Questions tagged [power]

The time rate of change of energy

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820 views

How do you find instantaneous power?

I was wondering is there a way to find instantaneous power when you are not given an equation relating work with time but only the numerical values of quantities like force, mass of body, acceleration ...
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2answers
246 views

Acceleration with zero initial power?

Power = Force x velocity At the initial instant, the velocity = 0, so the power is zero, even though there may be force, but zero power is taken from that force. So how can the object even start ...
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2answers
465 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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1answer
72 views

How can energy of petrol be compared to electricity?

When driving my car, which is powered by an electrical engine, I'm consuming between 10 and 17 kWh per 100 km (measured at the charging outlet). The car I had before consumed between 5 and 8 litres ...
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1answer
3k views

Relationship of Pump Power to Flow Rate

I read two different things about the relationship between power (P) of a pump and flow rate (Q). The most common thing that I found was simply that Q is directly proportional to P (power-of-pump). ...
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1answer
645 views

How does the voltage source provide energy in a circuit?

I understand that a voltage source drives the current to flow from the terminal with lower potential to the terminal with higher potential. However, if the electrons are gaining potential energy then ...
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2answers
182 views

Why does discharging a capacitor give a higher power output than using a battery?

Let's say you charge a capacitor with a 10V battery. The capacitor will have a potential difference (pd) of 10V once it is fully charged. If you then connect a light bulb to the capacitor, the pd ...
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2answers
140 views

Relative height or height from ground?

Ok so I encountered this question -> A 10 H.P. motor pumps out water from a well of depth 20m and fills a water tank of volume 22380 litres at a height of 10m from the ground. The running ...
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2answers
207 views

Why does Work Done Appear to be More than 100% Energy in a System? [closed]

Energy is equivalent to the work done on a motor. The work done on the motor is multiplied by more than 100% by recycling the energy the motor uses. Why then does the energy in the system appear to be ...
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2answers
70 views

Power in electrical circuits

Why does $P=i^2R$ apply only to transfer of electrical potential energy to thermal energy in a device with resistance while $P=iV$ apply to electrical energy transfers of all kinds?
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2answers
107 views

How can we show Power = $\mathbf{F}\cdot \mathbf{v}$? [duplicate]

How can we say that $$\text{Power} = \mathbf{F}\cdot \mathbf{v}$$ We know that small work done by a force $\mathbf{F}$ to displace an object by '$\mathbf{x}$' is $$W = \mathbf{F}\cdot \mathbf{x}$$ ...
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5answers
340 views

Why does a rocket engine have increasing power as the speed of the rocket increases? [duplicate]

Sorry to repeat this question, but I got no definite answer last time, so I am trying to state the question more clearly. Power = force x distance / time = force x velocity. So if we apply a small ...
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1answer
272 views

Infinitesimal work

I am a newbie in Physics (Senior on highschool) and our teacher wrote in a proof $$\dfrac{dK}{dt}=\dfrac{dW}{dt},$$ where $K$ is the Kinetic energy of a body and $W$ is the Work. So now that I am ...
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1answer
23 views

Experiment to determine cyclette power

I have a cyclette with 8 resistance levels set magnetically using a knob (a magnet is moved near the rotating disk thereby increasing its rotating resistance). The distance from the pedal to the ...
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4answers
372 views

infinite power from constant force?

P = W/t = Fd/t = Fv So, if we keep exerting a small constant force and the velocity keeps increasing to near infinite, we should be able to get near-infinite power from the small constant force. We ...
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4answers
60 views

Why does only one of the DC circuit power equations work for me here?

I know that $$ P=IV=I^2R=\frac{V^2}{R}$$ I solved for the current using the loop rule $$I=.47$$ but to solve for the Power output of the 16V battery I could only find the power by finding the ...
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1answer
144 views

Why is my solar panel more efficient as the temperature increases, rather than less efficient?

I have recently performed an experiment in which I placed a solar panel inside a closed box, together with a heater and a lamp (60W). I increased the temperature using a heater from 20ºC to 50ºC. The ...
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1answer
228 views

What is the energy output of Solar thermal collector?

I have a solar thermal collector. Its LCD screen shows an output of about 5KWH at the end of the day. How to interpret this number? Is this electrical KW (kilo watt)? If yes, isn't too low for a 1....
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1answer
74 views

Calculating the power output of car gives two different answers [closed]

Here is the problem: A 500 kg car accelerates from rest to 100 m/s over a distance of 400m with the average frictional force of 1200N. If it took the car 7.3 seconds to do this, what is the power ...
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2answers
222 views

what causes a decrease in velocity?

Let's assume that a car is travelling at a constant velocity (V) on a horizontal flat plane , hence the force of friction is balanced by the force applied by the engine. The power which the engine ...
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3answers
189 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
26 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
85 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
494 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
72 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
892 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
85 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 ...
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1answer
396 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
111 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
6k 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
123 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
565 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
442 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
49 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
107 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
67 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
413 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
4k 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
313 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]{...
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3answers
281 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 ...
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3answers
815 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
149 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
438 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
400 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
147 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
223 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
49 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 ...