# 7.7 Power  (Page 3/8)

 Page 3 / 8

## 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 $\text{1 hp}=\text{746 W}$ .

## Conceptual questions

Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on? (When off, it is a zero-watt device.) Explain in terms of the definition of power.

Explain, in terms of the definition of power, why energy consumption is sometimes listed in kilowatt-hours rather than joules. What is the relationship between these two energy units?

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please guys help, what is the difference between concave lens and convex lens
convex lens brings rays of light to a focus while concave diverges rays of light
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it depends on the size
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a lens which diverge the ray of light
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concave diverges light
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A diverging lens
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What is isotope
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each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element. "some elements have only one stable isotope
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A wave is a disturbance which travels through the medium transferring energy from one form to another without causing any permanent displacement of d medium itself
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Light is a form o wave
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Neurons communicate by sending message through nerves in coordination
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light has dual nature, particle as well as wave. when we want to explain phenomena like Interference of light, then we consider light as wave.
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Matthew
particles of light are like small packets of energy called photons, and flow or motion of photons is wave like
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light is just the energy of which photons emit
Matthew
the wave is how they travel
Matthew
photons do not emitt energy, they are energy. They are massless particles.
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a wave is a disturbance through the medium. Have you ever thrown a stone in still water? the disturbance produced travels in form of wave, the wave produced by throwing stone in still water are circular in nature.
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a photon does contain mass when in motion. it doesnt contain mass when at rest
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when would it ever be at rest
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a wave is a disturbance of which energy travels
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that's darkness. darkness has no mass because the photons within in aren't moving or producing energy
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Matthew
effective mass of photons only comes into picture when we consider it accelerating in gravitational field, mass of photon has no meaning as it is always travelling with speed of light and is never at rest. with that high speed, Energy and momentum are equivalent. and darkness is absense of photons.
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darkness is absense of light. not the presence of 'resting photons'. photons are never at rest.
Lalita
photons are present in darkness but don't give off any light because they are stationary with no mass or energy. once a force makes them move again they will gain mass and give off light
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this theory is presented in Einsteins theory of special relativity
Matthew
A.The velocity Vo for the streamline flow of liquid in a small tube depends on the radius r of the tube,the density and the viscosity iter of the liquid .use the dimensional analysis to obtain an expression for the velocity . B.Given that Vo =r square ×p all over 4×iter ×l
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A.The velocity Vo for the streamline flow of liquid in a small tube depends on the radius r of the tube,the density (rho)and the viscosity (iter)of the liquid. Use the method of dimensional analysis to obtain an expression for the velocity . B.Given that Vo =r square x p all over 4 x iter x l
True
Matthew, photons ARE light. there is no such thing as a photon that isn't moving. in fact the speed they move at is called C (for constant) in physics. through a vacuum they always travel at this speed no matter what. they can not slow down; except in another medium.
The reason why a photon can go at this speed is BECAUSE it had no mass. nothing can go this speed or faster because it needs to have no mass or negative mass. that's why it's called the constant.
when a photon hits something that is opaque, this is the only way to "stop"it. it isn't merely stopped but absorbed and turned into heat energy, then the remaining energy is reflected in different wavelengths. that reflection is what we call color. the darker something is, the less photons are ther
e. complete blackness is the absolute absence of photons altogether. I believe what you're referring to is not speed, but wavelength, which is indirectly proportional to the amount of energy a particular photon is made up of.
in order for a photon to have zero wavelength, it must (at least theoretically) have infinite energy.
about mass: you may have photons confused with electrons. elections have a mass so small that people say they are without mass, but they do. it is called electron mass or Me-.
you may also be getting electrons and photons confused because of the cherenkov effect. that is what happens when a particle travels faster than light IN THAT PARTICULAR MEDIUM. I emphasize that because no other particle besides photons can go the speed of c.
when a particle goes faster than light in a particular medium, a blue light is emitted, called cherenkov radiation. this is why nuclear reactors glow blue.
nuclear reactors release so much energy that when they emit electrons, those electrons are given enough energy to go faster than light in that medium (in this case water), releasing blue light. if you put the reactor in air or a vacuum, this effect wouldn't happen because the speed of light in air
is very close to c, which is the universal speed limit. I'd you did go faster than c, time would go backwards and you would have infinite theoretical mass and probably spagghettify, like with a black hole.
*if
*electrons
light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else.
Salim
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linear motion
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tension is equal to the weight of the object. so for example if something weighs 45 Newtons then the tension in the Rope holding it is 45 Newtons. and because it is in equilibrium if the object is 45N and there are three ropes holding it there would be 15 N of tension in each to equal the weight
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A prankster applies 450 V to an 80.0 µF capacitor and then tosses it to an unsuspecting victim. The victim’s finger is burned by the discharge of the capacitor through 0.200 g of flesh. What is the temperature increase of the flesh? Is it reasonable to assume no phase change?
what is mass
the quantity of matter that a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field.
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or in easier terms the amount of stuff in an object (stuff meaning whatever element or material that makes the object heavy) the object composition
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your mass never changes but your weight changes based on the gravitational pull of a system or planet. your mass is just the amount of matter with a certain object
Matthew
please can someone help, why a bats can fly in the night without heating anything, how does he managed an see in darkness?
Vincent
...hit an obstacle and has bounced back.
Odo
Hi Vincent. From what I can remember and what I've learnt, bats do not have a good eye sight and so they make use of waves, when they send out waves and they do not return back to them, they realise that the site is free of obstacles but if it sends a wave and it returns back, it means it hit an
Odo
obstacle and bounced back
Odo
So bats make use of waves in place of eye sight. I think but confirm from a few sites .
Odo
they us echolocation to make a sort of mental map. many sea animals use the same method. this can also be compared to sonar which works in the same way
Matthew
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Google Quantum Physics or refer to the text
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atoms are considered the smallest unit of matter. it is further divided into three subatomic particles: protons, neutrons, and electrons.
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a horizontal force of 10n is applied to a 4.0kg block that is at rest on a perfectly smooth, level surface.find the speed of the block and how far has it gone after 6.0s.
10n/4.0kg=2.5m/s^2 v=6.0sx2.5m/s^2= 15m/s (15m/s x 15m/s x 2)÷ (2.5m/s^2 )= 180m
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Denz
v=u+at, a=F/m=10/4, t=6, u=0, so v=15m/s, v2-u2=2as, so s=45m
praveen
a=F/m. 10n/4kg =2.5m/s^2 a=v/t but v=at = 2.5m/ s^2×6s=15m/s v^2=2as. but s=v^2/2a =15^2÷(2×2.5^2)=45m
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Denz
what is resistance of copper wire length
Naveedkhan