How does different machines (ie house blow dryer vs oven) use different voltages?

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I understand voltage is pressure and amps is current but how does different voltage work or more efficient for some items?

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It all depends on the power output needed by the appliance.

Power, usually referred to in the unit Watts for electricity, Watts = Volts * Amps.

By transmitting the electricity at a higher voltage you don’t need as many amps to deliver the same wattage. This can be more efficient because with higher amps you can lose more energy to heat in the wires, as the higher the current the more resistance it will experience from the conductors which will turn into heat, and you need thicker wires to carry more current.

If it plugs into your outlet, it uses the same voltage. That being said, some devices require a specialty outlet, although that’s typically for extra current, not voltage. Many devices will change (transform/convert/rectify) the voltage they receive to work with the internal circuits. This is what happens inside all those wall warts.

Now the question: why does a device work with different voltages?

Many electronic computational devices only use DC power, so you need to convert the AC from the wall into DC.

Within a device, certain components work at different voltages. A battery usually has a specific voltage that is set by chemistry. A CPU is designed to work at a lower voltage so that less heat is created from moving electrons around. Also, transistors are all rated for some maximum voltage before they burn out. The LEDs in a display are also optimized for a specific voltage. A lot of this is determined by material/physical constraints.

Other devices, like audio amplifiers require large voltage rails to support the large voltage swings required to power a loudspeaker at an appropriate volume, (typically around +/-50V). A tube amplifier will need to create even higher voltages (>100V) in order to correctly bias the tubes, otherwise they don’t work correctly.

An LED lightbulb is highly efficient and doesn’t require much energy. An incandescent bulb creates light from the resistive heat loss in the filament, so you need a high voltage to get enough light. A CRT television requires huge voltages to create the magnetic fields to aim the electrons shooting at the screen.

Motors, like in a hair dryer, or a washing machine, or a CD tray, come in many different shapes and sizes. There are AC motors, and DC motors, which have different properties and react differently to applied current. Choosing the right motor depends on required torque, speed, size, power consumption, etc etc. Again, different size motors are optimized for different voltages depending on where they will be used.

In all of these devices there are transformers that take the wall voltage and convert them to whatever the designers decided would work best. Some parts require high voltage/power to work correctly, other parts are very small and fragile, and would break if subjected to high voltages.

As a side note, In an ELI5 sense, yes, voltage is analogous to pressure, but they’re different. The units of voltage are actually joules/coulomb. Meaning, voltage measures the amount of energy gained or lost moving a coulomb of charge across said voltage change. To bring back to pressure, voltage measures the tendency for electrons to want to move to a higher voltage (because electrons are negatively charged), similar to how a rock will want to move downward towards the earth.

Electrical devices consume power measured in watts. Watts can be simplified as volts times amps. Wire has a voltage rating based on the insulation and an amperage rating based mostly on its diameter. I have a wall oven rated at 3100 watts. At 120 volts the oven draws 25.8 amps; at 240 volts it draw 12.9 amps and consequently can be wired safely with a smaller diameter wire which is cheaper to buy and easier to install.