot high-pressure gas. That gas runs through the coils on the back, where it is hotter than the kitchen, so it loses heat to the room and condenses into a liquid. The liquid is then forced through a narrow restriction into a low-pressure pipe inside the cold compartment, where it boils. Boiling absorbs heat, exactly as sweat cools skin, and the heat it absorbs comes out of your food. The now-gaseous refrigerant returns to the compressor and goes round again.
That is why the back of a fridge is warm, why a fridge with its door left open heats the room rather than cooling it — you are paying the compressor to move heat in a circle and adding its electrical waste on top — and why a fridge in a hot cupboard runs badly, because the coils cannot dump heat into air that is already warm.
Why does a pressure cooker cook faster?
Because water boils at 100 °C only at sea-level pressure, and in an open pan the food can never get hotter than the boiling water around it. A sealed cooker holds the steam in, the pressure rises, and water that is under more pressure has to be hotter before it can boil. A domestic cooker running about one atmosphere above the outside pushes the boiling point to roughly 120 °C.
Twenty degrees does not sound like much, but the rate of most cooking reactions roughly doubles for every 10 °C, so a stew that takes ninety minutes takes about twenty-five. It is also why cooking at altitude is slow: in Leh or La Paz the air pressure is low, water boils at around 87 °C, and rice can sit in boiling water indefinitely without becoming soft.
Denis Papin, a French physicist, built the first version in 1679 and called it a steam digester. He demonstrated it to the Royal Society by cooking bones into edible jelly.
Why does an induction hob heat the pan but not the glass under it?
Under the glass is a flat coil carrying alternating current at around 25 kilohertz. That produces a rapidly reversing magnetic field, and any electrically conducting object sitting in a changing magnetic field has current induced in it — loops of current with nowhere to go, called eddy currents. The metal's own resistance turns those currents into heat. The pan is the heating element.
The glass is not a conductor, so nothing is induced in it and it stays cool except where the pan warms it by contact. And the pan must be ferromagnetic — iron or magnetic stainless steel — because the effect is far too weak in aluminium or copper at these frequencies. The kitchen test is a fridge magnet: if it sticks to the base, the pan will work.
Water, dirt and glass
How does soap actually clean?
A soap molecule is a long chain with a split personality. One end is electrically charged and loves water; the other is a hydrocarbon tail that will not mix with water at all but mixes happily with grease.
Drop soap into greasy water and the tails bury themselves in the grease while the charged heads stay out in the water, so each blob of grease ends up wrapped in a ball of soap molecules with a water-friendly surface. That ball is called a micelle, it will not stick to your skin or the plate any more, and it rinses away carrying its cargo. Soap does not dissolve dirt. It gives dirt a handle that water can hold.
The same molecule does a second job: it lowers the surface tension of water so it can spread into fabric and into the pores of skin instead of beading up. And it is the reason twenty seconds of handwashing removes viruses — the tail slides into the fatty envelope around a virus like the influenza or coronavirus family and tears it apart. That is destruction, not just removal.
Soap is one of the oldest chemical products in existence. Babylonian tablets from about 2800 BCE describe boiling fats with ashes, which is exactly the modern reaction: fat plus a strong alkali gives soap plus glycerine.
Why does a mirror reverse left and right but not up and down?
It does not. This is the most satisfying wrong question in everyday physics.
A mirror reverses front and back — the axis pointing into the glass. Nothing else. Stand facing a mirror and your image's nose is nearer the glass than the back of its head, just as yours is, but the image is facing you, so its front-to-back axis points the opposite way to yours.
The apparent left-right swap comes from what you do next, which is imagine turning yourself around to face the same way as the image. You turn about a vertical axis, because that is how humans turn, and that turn is what swaps left and right. If you turned yourself upside down instead — cartwheeling about a horizontal axis to face the image's direction — you would say a mirror reverses top and bottom and leaves left and right alone.
The clean test: hold a page of text up to a mirror and it reads backwards. Now lie the page flat on a table and look at it in a mirror on the ceiling: the letters are upside down and not mirrored side to side. The mirror did the same thing both times. You changed which way you rotated the comparison.
