Why do computer chargers need those big adapters? Why can’t you just connect the devices to the power outlet with a cable?
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In addition to the other answers, one of the reasons the power brick is so big is not because of the lower voltage, but actually because of the current drawn at the lower voltage a laptop needs.
There’s a [‘triangle’](https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/dccircuits-dcp24.gif) used in physics calculations, which is:
Power (Watts)
over
Voltage * Current (Amps)
This is a quick way to convert between any of them – [power divided by voltage will give you current, divided by current will give you voltage etc.](https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/dccircuits-dcp25.gif) And it also helps to illustrate one thing about electricity – all three values are directly related to each other. If power is constant, reducing one of the other values means the other has to increase. (NB. the symbol for current is I, not A). Wattage (Power) is the measure of how much ‘work’ electricity can do, so you can adjust the other two values up and down but still get the same amount of power.
Every laptop has a different power rating, but they tend to average around 40-60 Watts for a cheap do-everything computer. At mains voltage, 60 Watts is nothing – at 120V, it’s half an amp, and at 240V, it’s one quarter of an amp (see how the triangle works? Double the voltage, halve the current). However, laptops on average use 19-20V. So 60 Watts at low voltage needs a current draw of 3 amps.
The thing with current is that it causes a heating effect. Higher current produces more heat. Chargers for small things like torches/flashlights are often very small because the device only wants a few hundred milliamps, meaning it doesn’t produce a lot of heat when charging. Even if the charger is producing a very low voltage, meaning there’s a big difference between the input and output, if the output current is very low, the charger can be very small – for some reason I remember how small the charger for my Game Boy was, partially because it was see-through and I could see the small cluster of electronics inside the casing, and it was surprisingly neat. From a 240V input to a 3V output, it only output 300mA, so it was small enough to fit in the palm of my hand.
Laptops needing many amps, however, need to be very careful with how they design the power supplies. If you cram the components too close together, they can easily overheat. So the power packs are larger to allow the components to be spaced apart enough to stay cool.
You’ll find that the amount of current a laptop needs directly affects how big its charger is – my work laptop maxes out at 90W, but my big gaming laptop needs a huge 255W at full load. The power brick for the latter is twice the size even though they both work at 19.5V. This is, again, to ensure that the brick doesn’t overheat when it’s pulling its maximum.
It’s also why laptops work at relatively high voltages for something battery powered – if the voltage goes lower, the current goes higher to do the same amount of work, so it would generate more heat. Running your average laptop at 5V, or 3V, would cause it to waste so much power as heat that it wouldn’t run on battery for long. 20V has become a comfortable value – high enough to limit the current from overheating things, but also low enough to be safe if something goes wrong. You can be seriously injured by mains voltage, but on the whole, 20V is safe for humans and you won’t get a nasty zap from it. There’s a lot of electrical safety components in the power brick to make sure that mains voltage never comes out of the low-voltage end if something breaks, which also takes up space in the charger, but every laptop charger has them. The difference in size is mostly for the amount of power they need to output.
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