# 7.7 Power  (Page 3/8)

 Page 3 / 8
$E=\text{Pt.}$

Electricity bills state the energy used in units of kilowatt-hours $\left(\text{kW}\cdot h\right),$ which is the product of power in kilowatts and time in hours. This unit is convenient because electrical power consumption at the kilowatt level for hours at a time is typical.

## Calculating energy costs

What is the cost of running a 0.200-kW computer 6.00 h per day for 30.0 d if the cost of electricity is \$0.120 per $\text{kW}\cdot h$ ?

Strategy

Cost is based on energy consumed; thus, we must find $E$ from $E=\text{Pt}$ and then calculate the cost. Because electrical energy is expressed in $\text{kW}\cdot h$ , at the start of a problem such as this it is convenient to convert the units into $\text{kW}$ and hours.

Solution

The energy consumed in $\text{kW}\cdot \text{h}$ is

$\begin{array}{lll}E& =& \text{Pt}=\left(0\text{.}\text{200}\phantom{\rule{0.20em}{0ex}}\text{kW}\right)\left(6\text{.}\text{00}\phantom{\rule{0.20em}{0ex}}\text{h/d}\right)\left(\text{30}\text{.}0\phantom{\rule{0.20em}{0ex}}\text{d}\right)\\ & =& \text{}\text{36}\text{.}\text{0 kW}\cdot \text{h,}\end{array}$

and the cost is simply given by

$\text{cost}=\left(\text{36.0 kW}\cdot h\right)\left(0.120 per kW\cdot h\right)=4.32 per month.$

Discussion

The cost of using the computer in this example is neither exorbitant nor negligible. It is clear that the cost is a combination of power and time. When both are high, such as for an air conditioner in the summer, the cost is high.

The motivation to save energy has become more compelling with its ever-increasing price. Armed with the knowledge that energy consumed is the product of power and time, you can estimate costs for yourself and make the necessary value judgments about where to save energy. Either power or time must be reduced. It is most cost-effective to limit the use of high-power devices that normally operate for long periods of time, such as water heaters and air conditioners. This would not include relatively high power devices like toasters, because they are on only a few minutes per day. It would also not include electric clocks, in spite of their 24-hour-per-day usage, because they are very low power devices. It is sometimes possible to use devices that have greater efficiencies—that is, devices that consume less power to accomplish the same task. One example is the compact fluorescent light bulb, which produces over four times more light per watt of power consumed than its incandescent cousin.

Modern civilization depends on energy, but current levels of energy consumption and production are not sustainable. The likelihood of a link between global warming and fossil fuel use (with its concomitant production of carbon dioxide), has made reduction in energy use as well as a shift to non-fossil fuels of the utmost importance. Even though energy in an isolated system is a conserved quantity, the final result of most energy transformations is waste heat transfer to the environment, which is no longer useful for doing work. As we will discuss in more detail in Thermodynamics , the potential for energy to produce useful work has been “degraded” in the energy transformation.

## Section summary

• Power is the rate at which work is done, or in equation form, for the average power $P$ for work $W$ done over a time $t$ , $P=W/t$ .
• The SI unit for power is the watt (W), where $\text{1 W}=\text{1 J/s}$ .
• The power of many devices such as electric motors is also often expressed in horsepower (hp), where $\text{1 hp}=\text{746 W}$ .

Determine the total force and the absolute pressure on the bottom of a swimming pool 28.0m by 8.5m whose uniform depth is 1 .8m.
how solve this problem?
Foday
P(pressure)=density ×depth×acceleration due to gravity Force =P×Area(28.0x8.5)
Fomukom
for the answer to complete, the units need specified why
That's just how the AP grades. Otherwise, you could be talking about m/s when the answer requires m/s^2. They need to know what you are referring to.
Kyle
Suppose a speck of dust in an electrostatic precipitator has 1.0000×1012 protons in it and has a net charge of –5.00 nC (a very large charge for a small speck). How many electrons does it have?
how would I work this problem
Alexia
how can you have not an integer number of protons? If, on the other hand it supposed to be 1e12, then 1.6e-19C/proton • 1e12 protons=1.6e-7 C is the charge of the protons in the speck, so the difference between this and 5e-9C is made up by electrons
Igor
what is angular velocity
angular velocity can be defined as the rate of change in radian over seconds.
Fidelis
Why does earth exert only a tiny downward pull?
hello
Islam
Why is light bright?
an 8.0 capacitor is connected by to the terminals of 60Hz whoes rms voltage is 150v. a.find the capacity reactance and rms to the circuit
thanks so much. i undersooth well
what is physics
is the study of matter in relation to energy
Kintu
physics can be defined as the natural science that deals with the study of motion through space,time along with its related concepts which are energy and force
Fidelis
a submersible pump is dropped a borehole and hits the level of water at the bottom of the borehole 5 seconds later.determine the level of water in the borehole
what is power?
power P = Work done per second W/ t. It means the more power, the stronger machine
Sphere
e.g. heart Uses 2 W per beat.
Rohit
A spherica, concave shaving mirror has a radius of curvature of 32 cm .what is the magnification of a persons face. when it is 12cm to the left of the vertex of the mirror
did you solve?
Shii
1.75cm
Ridwan
my name is Abu m.konnek I am a student of a electrical engineer and I want you to help me
Abu
the magnification k = f/(f-d) with focus f = R/2 =16 cm; d =12 cm k = 16/4 =4
Sphere
what do we call velocity
Kings
A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in a horizontal plane. A typical weather vane has a large cross-sectional area perpendicular to the direction the arrow is pointing, like a “One Way” street sign. The purpose of the weather vane is to indicate the direction of the wind. As wind blows pa
hi
Godfred
Godfred
If a prism is fully imersed in water then the ray of light will normally dispersed or their is any difference?
the same behavior thru the prism out or in water bud abbot
Ju
If this will experimented with a hollow(vaccum) prism in water then what will be result ?
Anurag

#### Get Jobilize Job Search Mobile App in your pocket Now! By By By Jazzycazz Jackson By OpenStax By Cath Yu By Sandy Yamane By OpenStax By Brianna Beck By Rhodes By Danielrosenberger By Janet Forrester By Gerr Zen