# Questions tagged [power]

The time rate of change of energy

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### How to determine the best fit battery for a battery powered application? [closed]

This is an attempt to get some scientific information about how to best use batteries (I'm going to refer to both single cells or assemblies of cells as batteries for the sake of this question) in ...
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### Can power (in the context of work and energy) ever be negative?

Consider this question which HAS definitely been asked but i ask you now to question your first inclination to just 'do the question' and explain the thought process. Restrictions: assume you only ...
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### How do microwave transformers avoid power loss through the skin effect?

I was reading about the skin effect and how the effect increases for higher frequencies. I also read that Litz wires can be used to counteract the effect, but usually only up 1 Mhz frequency. Is ...
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### 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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### Question about the relationship between resistive drag, dissipated power, and velocity

This is from a high school physics exam for 16 year olds that I did two years ago. I still don't understand it. The power $P$ required to move an object through a medium at constant speed depends ...
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### Units in the power equation

I have a question with the formula power = energy / time. I know it's a simple formula, but when substituting into this formula, Does the energy have to be in joules or watts (no kJ or something ...
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### Power of an engine

Suppose a car of mass $m$ starts out with an acceleration $a$ in the positive direction (say +ve x-axis), against a constant Resistive force $R$. At the instant it's velocity is $v$, what is power of ...
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### Converting lux to watts with known wavelength

I was wondering if there was a possible way to convert lux to watts accurately if the wavelength is known. I know that for the sun there is an approximate conversion of 0.0079W/m$^2$ per Lux using ...
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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 ...
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### 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 ...
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?