Everyday Physics

Your fridge
doesn't create
cold.

It steals heat from inside and dumps it out the back. The same four-stage cycle runs your air conditioner, your heat pump, and every supermarket freezer on Earth.

Refrigeration Cycle Thermodynamics Heat Pump 2nd Law

01 — The Rule

Cold isn't a thing. It's the absence of heat.

Heat flows naturally from hot to cold — always. You can't stop it, you can only work with it. A fridge doesn't make cold; it moves heat from inside to outside using energy. That's the entire trick.

Natural heat flow — hot to cold, no work needed
2nd Law of Thermodynamics
Heat never flows spontaneously from a cold body to a hot one. To reverse this — to move heat uphill from cold to warm — you must input work. That work is what your fridge's compressor does.
Why the back gets warm

Touch the back of your fridge. It's warm — sometimes hot. That warmth came from inside: from your food, your drinks, every item you put in.

The fridge didn't destroy that heat. It pumped it out. Your kitchen is fractionally warmer every time your compressor runs.

02 — The Cycle

Four stages. One loop. Runs forever.

A refrigerant fluid circulates continuously through four components. Click each stage to see what happens to the refrigerant — and where the heat goes.

Stage 01
Evaporation
Inside coils — refrigerant absorbs heat, boils to gas
Stage 02
Compression
Compressor squeezes gas — temperature surges
Stage 03
Condensation
Back coils — gas dumps heat, condenses to liquid
Stage 04
Expansion
Expansion valve — pressure drops, temperature plummets
Stage 1 — Evaporation: refrigerant absorbs heat from inside the fridge
−20°C Refrigerant temp in
−10°C Refrigerant temp out
Low Pressure
Liquid → Gas Refrigerant state

03 — The Compressor

The compressor is the heart of the machine.

When you compress a gas, its temperature rises — always. The compressor squeezes low-pressure cold gas into high-pressure hot gas, raising its temperature above room temperature. That's the trick that lets heat flow out of the fridge and into your kitchen.

Compressor speed — drag to change cooling rate
Compressor 50%
4°C Inside temp
38°C Back coil temp
150W Power draw
PV = nRT
Ideal gas law — compress volume → raise temperature
Why compression heats the gas

When you halve the volume of a gas, you do work on it. That energy has to go somewhere — it goes into the gas as heat, raising its temperature.

The compressor raises the refrigerant to ~60°C — hotter than your kitchen — so heat flows naturally from the refrigerant to the room.

Without this step, heat couldn't exit the system. The compressor is what pumps heat uphill.

04 — Same Principle

One cycle. Three applications.

The refrigeration cycle is direction-agnostic. Reverse where you care about the heat going, and you get a completely different appliance. Same physics, same four stages — wildly different use cases.

Fridge — moves heat out of a cold box into a warm room
Fridge
Moves heat from inside (cold box, ~4°C) to outside (kitchen, ~20°C). You pay for the compressor's work. For every 1 joule of electricity used, roughly 2–3 joules of heat are moved out. That ratio is the Coefficient of Performance (COP).
COP = 2–3
heat moved ÷ work input — fridge typical range

05 — The Discovery

From ice houses to mechanical cold.

Before mechanical refrigeration, keeping food cold meant cutting ice from frozen lakes in winter and storing it underground. The race to replace that with a machine took two centuries — and changed civilisation.

Jacob Perkins · 1834
American-born inventor Perkins filed the first patent for a vapour-compression refrigeration cycle in London in 1834 — the exact principle your fridge uses today.

His machine used ethyl ether as the refrigerant. It worked, but the era wasn't ready: ice was still cheaper than running a steam-powered compressor. The patent sat largely unused for 30 years.
Carl von Linde · 1876
German engineer Linde was the first to make refrigeration practical at industrial scale. His machine used ammonia as the refrigerant and could run continuously — revolutionising brewing, meat-packing, and food transport.

Linde's design is why we can ship food across continents and why supermarkets exist. He founded the company that became Linde plc — still one of the world's largest industrial gas companies today.
Timeline — from ice harvesting to household refrigerators

06 — The Full Picture

Heat pump. Not cold machine.

Every refrigeration device is really a heat pump. It doesn't generate cold — it relocates heat using work. The direction and destination of that heat determines whether you call it a fridge, an air conditioner, or a heat pump.

The full causal chain — from compressor work to cold food
Why your fridge works — in one sentence
A compressor forces refrigerant through a loop: it evaporates inside (absorbing heat from your food), gets compressed (heating up above room temperature), condenses outside (releasing that heat), and expands again (cooling back down) — endlessly.

The food inside doesn't get cold because cold was added. It gets cold because heat was removed — and dumped into your kitchen via the warm coils on the back.

The four-stage loop
① Evaporate — absorb heat inside, liquid → gas
② Compress — raise temperature above ambient
③ Condense — dump heat outside, gas → liquid
④ Expand — pressure drops, temperature plummets
↺ Repeat — indefinitely, until target temp reached
Try the simulator

Click through the four stages on the cycle diagram above, drag the compressor slider to see how speed affects temperatures, and toggle between fridge, AC, and heat pump to see the same physics in different contexts.