Questions tagged [power]

The time rate of change of energy

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Why is the general unit for energy (in terms of energy bills) $\rm kWh$? [closed]

Why is the unit for energy (in terms of energy bills) expressed as $$\text{time} \cdot \frac{\text{energy}}{\text{time}}$$ rather than just energy? Wouldn't it be better to express it in megajoules (...
flakpm's user avatar
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How do you calculate cumulative variable torques over 360 degrees and then calculate the horse power? [closed]

So, I often see the torque calculations for instantaneous torque at any moment, but what do you do when you have ten moments of rise and fall of torque within 360 degrees? I haven't see anything on ...
John Davis's user avatar
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2 answers
38 views

How is power dissipated proportion to current and current squared?

$P=IV$ and $P=I^2R$. Does this mean that $P$ is proportional to both $I$ and $I^2$? I don't understand this concept. And when should I use the various formulas for power? When another variable is ...
Mathguy3829's user avatar
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1 answer
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Efficiency of thermal (rankine) power plants based on ambient temperature?

so I'm preparing for an interview and this is a common question that they always ask, Say there are two power plants working on ideal rankine cycle One in Alaska (low ambient temperature) and one in ...
HungryChicken412's user avatar
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Time average of product of 2 function with same frequency

Hi I'm Reading a notebook on "light atom interactions" There is this calculation I don't understand of average power - $$P=\overline{-\boldsymbol{E} \cdot \frac{\mathrm{d} D}{\mathrm{~d} t}}=...
Yarden Sharabi's user avatar
1 vote
1 answer
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Calculating the efficiency of a solar cell based on the wattage of the lamp powering it [closed]

I am working to characterize some solar cells I have access to based on their material structure by measuring the IV curve of the cells. I can generally compare the cells by comparing their $P_{max-...
StudentOfMorpheus's user avatar
-1 votes
4 answers
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Power-time graphs for constant speed [closed]

i. What does the power-time graph for a car moving on a horizontal road at constant speed look like? Is it a horizontal line because the car engine is supplying a constant power, or is it zero because ...
Anaya's user avatar
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5 answers
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The car has maximum power, why not maximum force?

Suppose a car is moving up an inclined plane, it experiences resistive force proportional to its mass and square of speed. First, it carries a cart of twice its mass up an inclined plane and it can ...
PinkAura's user avatar
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1 answer
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Bulb glows full brightness at which time of $I$-$t$ graph? [closed]

I know that for first question,resistance increases.. In second question,it is asked to find the power when bulb glows at full brightness, I think it is NOT 1A*200V=200w What i think is that,we should ...
Neptotech -vishnu's user avatar
1 vote
0 answers
28 views

Difference between working at a constant rate and exerting a constant force [closed]

Let's assume that two trains of equal masses are moving along parallel paths. The engine of train 1 exerts a constant force, and the engine of train 2 works at a constant rate. If in time $t$ they ...
Nightwing 's user avatar
1 vote
2 answers
101 views

Why does the power loss in transmission cable increase when resistance is increased?

In transmission cables, why does power loss increase when length of conductor is increased? According to the formulas V=IR and P=I²R, When we increase the length, the resistance increases, while the ...
Hufaiza Hufaiza's user avatar
1 vote
2 answers
60 views

Calculating mechanical power of an electrical motor

The total power of a powered drill is 850 watts (the rated value written on the drill's casing) and the current at the supply voltage of 230 volts is 3.7 amperes according to the formula $I = P/U$. ...
allyson bryce's user avatar
0 votes
1 answer
35 views

Derivative of formula for battery's output power is not right

There is something about output power of battery in circuits that bothering me. I hope you help me. So we know the formula for power in electrical circuits is (current x voltage) or: $$ P=IV $$ and ...
rebin's user avatar
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1 answer
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Why is bandwidth not centred at resonant frequency in LCR series circuit?

When we calculate lower and higher frequency such that power becomes half in LCR circuit we end up with two value. For a complete derievation please see here. I have copied relevant information below: ...
ssr's user avatar
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1 answer
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Electricity: Ohm law vs Power in the Water analogy

I'm trying to understand electricity principles with the water analogy. I know this analogy is not perfect, but so far it has helped me the most. To recap: voltage (volt) is like water pressure ...
KiwiKiwi's user avatar
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1 answer
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Why can't take gravity = 10 why velocity is given? [closed]

What is this about? I have a question and there are two ways to solve it. One is wrong and other is right and I wanted to know why was the wrong one wrong. What do I want to know? In the wrong ...
Pankaj's user avatar
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1 answer
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Why does my 3rd method not show me that power = force dotted with velocity?

I have derived 3 expressions for the power that a force, $\vec{F}$, transfers to a body. (A) and (B) are consistent, but (C) isn't. Where have I gone wrong with (C)? EDIT: THE MISTAKE HAS NOW BEEN ...
Anis Manuchehri-Ramirez's user avatar
3 votes
1 answer
250 views

Is power of a battery constant?

