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Questions tagged [power]

The time rate of change of energy

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3answers
202 views

What does the equation $P=\vec{F}\cdot \vec{v}$ really mean?

Let's say that we have a car travelling at a constant velocity (V) on a flat plane , let's assume that it's applied force or the force applied from its engine ( Fapp ) balances the frictional force ( ...
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1answer
27 views

Why is the power for an object of mass m moving at a higher velocity greater than that for the same object moving at a lower velocity?

I understand the equation Fv but I will like a more intuitive answer concerning why the rate at which energy is transferred must be greater
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1answer
95 views

What is the relation between a fixed force and work?

How would you translate a constant force into work or energy? If I have a way to output 100KN continuously, how can I translate that into watts? Is there a formula for that translation?
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3answers
544 views

Question on work, energy and power

The engine of a train is working at a constant rate. The maximum speed of the train up a certain incline is $V_{1}$ and the maximum speed down the same incline is $V_{2}$. Then if the train moves on a ...
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1answer
75 views

Fixed voltage source & maximized power

The goal of a voltage source like a battery for example is that it will provide a fixed voltage regardless of the load resistance. The battery has internal resistance Ri If R_L≫R_i, the voltage ...
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1answer
1k views

Relation between power and energy

If I have an expression for the 'rated generator power' of a wind turbine, call it $P(t)$ how is the energy, that the wind turbine generator generate, related to the power $P(t)$? So: $$E(t)=\...
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0answers
89 views

Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
2
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1answer
456 views

Is there a difference between joules of energy per coulomb and volts?

There is a question in my textbook: A current of $7\ \mathrm{A}$ goes through the heating element in a hair-drier. The voltage of the heating element is $240\ \mathrm{V}$. b) How many joules ...
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3answers
128 views

Where to use $I^2R$ and $\frac{V^2}{R}$ for power [closed]

I was given the following question, Which of the following statement(s) is/are correct? (a) Resistance of the filament of the bulb is inversely proportional to the power of the bulb. (...
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4answers
7k views

Why a series-wired light bulbs consume less energy than a parallel-wired light bulbs? [closed]

Why a series-wired light bulbs consume less energy than a parallel-wired light bulbs ?
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1answer
129 views

How is power defined?

Isn't power defined by: $$P=\frac{dE}{dt}$$ (?) I see in all websites that power is the amount of work done per time. Is it represented only like this? Why?
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2answers
600 views

Calculations involving kilowatt and kilowatt hour

:) I am very confused with the calculation involving kWh and kW. I have tried searching my question on Google but couldn't find anything. Say, we have a device with a power rating of 1 kW. This ...
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1answer
514 views

Crate pulled on an incline with constant speed. What is the total work done?

A rope is attached to a 50.0-kg crate to pull it up a frictionless incline at constant speed to a height of 3-meters. Note that the force of gravity has two components (parallel and perpendicular ...
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3answers
54 views

Would we require energy to walk in a gravity free space?

If there was no gravity would our leg muscles lose energy to make us walk. When we walk we cover strides . when we lift our one leg from rest on ground accelerate it until it reaches top say to speed ...
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1answer
128 views

How many watts of power does the man use?

I was in the middle doing my homework perfectly fine until I encountered this question. How many watts of power does a man with a weight of 600.0 N have when he climbs a flight of stairs 3.0 m ...
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2answers
68 views

Does the same amount of laser energy ablate more material if delivered at higher power?

Consider laser cutting. Will 1 Joule of energy penetrate deeper into material if it is delivered at 100 Watts rather than 50 Watts? Consider alternative: suppose you can to cut up to 6 mm thick PMMA ...
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2answers
438 views

Does a rocket engine apply more power as the rocket's speed increases? [duplicate]

To clarify my level of knowledge, I'm a high school student all the way through AP Physics C: Mechanics. So, let's imagine that there is a rocket travelling through the vacuum of space (ignoring ...
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2answers
5k views

Why do power lines use high voltage?

