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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.

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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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A spark of static electricity, such as that you might receive from a doorknob on a cold dry day, may carry a few hundred watts of power. Explain why you are not injured by such a spark.

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Problems&Exercises

The Crab Nebula (see [link] ) pulsar is the remnant of a supernova that occurred in A.D. 1054. Using data from [link] , calculate the approximate factor by which the power output of this astronomical object has declined since its explosion.

A supernova explosion.
Crab Nebula (credit: ESO, via Wikimedia Commons)

2 × 10 10 size 12{2 times "10" rSup { size 8{-"10"} } } {}

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Suppose a star 1000 times brighter than our Sun (that is, emitting 1000 times the power) suddenly goes supernova. Using data from [link] : (a) By what factor does its power output increase? (b) How many times brighter than our entire Milky Way galaxy is the supernova? (c) Based on your answers, discuss whether it should be possible to observe supernovas in distant galaxies. Note that there are on the order of 10 11 size 12{"10" rSup { size 8{"11"} } } {} observable galaxies, the average brightness of which is somewhat less than our own galaxy.

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A person in good physical condition can put out 100 W of useful power for several hours at a stretch, perhaps by pedaling a mechanism that drives an electric generator. Neglecting any problems of generator efficiency and practical considerations such as resting time: (a) How many people would it take to run a 4.00-kW electric clothes dryer? (b) How many people would it take to replace a large electric power plant that generates 800 MW?

(a) 40

(b) 8 million

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What is the cost of operating a 3.00-W electric clock for a year if the cost of electricity is $0.0900 per kW h size 12{"kW" cdot h} {} ?

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A large household air conditioner may consume 15.0 kW of power. What is the cost of operating this air conditioner 3.00 h per day for 30.0 d if the cost of electricity is $0.110 per kW h size 12{"kW" cdot h} {} ?

$149

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(a) What is the average power consumption in watts of an appliance that uses 5 . 00 kW h size 12{5 "." "00 kW" cdot h} {} of energy per day? (b) How many joules of energy does this appliance consume in a year?

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(a) What is the average useful power output of a person who does 6 . 00 × 10 6 J size 12{6 "." "00" times "10" rSup { size 8{6} } " J"} {} of useful work in 8.00 h? (b) Working at this rate, how long will it take this person to lift 2000 kg of bricks 1.50 m to a platform? (Work done to lift his body can be omitted because it is not considered useful output here.)

(a) 208 W

(b) 141 s

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A 500-kg dragster accelerates from rest to a final speed of 110 m/s in 400 m (about a quarter of a mile) and encounters an average frictional force of 1200 N. What is its average power output in watts and horsepower if this takes 7.30 s?

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(a) How long will it take an 850-kg car with a useful power output of 40.0 hp (1 hp = 746 W) to reach a speed of 15.0 m/s, neglecting friction? (b) How long will this acceleration take if the car also climbs a 3.00-m-high hill in the process?

(a) 3.20 s

(b) 4.04 s

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(a) Find the useful power output of an elevator motor that lifts a 2500-kg load a height of 35.0 m in 12.0 s, if it also increases the speed from rest to 4.00 m/s. Note that the total mass of the counterbalanced system is 10,000 kg—so that only 2500 kg is raised in height, but the full 10,000 kg is accelerated. (b) What does it cost, if electricity is $0.0900 per kW h size 12{"kW" cdot h} {} ?

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(a) What is the available energy content, in joules, of a battery that operates a 2.00-W electric clock for 18 months? (b) How long can a battery that can supply 8 . 00 × 10 4 J size 12{8 "." "00" times "10" rSup { size 8{4} } " J"} {} run a pocket calculator that consumes energy at the rate of 1 . 00 × 10 3 W size 12{1 "." "00" times "10" rSup { size 8{-3} } " W"} {} ?

(a) 9 . 46 × 10 7 J size 12{9 "." "46" times "10" rSup { size 8{7} } " J"} {}

(b) 2 . 54 y size 12{2 "." "54" times "10" rSup { size 8{7} } " J"} {}

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(a) How long would it take a 1 . 50 × 10 5 size 12{1 "." "50" times "10" rSup { size 8{5} } } {} -kg airplane with engines that produce 100 MW of power to reach a speed of 250 m/s and an altitude of 12.0 km if air resistance were negligible? (b) If it actually takes 900 s, what is the power? (c) Given this power, what is the average force of air resistance if the airplane takes 1200 s? (Hint: You must find the distance the plane travels in 1200 s assuming constant acceleration.)

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Calculate the power output needed for a 950-kg car to climb a 2.00º slope at a constant 30.0 m/s while encountering wind resistance and friction totaling 600 N. Explicitly show how you follow the steps in the Problem-Solving Strategies for Energy .

