Physics of Everyday Objects

A pressure cooker: the flame sets the speed, the valve sets the temperature

A pressure cooker's cooking temperature is a genuinely testable piece of physics, and the trap in how it is usually asked is mistaking what controls speed for what controls the final temperature.

Water boils at a higher temperature under higher pressure, which is the entire principle a pressure cooker exploits: sealing the pot lets steam pressure build up inside, raising the boiling point of the water well above the normal 100°C and cooking food faster as a result. Two components govern that internal pressure directly: the size of the vent hole through which steam escapes, and the weight and design of the pressure-regulating valve or lid sitting on it. Together, these set how much pressure the system holds before releasing excess steam, and therefore set the final cooking temperature reached inside.

The flame's temperature is not what determines that final cooking temperature. A hotter flame mainly affects how quickly the target pressure, and therefore the target temperature, is reached, not what that final temperature actually is. Turn the flame up and the cooker gets to full pressure sooner; it does not cook any hotter once it gets there, because the valve and vent, not the flame, are what cap the temperature.

A thermos flask: blocking all three routes heat can travel

A vacuum (thermos) flask keeps a drink hot or cold by systematically blocking each of the three ways heat can move. The vacuum layer between its double walls removes essentially all the air, so there is no medium left for heat to travel by conduction (through a material) or convection (through a moving fluid). The inner wall's silvered, reflective coating then blocks the third route, radiation, by reflecting heat radiation back inward rather than letting the flask's contents radiate their heat away through the glass. A cork or plastic stopper handles the one path the flask body cannot: heat escaping through the open top, sealed off to stop convection currents forming there instead. No single feature does the whole job; each layer of the design closes off exactly one of the three heat-transfer mechanisms.

A refrigerator: moving heat requires doing work on the passing fluid, not just guiding it

A refrigerator does not "produce cold"; it moves heat from inside the cabinet to the room outside, and doing that against the natural direction of heat flow is precisely why it needs a compressor to force the process along. Its refrigerant is deliberately chosen to evaporate and condense at low, everyday pressures. In the standard vapour-compression cycle, the compressor pressurises the refrigerant gas, which heats it; that hot, high-pressure gas then releases heat to the room as it condenses into a liquid in the coils at the back. That liquid then passes through an expansion valve into the low-pressure evaporator coils inside the cabinet, where it evaporates back into a gas, absorbing heat from the food compartment as it does so and cooling it. The cycle then repeats. The whole system is, in effect, a heat pump: it does not destroy heat, it relocates it, and the compressor's mechanical work is what makes moving heat from a colder space to a warmer one physically possible.

Quick revision points

• A pressure cooker's final cooking temperature is set by the vent hole size and the valve/lid weight, which govern internal steam pressure. The flame's temperature affects only how fast that pressure (and temperature) is reached, not the final temperature itself.
• A vacuum flask blocks all three heat-transfer routes separately: the vacuum layer stops conduction and convection, the silvered coating stops radiation, and the stopper seals the open top against convection currents.
• A refrigerator does not produce cold, it relocates heat using a vapour-compression cycle: the compressor pressurises refrigerant gas (releasing heat outside as it condenses), then an expansion valve lets it evaporate inside (absorbing heat from the cabinet). The compressor's mechanical work is what makes moving heat against its natural direction possible.