Questions tagged [power]

The time rate of change of energy

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Discrepancy between two calculations for the power provided by a pump

I am a high school student and I am very confused in a question related to power. I was able to get the right answer but i am not satisfied with the results I get. The question is A motor pumps ...
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asking a question on POWER [closed]

I am a high school student and I am very confused in a question related to power I was able to get the right answer but i am not satisfied with the results I get . The question is "A motor pumps ...
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2answers
39 views

When to use Ohm's law as opposed to using power formula?

I am a mechanic trying to gain a better understanding of electrical theory on vehicles. I have a sound understanding of Ohm's law and also the power formula (Power = Voltage x Current) However, I am ...
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5answers
81 views

What is the power while accelerating a bicycle?

With respect to this question, the power on a accelerated body, that moves in the same direction of the acceleration, can be calculated by $W = Fv$, where $F$ is the external force and $v$ the ...
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1answer
28 views

Why Doesn't Thermal Radiation Render Thermionic Conversion Unable To Approach Carnot Efficiency?

Thermionic Conversion follows the classic Richardson-Dushmann Equation for thermionic current as a function of temperature squared: $$J_{RD} = A_0 T^2 \exp\left(-\frac{\phi}{k_B T}\right)$$ where $J_{...
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Single-mode laser oscillation in a resonator: time-dependent power

I have some issues with the time-dependent power of a single-mode laser oscillator. We are given the frequency $\nu$ of the resonator mode, the amplification of one period in the resonator is given by ...
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43 views

Noise power spectral density and $|X(\omega)|^2$. Why are those quantities associated usually ? Their definitions are different

I consider a generic signal $X(t)$. We could imagine to fix ideas $X(t)=\sqrt{R}I(t)$ ($R$ being a resistor and $I$ current flowing through it) The energy of this signal is defined as $E=\int_{-\infty}...
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4answers
65 views

Work Equals Torque? Horsepower, Pulleys

While reading one definition of torque, I saw its units are Newton-meter, which is the same as work. But sources usually make it a point to emphasize "even though both work and torque units are ...
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1answer
22 views

Instantaneous power for a variable force

I know that instantaneous power is defined as the time derivative of work done. For a constant, it is easy to prove that this is just the dot product of force and velocity. However, is Instantaneous ...
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35 views

Calculating the time to perform work on rolling shutters

I have rolling shutters, e.g.: I am trying to determine how much time it takes to raise/lower the shutters a specific amount. (Because of the physics involved, the time is non-linear relative to the ...
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2answers
69 views

Why does ground reaction force decrease at the start of a vertical jump?

I don't understand why reaction force would decrease at the start of a countermovement jump. I've tried looking at this diagram and considering moments and how lowering the position of the center of ...
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1answer
32 views

Power consumption of vacuum tube [closed]

What is the power consumption of a vacuum diode? (or vacuum tube). I'm interested only in the "Anode supply voltage", this source typically has values of around $100\ \text{V}$ but I don't ...
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3answers
58 views

Why does the power remain constant during the stepping up and stepping down of current in transformers?

I understand quite a bit of transformers, their structure and other concepts and formulas related to mutual induction. However I'm not able to explain why the electrical power has to be constant ...
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662 views

What's the point of an RMS value? [duplicate]

the RMS (root mean square) value of $f(x)$ is defined as: $$f(x)_{rms}=\sqrt{\frac{\int^b_a (f(x))^2dx}{b-a}}$$ Why do we do this very specific thing of taking the square, the mean, and then the ...
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2answers
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Power of water pump

Consider a water pump: Writing Newton's second law for water flowing out of a cylindrical cross section of it, $$ F= v \frac{dm}{dt}$$ Now, $$ P = F \cdot v$$ $$ P= v \frac{dm}{dt} v$$ $$ \frac{dm}{...
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Power Transmitted along the string by a sine wave [closed]

I have referred several sources which derived the equation for the power transmitted along a string by a sine wave. However, I could not find anywhere what exactly is it? "Along a string" ...
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3answers
196 views

Why can't we jump very high?