I have just read that using high voltage results in low current, which limits the energy losses caused by the resistance of the wires. What I don't understand is why it works this way. Does it have ...
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1answer
335 views

Derivation of the Lorentz transform of brightness ($dP/d\Omega$)

In the book 'Relativity made Relatively Easy' by A.Steane their is a derivation of the Lorentz transform of the 'brightness': $$ \newcommand{\p}[2]{\frac{\partial #1}{\partial #2}} \newcommand{\f}[2]{...
3
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3answers
331 views

RMS Values in AC Circuits

When do you use RMS values when calculating power? My physics textbook provides several equations for determining the power dissipated in AC circuits. For example, there are power equations using ...
3
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3answers
925 views

How many photons in a ultrafast laser pulse?

Photon has a specific wavelength $\lambda$. Imagine we created a mode-locked pulse, with $80\: \text{MHz}$ repetition rate, i.e. pulse are separated by $13\: \text{ns}$. The pulse duration is $4\: \...
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0answers
154 views

How to Calculate the total scattered radiation power and total radiation power.?

Let's suppose I have incident field $E^i_z$. The components of Scattered electric and magnetic field are $E^s_z$, $E^s_\phi$, $H^s_z$ and $H^s_\phi$. The electric and magnetic field components ...
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1answer
467 views

Work, energy and power [closed]

A load of mass $m$ slides down a smooth plane with inclination title from point $A$ to $B$. The same mass is then dropped from $A$ to $C$. Could anyone decipher this $\downarrow$. I've edited the ...
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1answer
1k views

How to find power required for rotating blades given the required shear cutting force [closed]

I want to design a sugar cane bud cutting machine. The rotating blades are connected to the output shaft of a speed reduction box ($30\,{\rm rpm}$). The shear force required for cutting is $1200\,{\rm ...
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2answers
416 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
161 views

If two or more different electric heaters have the same power consumption are they providing the same amount of heat to the room?

If I have a number of different electric heaters. Say one has coils which heat up, one is a fan heater and another is an oil heater in which the oil is heated by electric coils internally (They are ...
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2answers
2k views

work done is zero if displacement of point of application of force is zero but why can't we assume forces on COM and its displacement

suppose i jump from ground then work done by normal is zero as point of application displacement is zero(work is doneby internal forces) but during the pushing part COM has upwards displacement then ...
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1answer
246 views

Joule's law in constant cross-section conductors

I know that the work needed to move a charge $q$ from a point A to a point B in presence of a steady electric field $\vec{E}$ is $P = \displaystyle\frac{dW}{dt} = \displaystyle\frac{d}{dt}\left(q\...
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3answers
6k views

In the equation “Power=Force . Velocity”, if velocity is considered constant, how can force exist?

Power(P)=Force(F) . Velocity(V) "In the straightforward cases where a constant force moves an object at constant velocity, the power is just P = Fv. In a more general case where the velocity is not ...
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1answer
51 views

Wattage and wattage per hour discrepancy

Having moved to a new apartment recently, I checked the water heater and the boiler says that it has a 1500W heating unit, but the kWh said 2.5. Having only an extremely rudimentary in physics and ...
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1answer
42 views

work of body remaining still in gravitational field

How to quantify power output of a statite? It's a kind of (non-)satellite, that remains immobile in relation to the central body (e.g. hovering over one of the poles of a planet) through continuous ...
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4answers
2k views

Why doesn't increasing resistance increase brightness if $P=I^2\cdot R$

Light bulb brightness increases with power, $P$. So why doesn't increasing $R$ increase $P$ and hence increase brightness as $P=I^2\cdot R$ due to $P=I\cdot V$ and $V=I\cdot R$? I read increasing $R$...
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2answers
2k views

Sun constant formula derivation

In a book, I've to read for a class at my university, called 'Duurzame Energietechniek' (Dutch, translated: renewable energy) they gave a formula that looks like: $$\text{G}_\text{sc}\left(\text{n}\...
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2answers
4k views

