# Questions tagged [power]

The time rate of change of energy

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### Why are rotors like these on a cup anemometer not effective for wind turbines?

Intuitively, radial rotors (like these you would have on a wind speed meter) equipped with cups/cones/sails look like they should be very efficient at converting air flow into energy, given that they ...
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### Problem: find power required to keep metal attached to electromagnet

Suppose I need to find out the efficiency of electromagnet (solenoid) mounted on the table. I energize it so that $1\text{lb}$ steel object gets attached to it. I energize it with DC current from a ...
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### Where does the power delivered to car's wheels go?

Okay, so power is work/time. Most cases, when power is provided to something, energy is gained as kinetic energy or lost to friction. But in a car, the engine puts power ( torque x rpm/5252) to ...
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### Calculating Rocket Engine Power from Force and Burntime

I am struggling to relate power to force. The example that I am working on in my head is a model rocket. This model rocket (sans engine) has a mass of 50g. Its engine has a mass of 10g (for the ...
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### Why is the steady speed neglected when calculating work or power? [closed]

A driver notices that her 1080-kg car, when in neutral, slows down from 95 km/hr to 65 km/hr in about 7.0s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car ...
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### Power Generation from Gases [closed]

Can a heated gas be used to drive a turbine? I am not referring to steam heaters here. Why are already RTP gases e.g. argon or helium not heated and pressurised then used for power generation? *RTP ...
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### Power of conveyor belt required to accelerate a mass

Assume the following scenario: A conveyor belt is driven at velocity v by a motor. Sand drops vertically onto the conveyor belt at a rate of $m$ kg/s. What is the power required to move the belt at ...
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### Interpretation of real and imaginary parts of the Poynting vector in a vacuum

It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it. Then, its real part is the active power, while its imaginary ...
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### Potential difference and current

According to Ohm's law: $V=IR$. Therefore $V$ is directly proportional to $I$. Furthermore, $V=W/q$ and $q=IT$. Thus $V=W/IT$. Therefore $V$ is inversely proportional to $I$. How can $V$ be both ...
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### Power in different reference frames

I would like to reopen the question asked in this post because I am not quite satisfied with the accepted answer. Imagine observer A stationary (in world reference frame) and observer B moving with ...
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### How Much Power Is Required to Hover?

Suppose I have a spaceship that weighs 1,000 kilograms. I take it to the surface of the planet with a gravitational acceleration of 10 meters per second-squared. The planet has no atmosphere and I'm ...
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### How would you find the time taken for a steady current for a superconductor?

For part (c) I need to find the time taken for the solenoid to gain maximum magnetic field strength. However, since it uses superconducting wires, the total resistance is zero, so how would I be able ...
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### Contradicting relation between total power consumed and power consumed by each individual resistor in series and parallel configuration

In a given series configuration of n resistances of equal resistance r, I derived the relation bewteen total power and individual power consumption as follows: P(Total) = I^2 * R R= nr (equivalent ...
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### Is a Microwave oven without a faraday cage a health hazard?

Microwave ovens have something of a Faraday-cage/shield to keep from leaking (emitting) EM radiation. What would happen if the shield or the door was removed? No doubt it would be less efficient as ...
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### Power voltage relation

I have a question concerning the relation between power and voltage. We have a lamp with power of 36W when used with 12V. With what power does the lamp work if we connect it to a 6V battery? First,...
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### Power through an unknown element

I've been stumped by a pretty simple question about power through an element in a circuit for a few days, and I was hoping someone could clear it up. In the first diagram, the way I think of it is ...
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### Fan and venturi effect

A simple calculation shows the power $P$ you can get from wind at speed $v$ passing through an area $A$ is (in Watts) $$P = 1.23\,v^3 \frac{A}{2}.$$ Using the above equation and taking the Venturi ...
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### Behavior of force, velocity and acceleration when the power output becomes constant

This might look like a homework question, but really isn't. I am teaching myself some basic physics and am shamefully stuck in one place. I have designed the problem below to help myself understand ...
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### Calculate Retinal irradiance (W/cm² to log photons/cm²/s)

I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system. Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a ...
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### Basic question on Ohm's law

I have what I assume is a very basic question on Ohm's law. Let's say that we have $n$ equal light bulbs in a series with a battery. We know that $$U - IR - IR - \cdots -IR= U - nIR = 0.$$ Solving ...
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### Height and Velocity vertically falling body

Feynman wrote in his lecture From Newton’s Second Law we can easily tell how the object moves, and it is easy to find out how the velocity varies with time, namely, that it increases proportionally ...
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### Calculating the power for acceleration of an elevator [duplicate]

An elevator does need some kind of acceleration when it starts to rise up so there has to be s force acting on it. But it surely does not accelerate the whole time so after some distance or time it ...
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### Radiation EIRP impact on organisms

I read some documentation about how harmless the non ionizing radiation can be and it looks like the arguments are around the frequency(hence the energy) of the wave. In the sense that a particle with ...
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### Conversion between Power and Flow rate of Fan taking into consideration the geometry of the fan

I need to find the relation between two properties of a fan, that is between power and flow-rate. I expect such a formula to depend on the pressure and on the geometry of the fan. I have found in ...
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### Watts vs. volts amperes

What I understand: In simple DC circuits, this is a product of the current and voltage, such that 1 watt = 1 ampere x 1 volt I understand that a watt is a unit of power (change in energy per unit ...
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### Initial speed is zero and so is power?

If I want to accelerate something from standstill to max speed, with a constant force (acceleration and mass don't change), the equation P = F * v would say that in the beginning we use 0 W power. How ...
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### Power and intensity of a sound wave

So in the case of a point source of a sound wave in a sphere, I'm wondering why the Intensity decreases with $r^2$ because if i setup the formula the Power always increases with $r^2$. So these two ...