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Making connections: take-home experiment—electrical energy use inventory

1) Make a list of the power ratings on a range of appliances in your home or room. Explain why something like a toaster has a higher rating than a digital clock. Estimate the energy consumed by these appliances in an average day (by estimating their time of use). Some appliances might only state the operating current. If the household voltage is 120 V, then use P = IV size 12{P = ital "IV"} {} . 2) Check out the total wattage used in the rest rooms of your school’s floor or building. (You might need to assume the long fluorescent lights in use are rated at 32 W.) Suppose that the building was closed all weekend and that these lights were left on from 6 p.m. Friday until 8 a.m. Monday. What would this oversight cost? How about for an entire year of weekends?

Section summary

  • Electric power P size 12{P} {} is the rate (in watts) that energy is supplied by a source or dissipated by a device.
  • Three expressions for electrical power are
    P = IV, size 12{P = ital "IV,"} {}
    P = V 2 R , size 12{P = { {V rSup { size 8{2} } } over {R} } ","} {}

    and

    P = I 2 R . size 12{P = I rSup { size 8{2} } R"."} {}
  • The energy used by a device with a power P size 12{P} {} over a time t size 12{t} {} is E = Pt size 12{E = ital "Pt"} {} .

Conceptual questions

Why do incandescent lightbulbs grow dim late in their lives, particularly just before their filaments break?

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The power dissipated in a resistor is given by P = V 2 / R size 12{P = V rSup { size 8{2} } /R} {} , which means power decreases if resistance increases. Yet this power is also given by P = I 2 R size 12{P = I rSup { size 8{2} } R} {} , which means power increases if resistance increases. Explain why there is no contradiction here.

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Problem exercises

What is the power of a 1.00 × 10 2 MV lightning bolt having a current of 2.00 × 10 4 A ?

2 . 00 × 10 12 W size 12{2 "." "00"´"10" rSup { size 8{"12"} } " W"} {}

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What power is supplied to the starter motor of a large truck that draws 250 A of current from a 24.0-V battery hookup?

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A charge of 4.00 C of charge passes through a pocket calculator’s solar cells in 4.00 h. What is the power output, given the calculator’s voltage output is 3.00 V? (See [link] .)

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Photograph of a small calculator having a strip of solar cells just above the keys.
The strip of solar cells just above the keys of this calculator convert light to electricity to supply its energy needs. (credit: Evan-Amos, Wikimedia Commons)

How many watts does a flashlight that has 6.00 × 10 2 C pass through it in 0.500 h use if its voltage is 3.00 V?

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Find the power dissipated in each of these extension cords: (a) an extension cord having a 0 . 0600 - Ω size 12{0 "." "0600-" %OMEGA } {} resistance and through which 5.00 A is flowing; (b) a cheaper cord utilizing thinner wire and with a resistance of 0 . 300 Ω . size 12{0 "." "300" %OMEGA } {}

(a) 1.50 W

(b) 7.50 W

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Verify that the units of a volt-ampere are watts, as implied by the equation P = IV size 12{P = ital "IV"} {} .

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Show that the units 1 V 2 / Ω = 1 W size 12{1" V" rSup { size 8{2} } / %OMEGA =1" W"} {} , as implied by the equation P = V 2 / R size 12{P = V rSup { size 8{2} } /R} {} .

V 2 Ω = V 2 V/A = AV = C s J C = J s = 1 W

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Show that the units 1 A 2 Ω = 1 W size 12{1" V" rSup { size 8{2} } / %OMEGA =1" W"} {} , as implied by the equation P = I 2 R size 12{P = I rSup { size 8{2} } R} {} .

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Verify the energy unit equivalence that 1 kW h = 3 . 60 × 10 6 J size 12{1" kW" cdot "h = 3" "." "60"´"10" rSup { size 8{6} } " J"} {} .

1 kW h= 1 × 10 3 J 1 s 1 h 3600 s 1 h = 3 . 60 × 10 6 J size 12{1" kW" cdot "h=" left ( { {1 times "10" rSup { size 8{3} } " J"} over {"1 s"} } right ) left (1" h" right ) left ( { {"3600"" s"} over {"1 h"} } right )=3 "." "60" times "10" rSup { size 8{6} } " J"} {}

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Electrons in an X-ray tube are accelerated through 1.00 × 10 2 kV and directed toward a target to produce X-rays. Calculate the power of the electron beam in this tube if it has a current of 15.0 mA.

