Questions tagged [power]

The time rate of change of energy

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22 views

Instantaneous Power transmitted across a wave

As the power transmitted across a point in a string wave is given as $u \omega^2 A^2 v \cos^2{ \omega(t- \tfrac{x}{v})}$ and average power transmitted is given as $\tfrac{1}{2}u\,\omega^2A^2v$ ...
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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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Determine power of the engine [closed]

Upon a sloping roller and from a negligible height, sand is placed with a rhythm of $\frac{dm}{dt}$. Thanks to an engine that provides a power $W$, the speed of the roller is kept constant ($v_0$). ...
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19 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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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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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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19 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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32 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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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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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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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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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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820 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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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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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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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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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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32 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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Energy transmitted through a point in Wave motion

If the energy transmitted through a point is $uw^2A^2v \cos^2w(t-x/v)$ and energy transmitted for a wavelength is half of that,how come when u apply the formula to find the energy of a wavelength it ...
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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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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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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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62 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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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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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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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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56 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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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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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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160 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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53 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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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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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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148 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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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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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
70 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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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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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 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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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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114 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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50 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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112 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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