If somebody is able to output $3750$ $\text{N}$ lifting weight on the back doing squats, why can't they jump at $150$ $\text{km}/\text{h}$ without weight? With weight: $$(300 \text{ kg of load} + ...
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1answer
54 views

Motor's work and power against the gravity

Ok, I've read this answer: Why does holding something up cost energy while no work is being done? But more questions pop up. If there is no work being done does that mean that there is also no power ...
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7answers
335 views

Why does increasing voltage result in a decrease in current and not rather a decrease in power? [duplicate]

I recently read that in order to conduct electricity over vast distances via power lines they had to step up voltage which results in a decrease in current through the medium. This is reflected in the ...
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2answers
59 views

Why do current carrying wires with smaller area heat up more than those with larger area?

Consider two wires of length $L$ and resistivity $\rho$, and consider a fixed voltage source with voltage $V$. The first wire has cross sectional area $A_1$ and the second wire has cross sectional ...
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1answer
21 views

What is the meaning of power delivered by the battery after $t$ seconds?

I don't get it when a question asks power delivered by a battery after t seconds, are they talking about the total energy delivered by the battery in time t seconds divided by the time, or are they ...
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1answer
31 views

Derivation of the Gaussian Beam Pattern

I am currently reading this book by Mireille Levy. It says (on page 40) that the Gaussian beam pattern with half-power beamwidth $\beta$ is gien by: $$B(\theta)=A\text{exp}\left(-2\text{log}2\frac{\...
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29 views

Power required to reach X velocity in X time with drag

I wrote a program that adds force to a car like so: Engine Force = Power / Velocity​ Drag Force = -Velocity²​ Net Force = Engine Force - Drag Force = Power / Velocity - Velocity²​ I'd like to ...
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1answer
22 views

Is combustion engine power defined by speed and torque?

Of a combustion engine, the speed, torque, and power is measured at the crankshaft. For a measured pair of values for speed $N$ and torque $M$, can there be more than one value for power $P$? In other ...
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2answers
33 views

Rate of charging the capacitor

Why does the rate at which a capacitor in an RC circuit charges, only depend on $R$ and $C$? We know that for power $$P = \frac{V_{\text{emf}}^2}{R} e^{-t/RC}=\frac{QI}{C} $$ so why are we ignoring ...
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1answer
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If a light bulb is connected to different voltages, is there a single method to calculate the utilized power?

If a bulb is marked at 100W and 200V, we know that the bulb utilises a power of 100W when connected to a 200V potential difference. So, can we say that the bulb, when connected to twice the potential ...
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Linear relationship between air resistance and power consumption?

Suppose that the air resistance a car encounters is independent of its speed. When the car travels at $15\mathrm{~m/s}$, its engine delivers $20\mathrm{~hp}$ to its wheels. (a) What is the power ...
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24 views

Derivation of power transmitted in a wave [closed]

In the derivation of power transmitted by a travelling wave, we consider only the force on the left side of the element which has been assumed. The force on the right side or the net force are not ...
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0answers
33 views

In a tungsten filament bulb can be approximated as $P=a(R-b)^4$. Where does the parameter $b$ come from?

I did an experiment measuring the voltage drop across and current passing through a tungsten bulb at increasing input voltage. I then calculated the resistance and power from this data, which when ...
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1answer
20 views

Calculating solar radiation toward earth

This problem is from the book by Arthur Beiser, Concepts of Modern Physics, example 1.7. Solar energy reaches the earth at the rate of about $1.4$ kW per square meter of surface perpendicular to ...
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827 views

Why do (kilo)watt-hours double-count units per time?

A watt is defined as 1 joule per second and is therefore a measurement of energy transfer over a period of time. A kilowatt-hour is a measurement of energy equal to one kilowatt sustained over a ...
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2answers
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How loud really are ultrasound transducers?

There are a plethora of mini ultrasound transducers available. Many of which have a Sound Level SPL > 110dB. This seems extremely "loud". If you were to listen to one of these transducers, would they ...
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2answers
43 views

Is work-energy theorem valid for Power = force * velocity?

Work energy theorem states "work is said to be done when there is change in Kinetic energy", i.e. $W = ∆KE$ but as we know, $P=\vec F \cdot \vec v$, where a constant force $\vec F$ maintains a ...
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1answer
44 views

Why does Joule's Law of Heating defined by $H=i^2R$ only? [duplicate]

Joule's Law Of Heating states that for a conductor Heat loss is given by $$H=i^2Rt$$ Couldn't we substitute $$V=iR$$ and show that $H=(V^2/R)t$ ? If this is so, when transmitting power across large ...
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3answers
68 views

What is meant by transmission voltage?