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An electric water heater consumes 5.00 kW for 2.00 h per day. What is the cost of running it for one year if electricity costs 12.0 cents /kW h size 12{"12" "." 0" cents/kW" cdot h} {} ? See [link] .

Photograph of an electric hot water heater connected to the electric and water supply
On-demand electric hot water heater. Heat is supplied to water only when needed. (credit: aviddavid, Flickr)

$438/y

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Questions & Answers

advantages of CRO over ordinary voltmeter
Dismas Reply
what is the difference between displacement and distance?!
Daniel Reply
what is equilibrium
Sade Reply
If a system is said to be under equilibrium whenever there is no force act upon it... And it remain in its initial stage..
soniya
What is conductivity
Saud Reply
It is the ease with which electrical charges or heat can be transmitted through a material or a solution.
Cffrrcvccgg
how to find magnitude and direction
Arjune Reply
how to caclculate for speed
Arjune
derivation of ohms law
Kazeem Reply
derivation of resistance
Kazeem
R=v/I where R=resistor, v=voltage, I=current
Kazeem
magnitude
Arjune
A puck is moving on an air hockey table. Relative to an x, y coordinate system at time t 0 s, the x components of the puck’s ini￾tial velocity and acceleration are v0x 1.0 m/s and ax 2.0 m/s2 . The y components of the puck’s initial velocity and acceleration are v0y 2.0 m/s and ay 2.0
Arjune
Electric current is the flow of electrons
Kelly Reply
is there really flow of electrons exist?
babar
Yes It exists
Cffrrcvccgg
explain plz how electrons flow
babar
if electron flows from where first come and end the first one
babar
an electron will flow accross a conductor because or when it posseses kinectic energy
Cffrrcvccgg
electron can not flow jist trasmit electrical energy
ghulam
free electrons of conductor
ankita
electric means the flow heat current.
Serah Reply
electric means the flow of heat current in a circuit.
Serah
What is electric
Manasseh Reply
electric means?
ghulam
electric means the flow of heat current in a circuit.
Serah
electric means the flow of electric current through conductor
Sade
the continuos flow of electrons in a circuit is called electric
ANUBHA
electric means charge
ghulam
electric means current
Sade
a boy cycles continuously through a distance of 1.0km in 5minutes. calculate his average speed in ms-1(meter per second). how do I solve this
Jenny Reply
speed = distance/time be sure to convert the km to m and minutes to seconds check my utube video "mathwithmrv speed"
PhysicswithMrV
d=1.0km÷1000=0.001 t=5×60=300s s=d\t s=0.001/300=0.0000033m\s
Serah
A puck is moving on an air hockey table. Relative to an x, y coordinate system at time t 0 s, the x components of the puck’s ini￾tial velocity and acceleration are v0x 1.0 m/s and ax 2.0 m/s2 . The y components of the puck’s initial velocity and acceleration are v0y 2.0 m/s and ay 2.0
Arjune
D=1km=1000m t=5mins×60secs=300sec s=d/t=3.333m/s
Daniel
I think Daniel Glorious is ryt
Amalia
why we cannot use DC instead of AC in a transformer
kusshaf Reply
becuse the d .c cannot travel for long distance trnsmission
ghulam
what is physics
Chiwetalu Reply
branch of science which deals with matter energy and their relationship between them
ghulam
Life science
the
what is heat and temperature
Kazeem Reply
how does sound affect temperature
Clement Reply
sound is directly proportional to the temperature.
juny
how to solve wave question
Wisdom Reply
I would like to know how I am not at all smart when it comes to math. please explain so I can understand. sincerly
Emma
Just know d relationship btw 1)wave length 2)frequency and velocity
Talhatu
First of all, you are smart and you will get it👍🏽... v = f × wavelength see my youtube channel: "mathwithmrv" if you want to know how to rearrange equations using the balance method
PhysicswithMrV
nice self promotion though xD
Beatrax
thanks dear
Chuks
hi pls help me with this question A ball is projected vertically upwards from the top of a tower 60m high with a velocity of 30ms1.what is the maximum height above the ground level?how long does it take to reach the ground level?
mahmoud
what is scalar quantities
babatunde
scaler quantity are quanties that have only direction and no magnitude
Natsu
ice Point
babatunde
scalar quantity are quantities that have magnitude but no direction
Ehigiamusoe
Practice Key Terms 1

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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