Why is glass transparent when the sand it comes from is not?
Because transparency is about what a material's electrons can absorb, not about how solid it is.
Light passes through a material untouched unless the material has an available energy step that matches the energy of the light. In silicon dioxide the gap between the filled electron states and the empty ones is very wide — wider than the energy carried by any visible photon — so visible light finds nothing to grab it and goes straight through. Ultraviolet light does have enough energy, which is why ordinary window glass blocks most UV and you do not get sunburnt through a closed window.
Sand is the same substance and is not transparent because it is a heap of separate grains. Light entering a grain is refracted, leaves, enters the next at a different angle, and after a few hundred of these it has been scattered in every direction. The grains are transparent; the heap is not. Crushed glass, snow and clouds are all the same trick — clear material, scattered by interfaces.
Why does the drain under your sink have that U-bend?
It is a trap, and it is full of water on purpose. Sewers contain gases that are foul at best and explosive or toxic at worst, and a straight pipe from a sewer to your kitchen would deliver them. The U holds a plug of water, typically 50 to 75 millimetres deep, that the gas cannot get past. Every time you run the tap the water in the trap is replaced.
The design is credited to Alexander Cumming, a Scottish watchmaker, who patented an S-shaped trap for a flushing toilet in 1775 — one of the genuinely important public-health inventions, and one made by somebody with no connection to plumbing. A trap that dries out, in a room nobody has used for weeks, is the usual explanation for a bathroom that suddenly smells of drain.
Why is a toilet flush able to empty the bowl completely?
Because it is not pouring; it is starting a siphon. The flush dumps water in fast enough to fill the whole outlet channel behind the bowl. Once that channel is full and running over its high point, the falling column of water on the outlet side pulls the water behind it — atmospheric pressure pushes on the bowl surface and there is nothing to push back. The bowl empties in a few seconds, until air breaks into the channel and the siphon stops with a gurgle, leaving exactly enough water behind to reseal the trap.
Fire and light
What is on the tip of a match, and why do safety matches need the box?
An ordinary match head is an oxidiser — usually potassium chlorate — mixed with sulphur or a similar fuel, plus glue and fillers to control how fast it burns. The head carries its own oxygen supply, which is why it keeps burning in a draught that would blow out a spark.
The ignition source is on the box. The striking strip is red phosphorus mixed with powdered glass. Dragging the head across it does two things: the glass gives friction and heat, and a trace of red phosphorus is scraped off and converted by that heat into white phosphorus, which ignites in air at around 30 °C. That flash sets off the chlorate in the head.
Splitting the chemistry between head and box is the entire safety idea. The earlier "strike anywhere" matches carried white phosphorus in the head, so they lit in a pocket, in a box in transit, and in factories. They also gave the workers who made them a disfiguring bone disease called phossy jaw, from phosphorus vapour. The safety match, invented in Sweden in the 1840s and 1850s, was a response to that.
Why is a candle flame yellow at the top and blue at the bottom?
Two different things are burning.
At the base, near the wick, there is plenty of air and the fuel is burning cleanly and completely to carbon dioxide and water. The blue is light emitted by excited molecular fragments in that reaction — chemistry, not heat.
Higher up, the air has been used and the vapour breaks down before it can fully burn, leaving specks of solid carbon. Those specks are heated to around 1400 °C by the flame around them and they glow, exactly as a filament glows. The yellow is not a chemical colour. It is soot, incandescent. That is why a candle produces smoke if you cut the air supply, and why holding a cold spoon in the yellow region blackens it while holding it in the blue region does not.
The wick, by the way, does not burn. It draws liquid wax up by capillary action to the flame, where the heat turns it to vapour, and the vapour is what burns. Blow a candle out and the visible white thread is that vapour still rising — you can relight a candle by touching a flame to the smoke a few centimetres above the wick.
Why does an LED bulb last so much longer than the old one?