I had a question in class that went : $120$kW of power is generated at a power plant and is then transmitted. The resistance in the transmission lines is $0.4 \Omega$. Calculate the power lost as ...
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1answer
33 views

According to Lenz’s law, does actual number of turns in a transformer matter, or just turns ratio? If so, why?

I ‘m trying to make a transformer and I understand the basic concept of Lenz’s law, and I understand how the ratios work. I get that if you have a transformer with 100 turns on the primary and 10 on ...
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1answer
58 views

How does the area under a $F$-$v$ graph relate to power?

The Force-velocity graph for linear motion with constant acceleration from 0 to 15 m/s, followed by linear motion with constant power from 15 to 60 m/s is shown below I can then draw the Power-...
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1answer
22 views

How does increasing the battery voltage affect the power delivered to a light globe in a single light globe circuit?

I am doing a prac in my physics class and I would like to know how does increasing the battery voltage affect the power delivered to a light globe in a single light globe circuit?
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1answer
19 views

Is energy density of two $P$ W lamps the same for one $2P$ W lamp?

Do two lamps of power $P$ produce the same energy output as single lamp of power $2P$, assuming ideal environment? E.g. lamps do not occlude each other, distance between lamps is negligible, no ...
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1answer
79 views

Rate of energy dissipation in a wire carrying current cut in half [closed]

In my physics textbook, I have a sample problem stated as follows: You are given a length of Nichrome heating wire; it has a resistance R of 72 Ω. At what rate is energy dissipated in each of the ...
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Is there a graphical summary of which modulation technique to use in communication systems using Visible Light Communication techniques?

I am currently looking at various papers written (some listed at the bottom) that discuss how laser pulses can be modulated to send encoded messages to a receiver for communication systems. The more I ...
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1answer
42 views

Why is work measured linearly with the distance [closed]

If I run and push a car, the force considered is the one felt in my arms not in my legs? The distance the one marked on the field? If the energy outputs is constant and linear with time, why isn't ...
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28 views

What happens to radians in this calculation? [duplicate]

I rewrite N as kg m s^-2 and try to get Pmax, which is in Watts to kg m^2 s^-3 but when I do so I am left with an rad^2.
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1answer
35 views

Simulating a fan speed given power applied to it over time [closed]

I am driving a fan using a micro-controller. I want to test the micro-controller code without having fans physically attached to it so I thought I would model one programmatically. I want to model a ...
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1answer
57 views

Is the term “Power” meaningful? [closed]

As far as my intuition of Force goes, I believe it is a kind of 'field' (that was not the most precise of ways to describe it!) Like say, a force of 5 N acts on a 1 kg block. It produces an ...
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Radiant Intensity

Radiant intensity is power per unit area and area being perpendicular to propagation of energy. Moreover, intensity varies inversely with square of distance. My question is if a beam of light is ...
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1answer
53 views

The power generated from an hydroelectric plant is taken constant?

Using the energy equation for a flowing fluid, by doing a lot of assumptions on the flow behavior, (incompressible fluid, stationary flow, ideal flow...) and applying it to the inlet and outlet of a ...
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1answer
64 views

How same power is distributed in different voltages in power lines?

I have seen when a resistor is connected to a battery, it carries the same voltage across the resistor, if the resistance is changed the current changes but voltage remains the same making the battery ...
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3answers
168 views

Is power equal to $\vec{F}\cdot\vec{v}$ where $\vec{F}$ is force and $\vec{v}$ is velocity? [duplicate]

Can you please tell me when $P=\vec{F}\cdot\vec{v}$ is valid? Let us consider $\vec{F}$ as to be a function of time $t$. We know that $P=\frac{dW}{dt}$, where $W$ stands for the work done. $$P=\frac{...
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1answer
54 views

Why does not work done increase when power is increased? [closed]

Suppose, a man pushes a wooden block through a distance of 2 m in 2 seconds and an other man pushes the same block to the same distance in 1 second. According to my assumption, the first man must have ...

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