What Is Electrical Power? | Physics in Motion ♪♪ We’ve talked a lot about
electricity in this unit. We looked at what it is,
the forms it takes, how it behaves, and how we
build circuits to harness it. But where does
the electrical power we use in our everyday
lives come from? Yes, power plants. But there’s more
to it than that.For instance, take the
McDonough Atkinson Plant,
in Smyrna, Georgia,which is a new kind
of natural gas plant
that came
online in 2012
and generates five
times as much energy
as the coal plants
that it replaced.
Now, how much electricity
do you think we use in the U.S.
every year?In 2016,U.S. power plants generated
more than 412 million megawatt
hours of energy,and that was just for
the month of July.
So, an estimate would
be somewhere around
5 billion megawatt
hours per year.
The U.S. is the second greatest
user of energy in the world, after China. Any idea where all that
energy comes from? Oil? Dams?Well, overall,
the breakdown is that
about a third of our electricity
is generated from coal,
a third is generated
from natural gas,
and another 20% is
from nuclear plants.
Hydroelectric power
wind power about 5%,
and solar power about 1%.
And oil also about 1%.Is that surprising?Well, the ideal solution
for generating electricity
is a matter of a lot
of debate, as you know.
But, for now,
your main energy source
depends on
where you live.
In Iowa, for instance,about a third of the electrical
power comes from wind.
South Carolina gets most of
its power from nuclear plants.
Here in Georgia,the leading source of
energy is natural gas
from plants
like McDonough.
Being able to calculate
the amount of power that can be
generated is why you can
turn on your lights and power your home
reliably and safely. Let’s look at
how that’s done. Earlier in the series,we saw that work done or energy
output over a span of time
is called power,which is measured
in units of watts.
We defined electrical
potential difference, V,
as the work, W, that could
be done on a charge, Q.
Mathematically,work equals potential
difference times charge.
Think about Q.How do we know how much charge
is available to be worked on?
Well, it has to do with
a current in the circuit.
Currents, remember, is a
measure of how much charge
flows past a given
point in the circuit
over an amount of time,
which we can call T.
So, current equals
charge divided by time.
Notice that the charge, Q,
appears in both equations.
What happens if we
When you rearrange
the current equation,
you get charge equals the
current multiplied by the time.
We can then substitute
this equation
into the work equals
electric potential
times charge equation,to give us work equals
to electric potential
times current
multiplied by the time.
Or, when you divide
both sides by time,
work per unit time
equals voltage
multiplied by
the current.
But what is work
per unit time?
It’s power.Based on our knowledge
of what power is
and how circuits work,we’ve discovered the equation
form for electrical power.
Electrical power equals
voltage times current.
This may remind you of
another relationship
involving V and I,
Ohm’s law.
Ohm’s law means we can
write the equation
for an electrical power
in a few different ways.
Power in watts equals
voltage in volts
multiplied by
current in amps,
which equals current
squared times resistance,
which equals
voltage squared
divided by
the resistance.
So, that’s the math part. Now, let’s look at how
that translates into power that we can see. I’ve got three incandescent
light bulbs here. A dim one, a medium one,
and a bright one. Power is related to the
brightness of the bulb. For bulbs of
the same type, the higher the power rating,
the brighter the bulb. Now, I’m going to unscrew
the dimmest incandescent bulb and replace it with a bulb
that is a different type. An LED, which has one-fifth
the power rating of our brightest bulb. But, before I
turn it on, which incandescent bulb should
this LED bulb be closest to in brightness? Let’s see. It’s the brightest bulb. The one using five times
as much energy as the LED. So, the LED bulb is more than
five times more efficient than the brightest
incandescent bulb, allowing us to use
a fifth as much power to produce equivalent
brightness. In physics,
efficiency is the ratio of the total
energy output divided by the energy
input into a device. We can output power in
the form of light or heat. So, in terms of
lighting brightness, which of these two bulbs
is the most efficient because it loses
less power to heat? Yeah, it’s the LED. Energy efficiency
is important. Because even though energy
cannot be created or destroyed, according to the law of
conservation of energy, the amount we can use and the amount we
lose to wasted heat depends on how
efficient we are. Any idea how efficient
you are as an energy user? You can actually
find out through an online
personal energy audit. You might be really surprised
at what you discover. Before I show you
a quick example, we need to know
how to calculate the amount of
energy we use.We saw that
electrical work,
also known as
electrical energy,
divided by time
equals electrical power.
If we rearrange
this equation,
we see that electrical
energy equals
power multiplied
by the time.
We can substitute in
different units
to make them
more real to us.
Electrical energy
in kilowatt hours,
which is what you will
find on your power bill,
equals power
in kilowatts
multiplied by
the time in hours.
Let’s calculate how
much energy we use
with just a few devices,
as an example.
How many minutes a month
do I use my microwave?
two hours,
with a power rating of
1,100 watts or 1.1 kilowatts.
Over the course
of the month,
I use 2.2 kilowatt
hours of energy.
Let’s say I watch
100 hours a month of TV.
My TV has a power rating of
150 watts or .015 kilowatts.
Over the course
of a month,
I use 15 kilowatt
hours of energy.
What about the air
conditioning in my home?
A central air unit
for a typical home
has a power rating of
5,000 watts or 5 kilowatts.
If it runs an average
of three hours per day,
that’s about 450 kilowatt
hours per month.
When you add the energy used by
these three devices together,
you get 467.2 kilowatt hours
of energy used per month.
Let’s say you pay,
on average,
11 cents per kilowatt
hour of energy.
So, for me to use my
microwave, watch TV,
and run my air
conditioning each month,
it costs a
little over \$50.
So, just by doing audits
of those three activities, I see ways in which I can save
a lot of energy and money every month. Now, when you go through
your daily routine of turning on lights,
heating your food, and cooling your home, you’ll have a bit
more insight into how it all happens, and how you can affect the
amount of power you use. That’s it for this segment
of “Physics in Motion,” and we’ll see
you next time.For more
practice problems,
lab activities,
and note-taking guides,
check out the
“Physics in Motion” toolkit.

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1 thought on “What Is Electrical Power? | Physics in Motion”

1. francisco castillo says:

I love this channel 🙂