In my question I'm neglecting the impact of discharging. I'm trying to make sense of Watt's law: $$P=IV.$$ I always thought that the voltage and the power of a battery were constant features that ...
Роман Кирьянов's user avatar
2 votes
6 answers
218 views

What exactly is power and how does it work?

Lets say that a car engine can output a maximum power of $P$. Now initially, the car starts from rest would have zero velocity. Now the tires start slipping and a force of $f_k = \mu mg$ will act on ...
Dev Not Taken's user avatar
6 votes
1 answer
886 views

How much power does it take to maintain the high vacuums in advanced physics experiments?

The LIGO Gravitational-Wave Observatory and CERNs Large Hadron Collider both have some impressive ultra-high vacuum systems. For my project proposal I need to demonstrate some understanding of how ...
phil1008's user avatar
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1 answer
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How do static and rolling frictions factor into force/power needed to start and accelerate trains?

I am trying to understand how to calculate the necessary force to start moving a train and I was wondering about the roles of rolling and static friction into the equation. From several sources I read ...
Everyone's user avatar
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3 votes
4 answers
274 views

Why is the brain so "efficient"? [closed]

By efficient I mean, that why is the brain able to learn tasks, for example driving, at a much less energy cost than Machine learning models? From a quick google search it seems like it took about 55 ...
Leibniz's Alien's user avatar
-1 votes
1 answer
114 views

Energy lost in a resistor and its temperature

The power lost across a resistor is proportional to the square of the voltage drop across it. This means by increasing the voltage across the resistor, the energy lost in it in a given time grows non-...
seyyed mohammad saadatmand's user avatar
3 votes
0 answers
48 views

Air resistance due to Helium vs Air

I was reading this technical note by NASA on windage power loss in alternators. In the paper, the power loss due to fluid shear between two concentric cylinders (a rotor and a stator) is derived. The ...
E400Jack's user avatar
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1 answer
54 views

Power delivered to a Resistor is $V^2/R$. Does size of battery matter?

I had connected a thin Resistor wire to series combination of six coin cells or to a small sized 9V battery. The wire got hot in a normal way BUT when I connected the same resistance wire to a large ...
Shinnaaan's user avatar
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2 answers
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Would be possible to cover all the deserts with solar plates and satisfy the demand of electricity?

if we covered all the deserts with solar plates , would that be the solution to the energy crisis ? i mean put a solar plates in ALL the deserts of the earth to harvest all solar power
Jose Perez's user avatar
2 votes
2 answers
135 views

How can power be defined as the derivative of work with respect to time if work is not a function?

In thermodynamics, it is frequently said that work and heat are path functions with inexact differentials $\delta Q$, $\delta W$, so one can not talk about a change in work or a change in heat since ...
keska_learning's user avatar
0 votes
3 answers
117 views

Why is Power in an electric circuit equal to $VI$? [closed]

Where did this formula come from? Everyone I asked just told me to substitute values of in ohms law to derive this but no one told why is power equal to voltage * current. Part of the reason for this ...
potato's user avatar
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Energy Dissipated by Damper Infinitesimal Derivation

If we consider a damper (dashpot) element that exerts a force opposite the direction of motion proportional to the velocity, i.e. $$ \vec{F} = -c \vec{v}$$ Therefore, we can consider an infinitesimal ...
Jacob Ivanov's user avatar
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Power from CNO Cycle Fusion

There are all sorts of projects trying to get power from fusion reactions. The best known and funded are the Tokamaks, big expensive crush Deuterium and Tritium (He3 sometimes) plasmas and spew ...
MongoTheGeek's user avatar
0 votes
1 answer
76 views

Is power through a surface Lorentz invariant?

I am trying to solve this problem from Griffiths, but my point is not really to ask the answer of this problem : A point charge $q$ is moving velocity $v$ parallel to $x$ axis. determine the total ...
Mahammad Yusifov's user avatar
1 vote
1 answer
65 views

Why do we only consider tension pointing toward one direction when deriving the power of wave on a wire?

How about the tension pointing in the opposite direction? Does it not contribute to the work done by wave to make the wire move up and down too?
Maxine's user avatar
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2 answers
63 views

Can the brakes of a Semi-trailer truck overcome the torque of the engine? [closed]

On the average Semi-trailer truck, assuming an average load, can the engine overcome the brakes and continue spinning the wheels despite the operator applying the brakes fully? This would be assuming ...
nabeelr's user avatar
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2 answers
47 views

$I²R$ loss in case of transformer

Suppose we have a 100 V A.C. supply, and we connect a 1 ohm resistor with it. Now the current through the resistor will be 100 A. Now we connect a 1:2 step up transformer with the A.C supply, and then ...
Arbish Ali's user avatar
0 votes
0 answers
26 views

Why are partial derivatives of Energy and Co-Energy with respect to x opposite numbers?

