# Questions tagged [power]

The time rate of change of energy

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### High voltage power lines - clarification of energy loss

I've been having a bit of trouble understanding the high-voltage power lines. If I was sending power from $A \rightarrow B$, we have: Ohm's law $V = IR$ Power lost in the form of heat $P = I^2 R$ ...
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### Why isn't the product rule used in the definition of mechanical work?

Mechanical power is normally defined as $P = \mathrm{d}W/\mathrm{d}t$, and work is normally defined as $W = \vec F \cdot \vec x$. Today an undergrad pointed out a confusion he had from Griffiths' E&...
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### Why aren't (domestic) kettles insulated?

In my experience of buying and using kettles, I have come across none which are insulated. The obvious reasons as to why it would be beneficial being that heating time would be reduced, similarly, ...
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### Why is power dissipated in a circuit maximum when external resistance is equal to the internal resistance in the circuit? [closed]

[![question 24][1]][1]Suppose in a circuit the battery has emf 6V and internal resistance 3ohm. It is connected to an external resistance = R ohm. According to my ...
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### How to inject the maximum acoustic power into a block of granite?

I know that we can use transducers that are glued to a surface to achieve this. If I want, for example, to have 200 watts of actual acoustic power in the audible range in the granite, is a transducer ...
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### Does the 'mAh' rating of a battery have something to do with its power?

I'm curious about the 'mAh' of a battery: how can this impact the power of the battery? I've done some research on the internet, and most of the articles I found explain about the 'amount of charge ...
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Say I want to pump water from one container to another. The water levels are 3 meters apart, and I want to pump 10 litres per hour. I figure the mechanical power necessary, assuming no losses, is:  ...