# Tagged Questions

The time rate of change of energy

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### Bernoulli equation and pumps

I have a doubt on the use of Bernoulli equation for pumps. Consider the situation in the picture. I marked different points: $1$ on the surface of first tank, $2$ in the exit from first tank, $3$ ...
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### Is power still consumed when we switch off the lights?

I was having heated discussion with one of my friends. We were discussing Earth Hour and its role in saving electricity, and creating awareness. He stated that even if you turn off the lights, the ...
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### Elementary derivation of $P = UI$ starting from the definition of voltage as work done per unit charge

Consider the following elementary derivation of the formula $P = VI$ for Joule heating of a resistor $R$ for the dissipated power $P$ where the voltage over the resistor is $V$ and the current $I$. ...
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### How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
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### Different locations of a pump in a tube

I'm confused about pumps in fluid dynamics. As far as I understand, the basic effect of a pump which deliver a power $\mathcal{P}$ can be described with modified bernoulli equation between a point $A$...
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### Different interpretations of the power in a resistor law?

On this KhanAcademy article, under the Power in a resistor heading, it states three equations, all of which can be derived from $V=iR$ and $P=iV.$ They are: $P=Vi$ $P=i^2R$ $P=\frac{V^2}{R}$ Then ...
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### How much power to keep surface of aluminum plate at given temperature? [closed]

I want to heat one side of an aluminum plate enough to hold the other side of that plate $100K$ above ambient. I'm willing to assume that the heated side of the plate is "well" insulated (along with ...
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### What's the relation between output voltage and time to boil water given the same kettle?

An electric kettle rated 220V, 2000W needed 10 minutes to boil water when it is half filled with water in Singapore where the output voltage is 220V. Estimate the amount of time needed to do the same ...
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### Intensity of a standing sound wave at displacement nodes?

We know $$P = F\cdot v$$ where $v$ is the velocity vector. Since at the "displacement nodes" in a standing sound wave the velocity of the particles is always 0, the Power must be 0 and hence the ...
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### Power output of electrical hotplate

This sounds simple but i need some clarifications. I am using a hot plate to heat a solution in an oil bath. The power rating and maximum temperature of the hot plate is 1520 W and 500 oC ...
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### Nuclear to Electricity Energy Conversion

Currently nuclear power generates heat, which heats water into steam that turns conventional turbines. The energy conversion is as follows: photonic->heat->kinetic->electric This would result in low ...
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### 16 Volt on a 12 Volt LM317 Regulated Power Supply [migrated]

I recently purchased a 1,2-12V regulated power supply with an LM317 chip and accordingly, a 220V to 12V transformer. Because I did not need the added functionality of the "logic pen" facilitated by ...
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### Which power equation to use: $P = I^2 * R$ or $P = V^2 / R$?

Given are ideal max voltage $V = 200\;\mathrm{V}$ and max current $I = 5\;\mathrm{A}$. Therefore: ideal resistance is $$R = \frac VI = \frac{200 \;\mathrm{V}}{5\;\mathrm{A}} = 40 \;\mathrm{\Omega}$$...
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### Power generation with a long, horizontal pipe, using atmospheric pressure differences

Straight to the point. If I float a pipe starting from a point in the ocean that has the atmospheric pressure of 101 kPa (Call it Point A) all the way to another geographic position on the ocean with ...
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### Calculating the electrical power generated from a wind turbine [closed]

Heres the question: A wind turbine has blades that sweep an area of 2000 m^2. It convrts the power available in the wind to electrical power with a efficiency of 50%. What is the electrical power ...
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### How can the amplitude of the same acoustic wave be different in a interferometer?

I came up with an exercise on Halliday Resnick Krane that asked a question which consfuses me. I premit that I do not look for a solution of the exercise but for suggestions only regarding the ...
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### Optimal speed for the water wheel

The hydroelectricity plants extract the potential energy of highly deployed massive object (water) as it falls down. Without turbine, all that energy would be converted into speed (kinetic energy) at ...
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### Velocity over time applying constant power and considering air drag

I wanted to understand how velocity changes over time when applying a constant power to a body of mass $m$. I figured out that in this case, since $$P=d/dt(KE)=d/dt(\frac{1}{2}mv^2),$$ integrating I ...
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### Do we use rms values or peak value for calculating power of a transformer?

