# Questions tagged [power]

The time rate of change of energy

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### How to convert average horse power?

My question is " A locomotive engine draws a load $m$ pounds up an inclined plane making angle $a$ with horizontal, the coefficient of friction being $k$ . If starting from rest and moving ...
321 views

### Conducting ring free-falling in a variable magnetic field

Consider a free-falling conducting ring oriented horizontally to Earth's surface. It then enters a region with magnetic field $B = B_o(1+\lambda z)$ where $+z$ is the vertical direction upwards. By ...
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### How is radiance conserved? - Power/intensity misconception

Before getting to the main question I'd like to clear some pretty basic concepts regarding energy/power/intensity as my physics is pretty rusty. According to wiki In radiometry, radiant flux or ...
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### mechanical power in moving coordinate system

Suppose I have an object resting on top of a table. There are two forces acting on the object: gravitational pull and the supporting force from the table. It is pretty obvious, that the table is doing ...
36 views

### How to increase power in a geared system using P = (omega)(torque)

Could someone give me a basic, intuitive explanation on how to increase power in a geared system using the P = omega*torque formula? By geared system I mean two connected gears, where the first one ...
31 views

### what does the “T” really represent in the signal power equation

In the signal power equation $$P(g(t)) = \lim _{T\to \infty }{\frac{\left(\int_{-\frac{T}{2}}^{\frac{T}{2}}g\left(t \right)^2dt\ \right)}{T}}$$ What does the $T$ really represent ? From my ...
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### What is the sound power produced at a defined frequency by a woofer of given diameter and excursion?

Assuming an infinitely rigid woofer of a given diameter and oscillating with a given excursion at a given frequency, what is the SPL (sound pressure level) at 1 m distance? I found https://en....
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### How to devise a microwave power sensor?

I am fairly aware of the general thermistor/ diode/ thermocouple methods of sensing microwave power. Currently my requirements are simpler: A device that senses only microwave power incident on it - ...
71 views

Say we have a waterfall and we know that 10kg of water falls over the ledge per second. We are also given that the acceleration due to gravity is 10m/s^2, and the distance to the ground is 5m. We are ...
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### Electromagnetic plane wave interaction with metal plate

Given a plane wave with $\overrightarrow E$ field parallel to $z$ axis and a square metal plate in $xy$ plane. Suppose we know the power of the incident wave and the size of the plate, how can we ...
47 views

### “Reference dependence of fuel gauges” in classical mechanics

The power exercised by a force on an object is dependent on the velocity of the object and hence the frame of reference. I am wondering wherein the error lies with arguments such as the ones presented ...
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### Why does having a high input impedance and low output impedance cause minimal voltage drop when you connect two circuits?

Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to ...
106 views

### Wind Power in a Wind Tunnel

Two questions in one day! What? Okay, so the power equation for air flow is as follows: $$P = 1/2 \cdot \rho \cdot A \cdot V^3$$ In a wind tunnel due to the Bernoulli principle as the area ($A$) ...
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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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### Experiment to determine cyclette power

I have a cyclette with 8 resistance levels set magnetically using a knob (a magnet is moved near the rotating disk thereby increasing its rotating resistance). The distance from the pedal to the ...
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### Why does only one of the DC circuit power equations work for me here?

I know that $$P=IV=I^2R=\frac{V^2}{R}$$ I solved for the current using the loop rule $$I=.47$$ but to solve for the Power output of the 16V battery I could only find the power by finding the ...
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### What is the relation between a fixed force and work?

How would you translate a constant force into work or energy? If I have a way to output 100KN continuously, how can I translate that into watts? Is there a formula for that translation?
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### Question on work, energy and power

The engine of a train is working at a constant rate. The maximum speed of the train up a certain incline is $V_{1}$ and the maximum speed down the same incline is $V_{2}$. Then if the train moves on a ...
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### Fixed voltage source & maximized power

The goal of a voltage source like a battery for example is that it will provide a fixed voltage regardless of the load resistance. The battery has internal resistance Ri If R_L≫R_i, the voltage ...
195 views

### Why does potential energy time-derivative depend only on the mass flow?

This is a homework exercise, but I don't want specifically to tell me how to resolve the exercise. I want you to help me what's wrong here, and why I can't do it. Water is pumped from a lake to a ...
441 views

### Measuring output optical power of an LED

How do I measure the output optical power of an LED? I have a detector that measures output power of a laser incident on a small opening. The problem with using the LED with the same detector is that ...
35 views

### Transformer vs Inductor for lighting a bulb or any other appliance

There was a question in my school physics exam which is as follows: Consider a 100W bulb which operates at 50V dc. John wants to light this bulb by a 200V ac source. What component should he use, ...
302 views

### How much heat can be adsorbed in a solar parabolic trough?

How do I know the produced heat on the focal point of a solar parabolic trough per different radius if made from steel and also from glass?
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### Predicting top speed of a flywheel [duplicate]

I connect a DC motor to a flywheel with a given moment of inertia. I then supply that motor with a constant voltage and current for an infinite time interval. What do I need to know about my motor to ...