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Questions tagged [power]

The time rate of change of energy

2
votes
2answers
2k views

Sun constant formula derivation

In a book, I've to read for a class at my university, called 'Duurzame Energietechniek' (Dutch, translated: renewable energy) they gave a formula that looks like: $$\text{G}_\text{sc}\left(\text{n}\...
1
vote
2answers
3k views

Power required to move an object

Edited to better reflect the answer I was after * I have a trolley with laden weight of 110kg that needs to ascend an incline of 6 degrees at a speed of 10km/h. Provision is made to reach that speed ...
0
votes
1answer
33 views

Determining power required to close gap between moving objects

I am running a simulation of two bicycles traveling in the same direction in a single dimension. Bicycle A is $z$ meters behind bicycle B. The acceleration of each bicycle determined by the power (P) ...
28
votes
6answers
8k views

Can a gym be built to supply electricity to homes? [closed]

A moving magnet induces a current in a conductor, then shouldn't we be able to generate electricity through manual labour? I was thinking about building a gym that used magnets as weights. People ...
110
votes
12answers
42k views

Why don't we use weights to store energy?

One of the main reasons why we haven't switched to clean energy is the lack of efficient storage methods - But, why aren't we using dead weights to store energy and draw it back later when needed? As ...
2
votes
0answers
54 views

Why does an alternator slow down when the 3 phases are shorted?

I'm designing a small wind turbine and noticed that when i short the three wires coming out of the alternator, the alternator is breaking and the blade RPM is dropping. What is the explanation for ...
2
votes
4answers
1k views

Can I use the work done to calculate power of a motor?

I am trying to calculate the energy/ power required to move a plate up and down in a liquid. My approach is to calculate the forces that must be applied by the motor to push/pull the plate based on ...
0
votes
3answers
407 views

Power/torque ratio vs acceleration

Let there an object with determined mass which needs to be moved with a motor (lets say electrical). We have two available motors, which have exactly the same power (and other characteristics), but ...
1
vote
1answer
35 views

Power expression for a dynamic force

Given that: $$W=\int_{x_0}^x \vec{F}\cdot d\vec{x}$$ and assuming $F$ is time dependent, we get: $$\frac{dW}{dt} = \int_{x_0}^x \frac{d\vec{F}}{dt}\cdot d\vec{x}+\vec{F}(x,t)\cdot \vec{v}$$ ...
-1
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2answers
1k views

Power in Series Combination Of Appliances

The general formula for power, P for an appliance is P= (I^2)R where I is Current flowing across a resistance R Thus power, P is directly proportional to resistance, R of an appliance for a ...
0
votes
1answer
72 views

Using the formula for electric energy confusion $W = P * t$

I have this example question: A low energy lamp has the label 230V, 15W. It gives about the same amount light as a regular lamp at 75W. How much energy do we save by using the low energy lamp if it's ...
8
votes
4answers
2k views

Contradiction in Ohm's Law and relation $P=VI$

Ohm's law states that  electric current is directly proportional to voltage provided that physical conditions like temperature remain constant i.e. $$V = IR$$ On the other hand, $$\text{Power = ...
0
votes
2answers
2k views

Does the brightness/power of each individual bulb change when you add more bulbs in a parallel circuit?

I have asked this question on different sites and have received contradictory answers. Assume that there is an ideal voltage source and the resistance of each bulb is the same. I believe that the ...
2
votes
1answer
5k views

Why is the lamp with the lower power rating brighter in the series circuit?

If I have two lamps, both of them have different power values, and voltage value is constant , I've been told that the lamp that has less power would be brighter and make more light than the lamp that ...
0
votes
3answers
318 views

Power in a uniform circular motion

I've a circular motion and I want to find the power that the motion is generating. The formula's I got: $$\text{P}\left(t\right)=\text{W}'\left(t\right)=\text{E}'\left(t\right)$$ Where $\text{P}$ is ...
0
votes
1answer
291 views

Relationship Between Watts and Candela

I am currently working on a project that involves the use of LEDs. According to the datasheet, the LEDs I am using have a maximum brightness of 10,000 mcd. In my project, though, I will not be using ...
2
votes
1answer
40 views

Are large scale optical power delivery systems feasible?

I am attending an introductory course of optics and photonics and we just started to study optical fibers for digital communications. However, I have started wondering about using optical fibers to ...
-2
votes
2answers
290 views

AC vs DC & why we ended up using AC for power in our homes, etc [duplicate]

I think that DC is better than AC (and yes I do have a reason, check the comments). Also, if it (DC) is in-fact better than AC why did we use AC then?
1
vote
1answer
60 views

Rotational Power/Energy Mismatch

My question is pretty fundamental but has me stumped. Long story short I can't seem to calculate the correct required power to accelerate a mass to a set speed in a set distance. Every time I ...
0
votes
1answer
461 views

Why power dissipated over a resistor relies upon intensity not upon voltage

For computing the power dissipated over a resistor the correct formula is $P = R \times I^2$. Why, in other purposes equivalent, $P = \frac{V^2}{R}$ is not good? I can imagine that the actual current ...
0
votes
3answers
113 views

When do these equations apply in electrical circuits and when do they not?

My textbook tells me that in order to calculate the power (in watts) for an electrical circuit then the equation is $P=VI$ (volts times amps equals watts). However, I have read online that this ...
0
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0answers
30 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, ...
0
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4answers
408 views

Definition of Power from electricity and from energy

In the electricity topic, my physics book has defined Power as $P=IV$, $I$ being current and $V$ as voltage. But then later in the Energy topic, power has been defined as $P=\frac{W}{t}=\frac{F.d}{t}$...
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2answers
46 views

Velocity vs time curve of negatively accelerating motor

I have a hopefully interesting yet simple question. I want to plot the velocity vs time curve of a motor decelerating. The interesting bit is that the acceleration is going to be directly connected ...
1
vote
3answers
67 views

Does it make sense to compare mechanical wattages to electrical wattages?

