Questions tagged [power]

The time rate of change of energy

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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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Cross-correlation power spectra

I am trying to understand how cross-correlation power spectra (PS) are interpreted. If I have understood correctly, the following hold: A PS can either be an autocorrelation PS or a cross-correlation ...
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Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
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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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266 views

Adding a capacitor in parallel improves power factor of an inductive circuit?

I know that to reduce power loss during transmission, we require a power factor as close to one as possible. This can be done by making $Z$ tend to $R$ which can be done by increasing $X_c$ such that ...
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An electric bulb is rated 220 V and 100 W. When it is operated on 110 V the power consumed will be- [closed]

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V what will be the power consumed? The answer to the question is 25 W using the formula P=V^2/R. But if I use the formula P=VI I ...
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9k views

Calculating displacement amplitude of ultrasonic power transducer

I am not a physicist, but my current project drives me to some physics-related computations, hence seeking help. I have some ultrasonic transducers, 5938D-25LBPZT, for which very limited information ...
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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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472 views

How many watts of electricity can the human body withstand without being killed?

I'm talking about DC and/or AC. I've read about people surviving extremely high voltage shock (300kV) but that could be explained by extremely high resistance in the circuit that resulted insufficient ...
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1answer
140 views

Power of conveyor belt required to accelerate a mass

Assume the following scenario: A conveyor belt is driven at velocity v by a motor. Sand drops vertically onto the conveyor belt at a rate of $m$ kg/s. What is the power required to move the belt at ...
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1answer
27 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
196 views

Power and radiance scaling in Coherent Beam Combination

Generally, it's said that for incoherent laser beam combining, the intensity scales with N (where N = # of lasers), whereas the intensity scales with $N^2$ for coherent combining. $$I_{array,...
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55 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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765 views

Electric power transmission

If we want to transmit electic current for a long distance, we must minimize a heat that releases because of the resistanse. We cannot make a cable wide because it is expensive and it will be massive. ...
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1answer
51 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
55 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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12k views

How big is a 1kW fire?

What is the size/scale of a wood fire that is producing 1kW? I'd like to improve my ability to conceptualize various power scales.
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154 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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51 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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1answer
26 views

Harmonic driving of a non dissipatice circuit (LC for example): does the energy really sometimes goes back in the generator?

Let's assume you have a non dissipative circuit represented by a box: it involves only capacitance and inductance for example. You drive this box via a sinusoidal voltage generator. I call $U(t)$ ...
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32 views

How much solar power in kWH is there in the stratosphere during the day? [closed]

The stratosphere is above much of the atmosphere. People have talked about collecting solar energy there and beaming it back to Earth. How much power are we talking about?
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Power meter zero-offset calibration

I have to measure the output power of a laser diode. To this aim, I am going to use an integrating sphere connected to a photodetector and a power meter. Concerning the photodetector, electrical ...
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309 views

How to calculate total power generated?

I have a circuit with a piezoelectric transducer which converts mechanical energy to electrical. The ceramic is connected to a 1 Megaohm resistor and a voltmeter which reads voltage. If I want to ...
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2answers
144 views

power = force times velocity of object, what if the object is “abstract” such as “contact patch” of a tire?

Consider the case of an accelerating motorcycle, the engine converts potential energy into mechanical energy which is used to generate a Newton third law pair of forces at the rear tire's contact ...
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84 views

What does work really mean deeply? I think the definition of work is not convincing

Suppose we have two objects 1kg mass. The one of them moves by constant velocity 3m/s and then we apply to it force equals 1N so the done work is 3.5 joule. the other object is at rest and then apply ...
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Electric reactive power? Is it real or is the design accepted for calculation? [closed]

Do you agree that the reactive power definition is an acceptance for calculations? I disagree with this view. Then; How the electromagnet force or the power stored by the capacitors can be explained.
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302 views

How much heat can be adsorbed in a solar parabolic trough?

How do I know the produced heat on the focal point of a solar parabolic trough per different radius if made from steel and also from glass?
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583 views

Friction energy dissipation calculation

In a moving block, a variable force pushes it. Friction is acting against the movement. Below equation specify the amount of energy could be used to give motion: $$ E_1 = \int F_T(t) .v(t) .dt $$ ...
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How does a car gain kinetic energy?

I understand that the engine delivers power to the wheels and friction from the ground causes the wheels to roll. However, given the power (work per time) at the wheels, how does that energy become ...
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1answer
69 views

Energy power relation [closed]

If we have an energy equation like $E=A\sin(wt-kx)$ If we differentiate it w.r.t. time, we will get power. If we again differentiate the last equation, it will be termed as power per unit time. But ...
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Multiple postive solutions for resistor value selected to have a given power

Textbook Example: find the load resistance $R_L$ such that the power at the load is $500\times10^6~\text{watt}$. By Voltage Divider, voltage of load is: $$V_L = \frac{R_T}{R_L + R_T}V_s\tag{1}$$ ...
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192 views

What exactly is average power?