Because nothing in it is being deliberately destroyed. An incandescent bulb worked by heating a tungsten wire to about 2700 °C until it glowed, and at that temperature the metal steadily evaporates. The wire thins, the thinnest point gets hottest, and it breaks. The bulb's lifetime was set by how fast you were boiling away its own filament. About 95 per cent of the energy left as heat rather than light.
An LED makes light by a completely different route: electrons dropping across a junction in a semiconductor release their energy directly as photons of one colour, at a junction temperature under 100 °C. There is no filament to consume. What eventually kills an LED lamp is the small electronic driver circuit in its base, or the slow browning of the phosphor coating that converts blue to white — which is why LED lamps dim over years rather than failing suddenly.
Air
Why are there fans mounted high above the doors in canteens and old offices?
Because hot air, steam, smoke and cooking smells all rise, and the cheapest way to remove them is to take them from the top of the room while fresh air comes in low.
A kitchen or canteen generates a rising column of warm, humid, grease-laden air from every stove and every plate of food. Left alone it collects under the ceiling and gradually fills the room downward. A high-level exhaust fan set into the wall above the door pulls that top layer straight outside. Because the fan is removing air, the room goes very slightly below outside pressure, and replacement air is drawn in through the doorway and windows at floor level, where it is cooler and cleaner. The result is a continuous slow upward sweep through the room, which is exactly the direction the contamination is already travelling.
Putting the fan above the door specifically is not an accident either. It is the highest point in the wall that is structurally easy to cut, it is out of reach of people and of stacked furniture, and the doorway underneath is the largest guaranteed opening for makeup air, so extract and intake are on the same wall and cannot fight each other. Older buildings did the same job without electricity using a hinged transom window above the door, which let hot air out of a room while the door stayed shut for privacy.
Why does a ceiling fan cool you when it does not cool the room?
It does not lower the air temperature at all — in fact the motor adds a small amount of heat. What it does is remove the still layer of warm, humid air that your body builds up against its own skin.
You lose heat two ways here: by warming the air in contact with you, and by evaporating sweat. Both stall when that thin boundary layer becomes as warm and as damp as you are. Moving air strips the layer away continuously and replaces it with room air, so both mechanisms restart. The fan cools the person, not the place, which is why leaving one running in an empty room is pure waste, and why a fan stops helping once the air is hotter than your skin and already saturated.
Food
Why do onions make you cry?
Because a cut onion manufactures a chemical weapon in about ten seconds, and it is aimed at anything chewing it.
Intact onion cells keep two things apart: a sulphur compound built up from the soil, and an enzyme. Cutting breaks the walls and lets them mix. A chain of reactions produces syn-propanethial-S-oxide, a small, volatile, unstable molecule that drifts up to your eyes and reacts with the water film on them to make a weak sulphuric acid. Nerve endings in the cornea register that as pain, and the tear glands flood the eye to wash it out.
Everything that helps follows from that description. Chilling the onion slows the reaction and reduces how much evaporates. A sharp knife ruptures fewer cells than a blunt one crushing its way through. Cutting under running water or beside an extractor takes the gas away. Contact lenses help, and goggles help completely.
Why does bread go stale, and why does the microwave briefly fix it?
Stale bread is not dry bread. It is bread whose starch has recrystallised, a process called retrogradation. During baking the starch granules absorb water and their molecular chains unravel into a disordered gel, which is soft. As the loaf cools and sits, those chains slowly slide back into an ordered crystalline arrangement, squeezing water out as they go. The crumb becomes firm and crumbly even though the water is still inside the loaf.
This is why bread goes stale faster in the fridge than on the counter — the recrystallisation runs quickest just above freezing, around 4 °C, and is nearly halted by actual freezing. And it is why a few seconds in a microwave restores a stale slice: heating past about 60 °C melts those crystals back into the disordered form. The repair is temporary, because as soon as it cools the chains reorder, faster than before, and the slice ends up worse than it started.
What comes next
Everything on this page is a machine you are allowed to open. The next page is about the machines built specifically so you cannot — locks, safes, seals, and the ways a society decides that a particular person is who they say they are.