Questions tagged [power]

The time rate of change of energy

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1answer
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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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29 views

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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4answers
73 views

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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28 views

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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1answer
102 views

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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63 views

Power of a river

I'm trying to calculate the power of the Mississippi, using the section of the river as it passes through New Orleans. I have the flow rate of a river, turned into a mass flow rate of $16,992,000 \:\...
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A particle of mass m moves in a circle of radius $r$ has a velocity $v = v_0(1+e^{-γt})$ the power delivered to the particle by the force at $t=0$ [closed]

1) firstly ,I calculated the acc as func. of time $a= -V_0γt$ 2) then ,I calculated force = $-V_0γtm$ 3) then ,I calculated power =force.velocity= $-(V_0γtm).(V_0)$ But, ans given is. $ -(2V_0γtm).(...
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34 views

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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39 views

How to show that imaginary part of complex power is reactive power?

Suppose I have a voltage across an element $V(t) = A \cos(\omega t)$ and the current through it given by $I(t) = B \cos(\omega t + \phi)$. The instantaneous power is $$P(t) = V(t)I(t) = AB \cos(\...
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2answers
69 views

Why is the power unchanged by a transformer?

If we have a step up transformer then the voltage at the secondary side will be more than the voltage at the primary side. Since we all know that POWER = VOLTAGE * CURRENT and because voltage at the ...
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44 views

Power Ratings—Given a power value and nothing else, how can resistor values be calculated?

I am going through "Circuit Analysis for Dummies". On pg 18, it says, "If you calculate the power dissipated as 0.1 watts, then a 0.25-watt resistor can handle this amount of power. A 0.125-watt ...
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3answers
84 views

Work done by who/what when involving human body moving themselves (push up, stairs etc)

Let's say we do a push up. If asked to compute the work, it would be Mass(lifted) x G(gravity) x height moved. I get that. What confuses me is how/what force does the work? Part of me feels it ...
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1answer
21 views

Optimal radiator sizes?

Imagine a situation where we need to generate 10 GW of electrical power for about an hour. It would probably require large equipment, which would also produce waste heat. Lets further imagine that ...
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4answers
79 views

How is power of engine transferred to kinetic energy of car?

Okay, I understand that via the torque from the engine, the wheels push back on the ground, and static friction pushes forward, thus accelerating the car. However, the force of static friction does ...
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1answer
29 views

Total output power of a laser

What is the interpretation of and the difference between the total power and total output power of a pulsed laser? How are these parameters measured and which type of detectors are the most ...
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What is the unit of the Angular Power Spectrum $C_\ell$ of Galaxy Clustering and Weak Lensing?

If the matter power spectrum $P_m(k,z)$ is measured in $h/Mpc^3$ and $k$ is measured in $h/Mpc$, what about its Fourier Transform $C_\ell$ and the conjugate vector to $k$, $\ell$? Thank you so much
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31 views

Power consumption of an electric circuit

In an electric circuit with a resistor of resistance R there is energy conversion to other forms. If there is just one resistor then the voltage drop over the resistor is always equal to the battery ...
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1answer
44 views

Maximizing power, current, voltage

We're asked to maximize voltage across the resistor RL, then do the same for current and power. But to find a value of R that maximizes VL, I think we'd just say infinity. And to maximize current ...
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2answers
37 views

Confusion in usage of formulae for power dissipated in a circuit

When should we use $\frac {V^2}{R}$ and when to use $I^2R$. Because many times they ask about the power without giving details about series and parallel. I am really confused
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6 views

Power absorption - ferromagnetic resonance

Magnetic energy is given by: $$ E(t) = \mathbf{M}(t)\cdot \mathbf{H}(t)$$ Hence, the magnetic power becomes: $$P(t) = \mathbf{M}(t) \cdot \frac{\mathbf{H}(t)}{dt} + \mathbf{H}(t) \cdot \frac{\mathbf{...
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1answer
52 views

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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1answer
35 views

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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6answers
77 views

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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1answer
122 views

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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1answer
348 views

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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8 views

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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2answers
31 views

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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1answer
59 views

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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33 views

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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2answers
52 views

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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17 views

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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1answer
45 views

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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1answer
44 views

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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3answers
103 views

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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1answer
29 views

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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2answers
66 views

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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1answer
16 views

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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3answers
185 views

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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1answer
39 views

Energy in radar equation

In the book Skolnik, Introduction to Radar Systems, ed. 2, pag. 52 (sec. Transmitter Power) the radar equation in its simplest form is initially cited: $$R_{\max}=\left[\frac{P_t G A_e \sigma }{(4 \...
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Why can't we use air pressure to recharge an electric cars battery

If we use energy to accelerate a vehicle, a lot of that energy is lost to drag and friction, especially in an electric vehicle. Why can't we harvest some of that energy using an alternator and some ...
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2answers
81 views

How to derive $dW = dE$? [closed]

From the definition of power: $$P \ = \ \frac{dW}{dt} \ = \ \frac{dE}{dt}$$ Multiplying both sides by $dt$, we get: $$dW = dE$$ $1.$ What does this imply? $2.$ Are there any other (perhaps more ...
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1answer
59 views

What does this mean? World consume 14 trillion watts of power [closed]

I am not very clear about this statement. Does that mean in every second we are consuming that much energy?
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1answer
63 views

Why does current arrange itself in such a way as to minimize power loss in resistors connected in parallel?

In problem 2 in Problem Set 6, it is said: "Electricity prefers to flow in the way that minimizes energy loss to resistance." Using Lagrange multipliers I was able to show that assuming the above ...
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3answers
67 views

Why a Box moving at Constant Velocity isn't doing Work?

This was a question from an A-Level past paper and the question is pretty straightforward. I was in a hurry so I couldn't think much on this problem and chose $C$ as my answer, checked it on the ...
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1answer
95 views

Expression for net power consumed when resistances are connected in series in electric circuit

To find the expression for net power consumed, I did this :- $$ \text{Suppose some resistances } R_1, R_2, R_3, ... \text{ are connected in series in an electric circuit.} \\ \text{Let R be the ...
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5answers
3k views

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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7answers
99 views

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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1answer
41 views

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 ...
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41 views

Can the mains voltage affect laser intensity?

First of all in very new to the photonics field and i am only venturing into it because I have to troubleshoot an experiment and the issue seems to be the optics block. So i am using an instrument ...
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3answers
90 views

When high voltage leads to low current in power transmission system how its not applicable for household voltages?

When high voltage leads to low current in power transmission system using transformers,how increase in voltage in household circuits from 220 volts makes the current increase damaging the appliances, ...