Do we use rms values or peak value for calculating power of a transformer? I have an A levels exam tomorrow and I wasn't able to find any kind of explanation in the book.
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### Difference between $I^2R$ and $V^2/R$ and $VI$ for measuring power $P$

We use $I^2R$ or $V^2/R$ or $VI$ for measuring power $P$. Are all of these applicable for all circuits? I have seen in some circuit $V^2/R$ is not equal to $I^2R$. Why is that?
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### Power needed to raise a bucket at a constant speed by a pulley

This is the last section of this pulley exercise: The bucket in is filled with water so its total mass is now $45$ kg. The bucket is raised at a constant speed of $2.0$ $ms^{-1}$ using an ...
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### What does $P = Fv$ mean? [closed]

The engine of a car is developing a power of 90kW when it is moving on a horizontal road at a constant speed of 100 km h−1. Estimate the total horizontal force opposing the motion of the car. I ...
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### Question for power and Ohm's law [closed]

I have two questions specifically. Can the usage of Ohm's law be utilized to predict current, voltage in circuits such as parallel, complex and series? How would I explain this? How does the power ...
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### How do Calories (kcal) relate to watts?

In thinking about exercise and "burning calories" it occurred to me that there should be some fairly intuitive correlation between the biological energy conversion going on in a person's body with the ...
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### Is it possible to approximately know the wattage/luminosity of a lamp knowing distance and using a DSLR?

I'd like to know if it is possible to approximately know the wattage/luminosity of a lamp knowing distance and using a DSLR (digital single lens reflex camera) mounted on tripod. Maybe, when shooting ...
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### Why do people say that 'power flows' into or out of a component in an electric circuit?

Currently, I am learning about electric circuits, and how to apply the basic concepts of physics, like work and energy. But I keep running into the same phrase--on Wikipedia, my textbooks, everywhere--...
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### Rankine Cycle Reheat Mass Flow Rate

Having worked through many examples on the rankine cycle both non-reheat and reheat, i just have one question. Can the mass flow rate of steam be calculated given only the data below? Boiler ...
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### How is the dot product a generalization of multiplication?

I've seen an interesting explanation for lots of what I previously thought were unmotivated definitions in Newtonian mechanics, namely that power is always defined as effort times flow. But when ...
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### power of an engine with wind

I know the power of an engine to overcome air drag is $1/2 c_d ρAv^3$. If the speed of the car $25\ \mathrm{m s}^{-1}$ (relative to the ground) and it travels in opposite direction to a wind at a ...
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### What exactly is average power?

This question arises due to a problem I'd been solving recently: The question was to find the average power supplied by the water over the six hour period. Now I have read this post which ...
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### $P = \frac {v^2}{R}$ derivation confusion [duplicate]

I have always had a confusion of why we use $P = V I$ or $P = I^2 R$ and not $P = \frac {V^2}{R}$ for relating to power loss due to heat in high tension lines. I know there are a lot of ...
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### Could we use solar energy with endothermic reactions to fight global warming? [duplicate]

Im in holidays in Malaysia and it is god damn hot. You cannot leave your hotel/car and go out. All this because of the sun. I we were to capture all the heat (solar panels, ...) would this help make ...
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### Relationship among heat generated, power, current and voltage

Can you help me understand the relationship between current, voltage and heat generated? I tried two sets of heated gloves. One uses a battery which generates 7.4 volts x 2 amperes (14.8 watts of ...
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### Would a thinner atmosphere (such as high altitude) yield better solar panel energy generation?

I am curious. If you were to put a solar panel up in high altitude or on the surface of the moon, both of which have much less atmosphere to reflect/refract light, would that solar panel produce more ...
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### In 3 three power system, can damage occurs from incorrectly connected phases?

A three phase generator, no grid involved, phase 1 connected to phase 3 on the load, and phase connected to phase 1 on load, what is going to happen?
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### Opposing currents in a Ciruit

If I have two currents flowing in opposite directions in a wire where both currents are equal magnitude and their is a bulb on the same wire does the bulb light up at all or is it bright/dim ?
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### In which direction should flow of electric current be taken while solving problems?

Consider a simple circuit with a battery of $\theta\ \text V$s, and two resistors of $R_1 \ \Omega$s and $R_2\ \Omega$s connected in series. Let us assume that $R_1$ is connected nearer to the ...
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### What is the power given by centripetal force?(in circular motion)

A particle of mass $m$ is moving in a circular path of constant radius such that the centripetal acceleration is varying with time as $a_c = k^2rt^2$ where $k$ is constant. The power given to the ...
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### Voltage and Current in transformers

In transformers, the ratio of the voltages equals the ratio of the turns - so double the output coil's turns and the output voltage doubles. Then, in order to conserve energy, current halves. This ...
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### Laser power consumption vs power output [closed]

I have a 5 mW laser pen powered by a 1000 mAh 3.7 volt battery that takes over 2 hours to recharge. It drains it in 15 minutes. Why is it only called 5 milliwatts output if it is consuming 14.8 W?
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### How much power can nuclear power plants produce? [closed]

I looked at the power plants with the highest capacity and hydroelectric plants seem to dominate the list. Surely nuclear plants could be far more powerful? I assume they are limited because of ...