In a system where the flux linkage $\lambda = \lambda(x, i)$ is a function of dispalcement $x$ and current $i$. Its energy is defined as $$ W = \int_{0}^{\lambda} i(\lambda', x)d\lambda'. $$ Its co-...
zifan ying's user avatar
1 vote
1 answer
48 views

The Curie Temperature Motor Conundrum

Suppose we have a special motor with a rotor made of nickel. The shape of the rotor is such that it has a high surface area for exchanging heat with an applied flame or with a stream of cooling water. ...
ADesilets's user avatar
-1 votes
1 answer
94 views

Distance-velocity relation for a body accelerated with constant power

A body of mass $m$ starting from rest from the origin ($x(0)=0$ and $v(0)=0$ at $t=0$) moves along the x-axis under the influence of a force $F$ that exerts a constant power $P$. Question: How to find ...
Arghya Deb's user avatar
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0 answers
76 views

Why is virtual velocity defined with only variation in generalized velocities?

Jourdain's principle employed in Vehicle Dynamics (Road Vehicle Dynamics, Georg Rill) states that virtual power of $k$ rigid bodies vanishes, i.e. $$ \sum\limits_1^k \delta v_i(y,z)\cdot F_i^c(y,z) + \...
random_acct's user avatar
0 votes
2 answers
36 views

Why is the Power at the receiving antenna (as per Friis transmission equation) proportional to wavelength squared? [closed]

Power at the receiving antenna is given by the Friis transmission formula, which says that the received power is proportional to the wavelength squared. But energy carried by an electromagnetic wave E=...
Farhan S's user avatar
0 votes
0 answers
47 views

Does rocket increase power in own reference frame as speed increase? [duplicate]

In reference frame moving with rocket exhaust speed is constant, so what is with power as speed increase? If power is const. in this frame, how is possible that at frame at rest power rise? Shouldn't ...
user707264's user avatar
-2 votes
2 answers
114 views

What really determines punching force?

When it’s $F=ma$ why do some fighter like Francis Ngannou has the hardest punch although there are other fighters at same weight with more velocity. Does muscle strength has something to do with that? ...
Modie's user avatar
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0 votes
0 answers
40 views

Does $P=VI$ contradict $V=IR$? [duplicate]

Imagine there's a dam generating power (P) that needs to be transported 100km away. Engineer Pedro suggests increasing the voltage to reduce current acording to P=VI, while Engineer Juan suggests ...
Manuel Lopez Anido's user avatar
-2 votes
2 answers
151 views

Why Is Wind Energy Velocity Cubed Instead of Squared?

Formula for everything seems to be squared, for example, kinetic energy $KE = \frac{1}{2}mv^2$ But for wind energy, this formula is cubed, e.g. $\frac{1}{2}\rho A v^3$ I couldn't find any results for ...
burner8268's user avatar
1 vote
1 answer
85 views

Poynting vector and plane waves

Suppose the electromagnetic field at a point is a superposition to two plane waves $(\vec E_1, \vec H_1)$ and $(\vec E_2, \vec H_2)$. If the two plane-waves have different frequencies, the (time-...
Varidhi Shayana's user avatar
0 votes
2 answers
87 views

Why does force decrease with increase in velocity in case power was constant?

Suppose an internal combustion engine burns the same amount of fuel every cycle (regardless of engine or car speed ) that means it creates the same pressure every cycle and the force on the piston due ...
John greg's user avatar
1 vote
0 answers
83 views

How to find acceleration given power, velocity and mass?

Suppose we have an ideal vehicle with mass $m$ and power output $P$. Under ideal conditions, without friction, when the engine is running, $P$ is the time derivative of the kinetic energy of the ...
Dartnix's user avatar
  • 11
-3 votes
2 answers
97 views

How many typical nuclear power plants does it take to power 1 billion typical cellphones per year? [closed]

Not asking for an exact answer clearly. Let us say that a cellphone user typically watch like 2 hours of video per day and it is say on an iPhone of your choice. How much power would be needed to ...
mjs's user avatar
  • 93
0 votes
1 answer
157 views

Showing that the power is $P=UI$ using the Poynting vector

If I have a circuit with a resistor I want to show that the power delivered to the resistor is UI using Poynting vector. To do this I must integrate the pointing vector over a surface that contains ...
user394334's user avatar
-1 votes
2 answers
141 views

How high voltage and low current is beneficial for power transmission which is contradicting $V²/R$? [duplicate]

I have read that during power transmission high voltage is used because for same power output, high voltage produce low current and due to low current, according to I²R, heat loss through cables are ...
ADITYA DAS's user avatar
1 vote
1 answer
154 views

How do you calculate the power needed to pump water with a hand water pump?

I have a doubt about the way in which I could calculate the power requiered to pump water with a hand water pump like the next one: I´ve seen that many people use the equation below: ...
August's user avatar
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