Identify knowns: m = 950 kg size 12{m="950"`"kg"} {} , slope angle θ = 2.00º , v = 3.00 m/s , f = 600 N size 12{f="600"`N} {}

Identify unknowns: power P size 12{P} {} of the car, force F size 12{F} {} that car applies to road

Solve for unknown:

P = W t = Fd t = F d t = Fv , size 12{P= { {W} over {t} } = { { ital "Fd"} over {t} } =F left ( { {d} over {t} } right )= ital "Fv",} {}

where F size 12{F} {} is parallel to the incline and must oppose the resistive forces and the force of gravity:

F = f + w = 600 N + mg sin θ size 12{F=f+w="600 N"+ ital "mg""sin"θ} {}

Insert this into the expression for power and solve:

P = f + mg sin θ v = 600 N + 950 kg 9.80 m/s 2 sin 2º ( 30.0 m/s ) = 2.77 × 10 4 W alignl { stack { size 12{P= left (f+ ital "mg""sin"θ right )v} {} #size 12{ {}= left ["600 N"+ left ("950 kg" right ) left (9 "." "80 m/s" rSup { size 8{2} } right )"sin2"° right ] \( "30" "." "0 m/s" \) } {} #=2 "." "77" times "10" rSup { size 8{4} } `W {} } } {}

About 28 kW (or about 37 hp) is reasonable for a car to climb a gentle incline.

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(a) Calculate the power per square meter reaching Earth’s upper atmosphere from the Sun. (Take the power output of the Sun to be 4 . 00 × 10 26 W . ) (b) Part of this is absorbed and reflected by the atmosphere, so that a maximum of 1 . 30 kW/m 2 reaches Earth’s surface. Calculate the area in km 2 of solar energy collectors needed to replace an electric power plant that generates 750 MW if the collectors convert an average of 2.00% of the maximum power into electricity. (This small conversion efficiency is due to the devices themselves, and the fact that the sun is directly overhead only briefly.) With the same assumptions, what area would be needed to meet the United States’ energy needs ( 1 . 05 × 10 20 J ) ? size 12{ \( 1 "." "05" times "10" rSup { size 8{"20"} } " J" \) ?} {} Australia’s energy needs ( 5 . 4 × 10 18 J)? size 12{ \( 5 "." 4 times "10" rSup { size 8{"18"} } " J" \) ?} {} China’s energy needs ( 6 . 3 × 10 19 J)? size 12{ \( 6 "." 3 times "10" rSup { size 8{"19"} } " J" \) ?} {} (These energy consumption values are from 2006.)

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Questions & Answers

physiological Psychology
Nwosu Reply
How can I develope my cognitive domain
Amanyire Reply
why is communication effective
Dakolo Reply
Communication is effective because it allows individuals to share ideas, thoughts, and information with others.
effective communication can lead to improved outcomes in various settings, including personal relationships, business environments, and educational settings. By communicating effectively, individuals can negotiate effectively, solve problems collaboratively, and work towards common goals.
it starts up serve and return practice/assessments.it helps find voice talking therapy also assessments through relaxed conversation.
miss
Every time someone flushes a toilet in the apartment building, the person begins to jumb back automatically after hearing the flush, before the water temperature changes. Identify the types of learning, if it is classical conditioning identify the NS, UCS, CS and CR. If it is operant conditioning, identify the type of consequence positive reinforcement, negative reinforcement or punishment
Wekolamo Reply
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Wekolamo
because it helps many people around the world to understand how to interact with other people and understand them well, for example at work (job).
Manix Reply
Agreed 👍 There are many parts of our brains and behaviors, we really need to get to know. Blessings for everyone and happy Sunday!
ARC
A child is a member of community not society elucidate ?
JESSY Reply
Isn't practices worldwide, be it psychology, be it science. isn't much just a false belief of control over something the mind cannot truly comprehend?
Simon Reply
compare and contrast skinner's perspective on personality development on freud
namakula Reply
Skinner skipped the whole unconscious phenomenon and rather emphasized on classical conditioning
war
explain how nature and nurture affect the development and later the productivity of an individual.
Amesalu Reply
nature is an hereditary factor while nurture is an environmental factor which constitute an individual personality. so if an individual's parent has a deviant behavior and was also brought up in an deviant environment, observation of the behavior and the inborn trait we make the individual deviant.
Samuel
I am taking this course because I am hoping that I could somehow learn more about my chosen field of interest and due to the fact that being a PsyD really ignites my passion as an individual the more I hope to learn about developing and literally explore the complexity of my critical thinking skills
Zyryn Reply
good👍
Jonathan
and having a good philosophy of the world is like a sandwich and a peanut butter 👍
Jonathan
generally amnesi how long yrs memory loss
Kelu Reply
interpersonal relationships
Abdulfatai Reply
What would be the best educational aid(s) for gifted kids/savants?
Heidi Reply
treat them normal, if they want help then give them. that will make everyone happy
Saurabh
What are the treatment for autism?
Magret Reply
hello. autism is a umbrella term. autistic kids have different disorder overlapping. for example. a kid may show symptoms of ADHD and also learning disabilities. before treatment please make sure the kid doesn't have physical disabilities like hearing..vision..speech problem. sometimes these
Jharna
continue.. sometimes due to these physical problems..the diagnosis may be misdiagnosed. treatment for autism. well it depends on the severity. since autistic kids have problems in communicating and adopting to the environment.. it's best to expose the child in situations where the child
Jharna
child interact with other kids under doc supervision. play therapy. speech therapy. Engaging in different activities that activate most parts of the brain.. like drawing..painting. matching color board game. string and beads game. the more you interact with the child the more effective
Jharna
results you'll get.. please consult a therapist to know what suits best on your child. and last as a parent. I know sometimes it's overwhelming to guide a special kid. but trust the process and be strong and patient as a parent.
Jharna
what is learning? discuss how any three scholars propounded on how learning is
Grace Reply
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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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