# Questions tagged [power]

The time rate of change of energy

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### Is there a graphical summary of which modulation technique to use in communication systems using Visible Light Communication techniques?

I am currently looking at various papers written (some listed at the bottom) that discuss how laser pulses can be modulated to send encoded messages to a receiver for communication systems. The more I ...
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### Cross-correlation power spectra

I am trying to understand how cross-correlation power spectra (PS) are interpreted. If I have understood correctly, the following hold: A PS can either be an autocorrelation PS or a cross-correlation ...
237 views

### Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
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### Why is work measured linearly with the distance [closed]

If I run and push a car, the force considered is the one felt in my arms not in my legs? The distance the one marked on the field? If the energy outputs is constant and linear with time, why isn't ...
266 views

### Adding a capacitor in parallel improves power factor of an inductive circuit?

I know that to reduce power loss during transmission, we require a power factor as close to one as possible. This can be done by making $Z$ tend to $R$ which can be done by increasing $X_c$ such that ...
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### An electric bulb is rated 220 V and 100 W. When it is operated on 110 V the power consumed will be- [closed]

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V what will be the power consumed? The answer to the question is 25 W using the formula P=V^2/R. But if I use the formula P=VI I ...
9k views

### Calculating displacement amplitude of ultrasonic power transducer

I am not a physicist, but my current project drives me to some physics-related computations, hence seeking help. I have some ultrasonic transducers, 5938D-25LBPZT, for which very limited information ...
28 views

### What happens to radians in this calculation? [duplicate]

I rewrite N as kg m s^-2 and try to get Pmax, which is in Watts to kg m^2 s^-3 but when I do so I am left with an rad^2.
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### How many watts of electricity can the human body withstand without being killed?

I'm talking about DC and/or AC. I've read about people surviving extremely high voltage shock (300kV) but that could be explained by extremely high resistance in the circuit that resulted insufficient ...
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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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### Simulating a fan speed given power applied to it over time [closed]

I am driving a fan using a micro-controller. I want to test the micro-controller code without having fans physically attached to it so I thought I would model one programmatically. I want to model a ...
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### Why does not work done increase when power is increased? [closed]

Suppose, a man pushes a wooden block through a distance of 2 m in 2 seconds and an other man pushes the same block to the same distance in 1 second. According to my assumption, the first man must have ...
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### Harmonic driving of a non dissipatice circuit (LC for example): does the energy really sometimes goes back in the generator?

Let's assume you have a non dissipative circuit represented by a box: it involves only capacitance and inductance for example. You drive this box via a sinusoidal voltage generator. I call $U(t)$ ...
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### How much solar power in kWH is there in the stratosphere during the day? [closed]

The stratosphere is above much of the atmosphere. People have talked about collecting solar energy there and beaming it back to Earth. How much power are we talking about?
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### Power meter zero-offset calibration

I have to measure the output power of a laser diode. To this aim, I am going to use an integrating sphere connected to a photodetector and a power meter. Concerning the photodetector, electrical ...
309 views

### How to calculate total power generated?

I have a circuit with a piezoelectric transducer which converts mechanical energy to electrical. The ceramic is connected to a 1 Megaohm resistor and a voltmeter which reads voltage. If I want to ...
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### power = force times velocity of object, what if the object is “abstract” such as “contact patch” of a tire?

Consider the case of an accelerating motorcycle, the engine converts potential energy into mechanical energy which is used to generate a Newton third law pair of forces at the rear tire's contact ...
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### What does work really mean deeply? I think the definition of work is not convincing

Suppose we have two objects 1kg mass. The one of them moves by constant velocity 3m/s and then we apply to it force equals 1N so the done work is 3.5 joule. the other object is at rest and then apply ...
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### Electric reactive power? Is it real or is the design accepted for calculation? [closed]

Do you agree that the reactive power definition is an acceptance for calculations? I disagree with this view. Then; How the electromagnet force or the power stored by the capacitors can be explained.
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### 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?
583 views

### Friction energy dissipation calculation

In a moving block, a variable force pushes it. Friction is acting against the movement. Below equation specify the amount of energy could be used to give motion: $$E_1 = \int F_T(t) .v(t) .dt$$ ...
3k views

### How does a car gain kinetic energy?

I understand that the engine delivers power to the wheels and friction from the ground causes the wheels to roll. However, given the power (work per time) at the wheels, how does that energy become ...
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### Energy power relation [closed]

If we have an energy equation like $E=A\sin(wt-kx)$ If we differentiate it w.r.t. time, we will get power. If we again differentiate the last equation, it will be termed as power per unit time. But ...
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### Multiple postive solutions for resistor value selected to have a given power

Textbook Example: find the load resistance $R_L$ such that the power at the load is $500\times10^6~\text{watt}$. By Voltage Divider, voltage of load is: $$V_L = \frac{R_T}{R_L + R_T}V_s\tag{1}$$ ...
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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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### Isn't the equation for power more complicated? [closed]

I’ve been told that work is equal to $Fs$ and power is work done per second $dW/dt$ so looking at the equation, isn't power more like this? $\frac{dF}{dt}s + F\frac{ds}{dt}$
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### Why doesn't NASA try using high-explosives to fuel rockets?

For a smoothly burning liquid-vapor mixture, I understand that burning hydrogen and oxygen is the best possible solution, not just for now but even theoretically. (?) But I read about ideas, that ...
11k views

### Why does a higher wattage incandescent light bulb have a lower resistance value than a lower wattage incandescent light bulb?

I am an electrician and know through experience that resistance in an electrical circuit causes heat. An incandescent light bulb's light is a by-product of heat, so why does a 100w bulb have a lower ...
429 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 ...
361 views

### Energy consumption from acceleration

I have an interesting problem. I have a machine which has to lift 35 kg (this is the total mass of the object and the lifting arm). The lifting is done by a servo motor. Now I want to calculate the ...
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### 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 ...
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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 ...
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 ...