Today I'm going to talk about watts and electrical power.
In my previous video about voltage, I talked about how amperes is the number of coulombs
flowing per second, and volts is the number of joules
of energy transferred for each coulomb of charge that flows.
I had an example set up where I had a power
supply connected to a motor, and because I had 1.8 amps
flowing, and for every amp 1 joule was being delivered, this meant that 1.8 joules per
second was getting delivered to the motor.
This is what electrical power is.
Power is the rate at which energy gets supplied, or used up.
Instead of constantly saying joules per second, we have a unit that we use for power, called
the watt.
1 watt is equal to 1 joule being transferred
per second.
So for our example with the motor, 1.8 joules per
second means that 1.8 watts of power are being delivered to the motor.
If we increase the voltage to 2 volts, more current flows and more energy gets transferred
for each unit of charge, so now we are getting 4 joules per
second, or 4 watts of power being supplied to the motor.
Since more work is being done per second, the motor
spins faster.
Now in real life, no engineer is going to waste time converting volts to joules per
coulomb, and amps into joules per second.
You can instantly calculate power with this very simple shortcut.
Voltage x current = power.
Let's try this formula out with a real life example.
This is supposed to be a 40 watt light bulb, but I want to
see how much power it's actually drawing in real life.
I'm playing with high voltage mains electricity here,
do NOT try to replicate this experiment at home.
I'm using a standard north american wall outlet that
supplies 120 volts, and my multimeter is showing that we have exactly 119.6 volts going to
the light bulb.
Now I'm going to switch my multimeter to measure current, and we can see the light bulb is
drawing 337 milliamps from the wall outlet.
119.6 volts x 337 milliamps means that this 40 watt light bulb is actually drawing
40.3 watts, so it's converting 40.3 joules of energy into light
and heat every second.
Now I want to really emphasize that it is voltage AND current that leads to power.
Voltage and current can be combined in different ways to get the job
done.
For example you could design an LED lighting system
that runs from 12 volts, and make it so that it draws 10 times more current, and you'll
still get the same 40 watt draw.
And since LEDs are more efficient you'll get also get more light from every watt.
But not every situation is going to be better off with low
voltage.
For example, a high powered robot might actually be
more efficient with a higher battery voltage.
Speaking of LEDs, in my previous video on basic electricity, we had an example with
a low current, roughly 20mA, flowing through a resistor and an LED.
And the resistor got so hot that it went up in smoke!
Now that we know about electrical power, we can
explain why this happened.
We know that voltage x current = power.
And from my previous video about resistance and Ohm's law, we know that
voltage / resistance = current.
So let's substitute the current term in our power equation with Ohm's law and see what
happens!
Using a little algebra, you can see that another way to calculate power is voltage x voltage/resistance.
Or power = Vsquared/R. This equation will let
us find out the amount of power being delivered to a resistor
when all we know is the voltage and the resistance.
I was powering the system with 140V, and the LED had a 3 volt drop across it.
So this meant that the resistor must have had 137V across it.
The power being delivered to the resistor, in this case heat, is given by Vsquared divided
by R. 137Vx137V / 6800R = 2.76 watts of heat.
The resistor I was using was only rated for 1/4 of a watt of heat, so no wonder it went
up in smoke!
If we wanted something that could handle the 2.76
watts of power, we'd need something a lot bigger,
something that could dissipate more heat, like this resistor that is rated for up to
5 watts.
Nearly everything in electronics will have a power rating, which basically means how
much power can it handle before it goes up in smoke.
As you learn more about electronics, you will find that things like diodes
and transistors will all have their power ratings listed in a datasheet.
And of course, other things will have power ratings!
Like light bulbs, electric heaters, microwaves, and this refers to how much energy they are
converting per second.
Finally I want to make something very clear.
I've intentionally shown you a lot of high voltage examples
because watching things catch fire is really fun, and I think it's a good way to visualize
electrical power and what it can do.
But at home you don't need to have a large collection of big resistors like these in
order to play with electronics.
Most of the circuitry you play with at home will run off of low voltages like 12 volts
or less, and have low currents of a few milliamps.
So simple quarter watt resistors like this are good
enough for almost anything, as long as you pick the right resistance value.
And by now you should know enough about electricity to build some pretty
cool things...
Thank you for watching, don't forget to subscribe and thumbs up, and check out the website if
you've been enjoying this series on basic electricity.