This question arises due to a problem I'd been solving recently: The question was to find the average power supplied by the water over the six hour period. Now I have read this post which ...
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Isn't the equation for power more complicated? [closed]

I’ve been told that work is equal to $Fs$ and power is work done per second $dW/dt$ so looking at the equation, isn't power more like this? $\frac{dF}{dt}s + F\frac{ds}{dt}$
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Why doesn't NASA try using high-explosives to fuel rockets?

For a smoothly burning liquid-vapor mixture, I understand that burning hydrogen and oxygen is the best possible solution, not just for now but even theoretically. (?) But I read about ideas, that ...
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5answers
11k views

Why does a higher wattage incandescent light bulb have a lower resistance value than a lower wattage incandescent light bulb?

I am an electrician and know through experience that resistance in an electrical circuit causes heat. An incandescent light bulb's light is a by-product of heat, so why does a 100w bulb have a lower ...
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429 views

Measuring output optical power of an LED

How do I measure the output optical power of an LED? I have a detector that measures output power of a laser incident on a small opening. The problem with using the LED with the same detector is that ...
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3answers
361 views

Energy consumption from acceleration

I have an interesting problem. I have a machine which has to lift 35 kg (this is the total mass of the object and the lifting arm). The lifting is done by a servo motor. Now I want to calculate the ...
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2answers
187 views

Why does potential energy time-derivative depend only on the mass flow?

This is a homework exercise, but I don't want specifically to tell me how to resolve the exercise. I want you to help me what's wrong here, and why I can't do it. Water is pumped from a lake to a ...
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1answer
21 views

Why are rotors like these on a cup anemometer not effective for wind turbines?

Intuitively, radial rotors (like these you would have on a wind speed meter) equipped with cups/cones/sails look like they should be very efficient at converting air flow into energy, given that they ...
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1answer
33 views

Problem: find power required to keep metal attached to electromagnet

Suppose I need to find out the efficiency of electromagnet (solenoid) mounted on the table. I energize it so that $1\text{lb}$ steel object gets attached to it. I energize it with DC current from a ...
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2answers
66 views

Power of a river

I'm trying to calculate the power of the Mississippi, using the section of the river as it passes through New Orleans. I have the flow rate of a river, turned into a mass flow rate of $16,992,000 \:\...
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4answers
112 views

Where does the power delivered to car's wheels go?

Okay, so power is work/time. Most cases, when power is provided to something, energy is gained as kinetic energy or lost to friction. But in a car, the engine puts power ( torque x rpm/5252) to ...
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1answer
48 views

Calculating Rocket Engine Power from Force and Burntime

I am struggling to relate power to force. The example that I am working on in my head is a model rocket. This model rocket (sans engine) has a mass of 50g. Its engine has a mass of 10g (for the ...
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1answer
110 views

Why is the steady speed neglected when calculating work or power? [closed]

A driver notices that her 1080-kg car, when in neutral, slows down from 95 km/hr to 65 km/hr in about 7.0s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car ...
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36 views

Power Generation from Gases [closed]

Can a heated gas be used to drive a turbine? I am not referring to steam heaters here. Why are already RTP gases e.g. argon or helium not heated and pressurised then used for power generation? *RTP ...
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106 views

Work done by who/what when involving human body moving themselves (push up, stairs etc)

Let's say we do a push up. If asked to compute the work, it would be Mass(lifted) x G(gravity) x height moved. I get that. What confuses me is how/what force does the work? Part of me feels it ...
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1k views

How much power in a lightning strike?

The Biblical story of 1 Kings 18:20-40 relates how a lightning strike ignited a bulls carcass, as well as a wood pyre, both of which had been thoroughly wetted with abundant water. Is there enough ...
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5answers
13k views

Why does a 60W bulb glow brighter than a 100W bulb in a series?

In my physics class I have this problem that shows two lightbulbs, one 60W and one 100W in series, connected to a 120V battery. The problems are: Which bulb is brighter? (A: 60W) Calculate the ...
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46 views

How to show that imaginary part of complex power is reactive power?

Suppose I have a voltage across an element $V(t) = A \cos(\omega t)$ and the current through it given by $I(t) = B \cos(\omega t + \phi)$. The instantaneous power is $$P(t) = V(t)I(t) = AB \cos(\...

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