Physik
45 cards from across every topic, each citing a source. Tap one to open it in the feed.
- A fair-weather cloud weighs as much as a hundred elephants.
A typical cumulus cloud holds about half a gram of water per cubic meter. For a cloud one kilometer on each side, a …
- A kettle is loudest just before it goes quiet.
At the bottom the water reaches 100 degrees while the top is still at 70. Steam bubbles form there, rise into cooler …
- A thermos works because there's nothing inside it.
Heat travels three ways: conduction, convection and radiation. A thermos has two walls with a vacuum between them. With …
- A whip crack is a sonic boom.
The whip does not crack because it hits something. A wave runs from handle to tip, and because the cord keeps getting …
- An induction hob heats the pan, not the surface.
Beneath the glass sits a copper coil carrying alternating current at roughly 25 to 50 kilohertz. Its magnetic field …
- Beneath your feet it's as hot as the Sun's surface.
Earth's inner core is a ball of solid iron and nickel with a radius of about 1,220 kilometers. Its temperature is …
- Check the units before you trust the number.
Every meaningful physical equation has the same units on both sides: metres stay metres, seconds stay seconds. If your …
- Colder than minus 273.15 degrees is impossible.
Temperature measures how much particles move. At −273.15 °C, which is 0 kelvin, thermal motion is at its minimum …
- Contrails are clouds made of exhaust water.
Burning kerosene produces water vapour. At 10 kilometres altitude it is about minus 50 degrees; the hot vapour cools …
- Counting seconds tells you how far away the storm is.
Sound moves through 20 °C air at about 343 metres per second; light arrives practically instantly. When you see …
- Every banana is slightly radioactive.
Bananas are rich in potassium, and about 0.012 % of it is the radioactive isotope potassium-40. One banana delivers …
- Helium doesn't raise your pitch. Seriously.
With helium in your lungs, your vocal cords vibrate at exactly the same rate, so the pitch stays put. But sound travels …
- High tide happens on the side facing away from the Moon too.
The Moon pulls the near-side water more strongly than Earth's centre, and the centre more strongly than the far-side …
- How a nuclear plant makes electricity: with a steam engine.
In the reactor, uranium nuclei split and heat a coolant, usually water. That makes steam, directly or through a heat …
- How an MRI looks inside you without X-rays.
A magnetic field of typically 1.5 or 3 tesla lines up the hydrogen nuclei in your body. Radio pulses tip them over; as …
- In some metals, current flows forever without loss.
In 1911 Heike Kamerlingh Onnes cooled mercury to 4.2 kelvin and watched its electrical resistance drop abruptly to …
- Jittering pollen proved that atoms exist.
In 1827 the botanist Robert Brown watched pollen particles twitch in water under his microscope, with nothing alive …
- Lightning is five times hotter than the Sun's surface.
A lightning channel heats the air to around 30,000 °C, roughly five times the Sun's surface temperature. The air …
- More energy out than in? Look for the hidden source.
Two laws of thermodynamics have never been broken: energy does not arise from nothing, and heat flows by itself only …
- No single water molecule is wet.
Wetness only appears when billions of molecules act together. An ant colony builds roads though no ant knows the plan …
- No straw on Earth works beyond ten metres in length.
When you suck, you create no pull, only low pressure: the air pressure outside pushes the drink upward. That pressure …
- No tree can grow taller than about 130 metres.
A tree does not pump water. Evaporation in the leaves draws it up from the roots as one continuous thread. With every …
- No wind turbine can harvest more than 59 percent of the wind.
In 1919 Albert Betz showed that a turbine can't stop the air completely, or it would pile up and nothing would flow …
- No, old window glass hasn't flowed downward.
A popular myth says glass is a slow liquid, which is why old church windows are thicker at the bottom. In fact, glass …
- Nobody sees the same rainbow you do.
Sunlight enters raindrops, reflects inside them and fans out into colours on the way out. Red light leaves the drop at …
- On Everest, water boils at about 70 degrees.
Water boils when its vapour pressure matches the air pressure. At sea level that happens at 100 °C. The higher you …
- On the hottest pan, a water drop evaporates slowest.
At 150 °C a drop hisses away instantly. From about 193 °C the opposite happens: the underside flashes into steam and …
- Physics still isn't sure why ice is slippery.
The textbook story, that the pressure of a skate blade melts the ice, was refuted in 1939: it would make skating below …
- Planes fly because their wings shove air downward.
A wing is tilted slightly and curved on top. The oncoming air follows the surface and is deflected downward behind the …
- Ten tonnes of air are resting on your desk.
Standard air pressure at sea level is 101,325 pascals, about 1 kilogram of force per square centimetre. On one square …
- The Sun loses four million tonnes every second.
In the Sun's core, about 600 million tonnes of hydrogen fuse into helium every second. Roughly 0.7 % of that mass …
- The blue glow in a reactor pool comes from particles outrunning light.
Nothing beats light in a vacuum. In water, though, light crawls at only 75 percent of its speed, and charged particles …
- The compass has pointed south many times before.
Earth's magnetic field is generated by flowing liquid iron in the outer core. Over geological history it has flipped …
- The electrons in your cable crawl slower than a snail.
With one ampere flowing through a 2-millimetre copper wire, the electrons drift on average only 23 micrometres per …
- The sky is blue because air loves to scatter blue.
Air molecules are far smaller than light waves and scatter short wavelengths much more than long ones, proportional to …
- The sunlight on your skin is eight minutes old.
Light travels through vacuum at exactly 299,792,458 metres per second. That number isn't measured, it's fixed: since …
- Underwater, sound travels more than four times faster.
In air at 20 °C, sound covers 343 metres per second. In fresh water it is 1,481 m/s, in seawater about 1,500 m/s. The …
- Water is heaviest at 4 degrees. That saves fish.
Almost every substance gets denser as it cools. Water only does so down to 4 °C. Below that, hydrogen bonds arrange the …
- Why airliners cruise at 11 km instead of 3 km.
At 11 km the air has only about a quarter of its density at ground level. Less air means less drag, so the aircraft …
- Why giant movie ants would collapse on the spot.
Double a body's size and its surface area quadruples, but its volume and weight increase eightfold. Legs support weight …
- Why solar cells can never use 100 percent of the light.
A silicon cell only uses photons with energy above its band gap; red and infrared photons are wasted. Blue photons …
- Why the microwave heats your soup but not the plate.
A microwave oven emits radio waves at 2.45 GHz. Water molecules are electric dipoles, so they flip back and forth about …
- Why your phone ignores gloves but feels wet fingers.
Modern displays are capacitive touchscreens: beneath the glass lies a grid of transparent electrodes that maintains a …
- Without Einstein, your sat-nav would drift 10 km per day.
GPS satellites carry atomic clocks and orbit at about 14,000 km/h, 20,000 km up. Their speed makes the clocks run …
- Without the greenhouse effect, Earth would be an ice ball.
Sunlight warms the ground, which sends the energy back as infrared radiation. Water vapor, CO₂ and methane let sunlight …