Power required to move an object

Edited to better reflect the answer I was after * I have a trolley with laden weight of 110kg that needs to ascend an incline of 6 degrees at a speed of 10km/h. Provision is made to reach that speed ...
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1answer
34 views

Determining power required to close gap between moving objects

I am running a simulation of two bicycles traveling in the same direction in a single dimension. Bicycle A is $z$ meters behind bicycle B. The acceleration of each bicycle determined by the power (P) ...
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6answers
8k views

Can a gym be built to supply electricity to homes? [closed]

A moving magnet induces a current in a conductor, then shouldn't we be able to generate electricity through manual labour? I was thinking about building a gym that used magnets as weights. People ...
111
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12answers
47k views

Why don't we use weights to store energy?

One of the main reasons why we haven't switched to clean energy is the lack of efficient storage methods - But, why aren't we using dead weights to store energy and draw it back later when needed? As ...
2
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0answers
54 views

Why does an alternator slow down when the 3 phases are shorted?

I'm designing a small wind turbine and noticed that when i short the three wires coming out of the alternator, the alternator is breaking and the blade RPM is dropping. What is the explanation for ...
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4answers
1k views

Can I use the work done to calculate power of a motor?

I am trying to calculate the energy/ power required to move a plate up and down in a liquid. My approach is to calculate the forces that must be applied by the motor to push/pull the plate based on ...
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3answers
438 views

Power/torque ratio vs acceleration

Let there an object with determined mass which needs to be moved with a motor (lets say electrical). We have two available motors, which have exactly the same power (and other characteristics), but ...
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1answer
38 views

Power expression for a dynamic force

Given that: $$W=\int_{x_0}^x \vec{F}\cdot d\vec{x}$$ and assuming $F$ is time dependent, we get: $$\frac{dW}{dt} = \int_{x_0}^x \frac{d\vec{F}}{dt}\cdot d\vec{x}+\vec{F}(x,t)\cdot \vec{v}$$ ...
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2answers
2k views

Power in Series Combination Of Appliances

The general formula for power, P for an appliance is P= (I^2)R where I is Current flowing across a resistance R Thus power, P is directly proportional to resistance, R of an appliance for a ...
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1answer
72 views

Using the formula for electric energy confusion $W = P * t$

I have this example question: A low energy lamp has the label 230V, 15W. It gives about the same amount light as a regular lamp at 75W. How much energy do we save by using the low energy lamp if it's ...
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4answers
3k views

Contradiction in Ohm's Law and relation $P=VI$

Ohm's law states that  electric current is directly proportional to voltage provided that physical conditions like temperature remain constant i.e. $$V = IR$$ On the other hand, $$\text{Power = ...
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2answers
2k views

Does the brightness/power of each individual bulb change when you add more bulbs in a parallel circuit?

I have asked this question on different sites and have received contradictory answers. Assume that there is an ideal voltage source and the resistance of each bulb is the same. I believe that the ...
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1answer
6k views

Why is the lamp with the lower power rating brighter in the series circuit?

If I have two lamps, both of them have different power values, and voltage value is constant , I've been told that the lamp that has less power would be brighter and make more light than the lamp that ...
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3answers
371 views

Power in a uniform circular motion

I've a circular motion and I want to find the power that the motion is generating. The formula's I got: $$\text{P}\left(t\right)=\text{W}'\left(t\right)=\text{E}'\left(t\right)$$ Where $\text{P}$ is ...
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1answer
320 views

Relationship Between Watts and Candela

I am currently working on a project that involves the use of LEDs. According to the datasheet, the LEDs I am using have a maximum brightness of 10,000 mcd. In my project, though, I will not be using ...
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1answer
40 views

Are large scale optical power delivery systems feasible?

I am attending an introductory course of optics and photonics and we just started to study optical fibers for digital communications. However, I have started wondering about using optical fibers to ...
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2answers
323 views

AC vs DC & why we ended up using AC for power in our homes, etc [duplicate]

I think that DC is better than AC (and yes I do have a reason, check the comments). Also, if it (DC) is in-fact better than AC why did we use AC then?