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  • David Peterson

Voltage vs Power - Different, But How?

When we read the data sheets and instructions for electrical devices, there are a few words that we might expect to see - voltage, current, and power consumption.


Cell phone chargers, air compressors, light bulbs, motors - every one of them has a specific input rating.

If you look at the entire range of input voltages for devices you use around the house or the shop, the range of voltage is usually from a minimum of about 5 volts, and a maximum of around 240 volts (like for an oven or dryer).


If you think about that range, it's really not that big in electrical terms.


Compare those numbers to power ratings. Some smaller plug-in power supplies may be at low as 0.5 watts or less. Even mid-range devices like a countertop griddle may consume 1500 watts. A dryer may be close to 6000 watts.


Looking at those two ranges, the voltage doesn't really seem to have nearly as wide of a swing as the power.


We sometimes get into the habit of referring to certain devices by power or by voltage - here's an example.


-- You would buy a 60 watt light bulb, or maybe a 60 watt equivalent LED bulb, but you wouldn't go the store and ask for a 120 volt light bulb.

-- A car battery, on the other hand, would never be called a "9600 watt battery". It's a 12 volt battery. (By the way, that is the power output capacity of a car battery with 800 cca)


So it's okay to just use voltage sometimes, but power other times?


What's the difference?


Voltage

The number of volts is the amount of driving force for the electrons. The only downside with simply referring to voltage is that it doesn't tell what's actually happening - you actually don't need much force to get a lot of work done.


Think of an analogy with water, where the pressure is like voltage.


Let's say you turn on the water in the sink, and then suddenly you have to leave the room and you forget (maybe you have little kids). There's a good chance that the sink will overflow and water will begin to spill on the floor.

The amount of pressure driving the water over the edge of the sink is very small, but the velocity of the water from the faucet controls the amount of water overflowing the sink.


Now instead, imagine the top of a hydro dam or reservoir. If we fill up that reservoir, the exact same thing will happen - the water will spill over the top. Except now, it's a RIVER feeding the reservoir.


In both cases, the pressure driving the water flow was very small. But the amount of work accomplished by the water in the sink is far less than the work accomplished by the water over the edge of the dam.


If you don't believe that, try laying at the bottom of the spillway. You'll agree that the sink overflow is pretty tame.


Since the voltage is the same, the difference must be the rate of flow.


So you can see that voltage alone doesn't paint a complete picture of the electrical capacity.


Power

The power rating is the more accurate combination of both the voltage - the driving force - and the speed of motion of the electrons.


That electron speed is called 'current' just like the flow of the river.


If we combine both the force and the flow rate, technically we multiply them, we will be able to figure out how fast the work is getting done.


Power is the rate at which work is accomplished - specifically, it's one joule every second.


A large power can come from two things:

  1. You could have a very small force of voltage, but provide a massive path for electrons so a large current.

  2. You may alternatively have a large driving voltage, but a small current path.

If both the voltage and current are very small, you can have barely any energy consumed.

For circuit board applications, the power may be down in the millionths of watts (microwatts, like 0.000001 watts).

Now, if you have both a huge voltage AND a huge current... You will have a tremendous amount of power!

Grand Coulee Dam powerhouses generate 6.8 billion watts combined. That's gigawatts or 6,800,000,000 watts.


Remember though, even if the power rating is known, you still need to know either the voltage OR the current to be able to fully understand the situation.


Most electrical devices will tell you at least two of the three properties - voltage, current, or power. Sometimes all three.


Summary

As with anything in life, we sometimes have to adopt common terms or we risk confusion or ridicule.


It may be more accurate to always state both the voltage and power for everything, but often, we can simply use one of those terms as long as it's the accepted norm. Like the light bulb or the car battery from before.


But when we truly need to understand electricity, we don't want to leave out a property that could lead to injury. Knowing voltage without current can be dangerous. Knowing power without voltage can also be dangerous.


The relationship between voltage, current, and power gives us everything we need to know how to get the work done at the right rate, the right way!

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