Questions tagged [power]

The time rate of change of energy

581 questions
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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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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 ...
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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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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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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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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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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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 ...
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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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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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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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 ...
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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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 ...
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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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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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 ...
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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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 ...
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 ...
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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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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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?
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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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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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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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 ...
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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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 ...
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\...