A smartphone might produce 1 watt of power. A voltage of 1 volt producing 1 ampere of current over 1 second is 1 watt of power. (Did I do that right?) Or a kid pushing a toy block might produce 1 ...
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votes
1answer
99 views

Impact force of a pillow?

My friend and I just had a discussion about how dangerous it would be to get hit in the face with a pillow thrown out of a train. If we say the train is traveling at 100km/h and the pillows weight is ...
7
votes
1answer
1k views

How could this small nuclear reactor power two jet engines?

In the 1950s, the US was working on an Aircraft Nuclear Propulsion and it is stated that they successfuly operated two modified General Electric J47 jet engines using a 2.5 MW (thermal) nuclear ...
3
votes
0answers
118 views

What do I measure with power meter?

I modulate directly a laser diode by setting the pump current periodically from 0 A to $I_{on}$ A. I do it for different duty cycles and different frequencies. But I need to keep my average power of ...
1
vote
1answer
295 views

Finding out how long a light bulb will light [closed]

A cylindrical wire used to form a light bulb filament has radius 3.7 micrometers and length 1.7 cm. The resistivity of the wire is 5.25 * 10^-5 ohm meters. The light bulb is connected to a 12V battery....
1
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0answers
46 views

Can global warming be exploited as a useable energy source? [closed]

I saw an Internet video about global warming, specifically how well the comedic giant ice cube idea could actually work. But then I thought about (other) science-fiction cartoons in the future. How ...
2
votes
2answers
746 views

What is the physical meaning of the zeroth component of the Minkowski force 4-vector?

In David Griffiths' Intro to Electrodynamics, section on ED and relativity, he poses a simple problem regarding the zeroth component of the Minkowski force 4-vector for a charge $q$ moving with ...
0
votes
1answer
261 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?
7
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1answer
1k views

How much power and energy is (actually) in a 230 dB “click” from a whale?

In the BBC Science in Action podcast the acoustic power of a 1 millisecond "click" from a whale was said to be 230 dB. The related BBC article mentions this number and elaborates: The blue whale is ...
1
vote
1answer
146 views

Power and radiance scaling in Coherent Beam Combination

Generally, it's said that for incoherent laser beam combining, the intensity scales with N (where N = # of lasers), whereas the intensity scales with $N^2$ for coherent combining. $$I_{array,...
2
votes
1answer
1k views

What is the relation between Work and Power? [closed]

If I lift 40 kg 6 times 60 centimeters, is that 40*6*0,6*9,82 Joule? And then if I take the time into account, I will know the Watts? Or did I misunderstand? The background is that I read that you ...
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votes
1answer
2k views

Why Ohm's law is followed inside the house? [closed]

In power transmission lines, as power remains constant, according to $P=VI$, current decreases when voltage increases, thus Ohm's law is not followed. But inside our house current increases when ...
0
votes
3answers
185 views

How feasible is it to power your home with a bicycle? [closed]

This article claims "60 Minutes On This Bicycle Can Power Your Home For 24 Hours!" How feasible is this claim? Sounds great - but the skeptic in me says "yeah, right." Hope this is an acceptable ...
24
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3answers
2k views

Is there a theoretical lower limit to watts needed for lumens?

For example 100 watt incandescent bulb produces 1600 lumens. Led 23 watt can produce 1600 lumens. Is there minimum amount of watts needed ?
-1
votes
1answer
3k views

What is the maximum amount of electricity I can draw in 48 hours from a domestic supply? [closed]

An electricity company is offering free electricity for a 48 hour period every week. The rate the rest of the time is very expensive. How much power can be drawn from a domestic UK supply? Would it ...
0
votes
2answers
387 views

What is the difference between kilo-watt and kilo-volt-ampere? [closed]

What is Kilo-Volt_Ampere used for? What is kW used for?
1
vote
1answer
297 views

Isobaric process with non-boundary work?

I came up with a doubt on isobaric processes. Firstly, in any isobaric process, (irreversible or not) the following hold $$Q=n c_p (T_B-T_A)\tag{1}$$ $$W=p (V_B-V_A)=n R(T_B-T_A)\tag{2}$$ Is that ...
1
vote
1answer
290 views

Can the intensity of a sound wave in a minimum be greater than intensity in a maximum?

Suppose to have two speakers $A$ and $B$ one in front of the other, producing coherent sound waves with the same frequency $f$ and power $P$. In different positions on the line connecting the speakers ...
0
votes
1answer
328 views

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 ...
2
votes
1answer
42 views

Won't the Power be dissipated in the first quadrant of this graph?

In the following picture: In the first quadrant of the V-I graph(for an Independent Current Source, where the voltage supplied is positive on the top) won't the power be dissipated instead of being ...
-4
votes
1answer
201 views

Could an infinite spinning windmill create endless electricity? [closed]

This is not so much a question about if this idea is physically possibly, but more a question of if it was possible. If a magnet could somehow push a windmill around at a constant speed continuously, ...
0
votes
1answer
2k views

Bernoulli equation and pumps [closed]

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, $3$ ...
0
votes
2answers
100 views

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 $I$. ...
3
votes
0answers
70 views

Geometric frustration in power transmission networks

I read this article concerning the detection of Braess Paradox on power transmission networks: http://phys.org/news/2012-10-power-grid-blackouts-braess-paradox.html Here is a quote from the article: ...
2
votes
0answers
127 views

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 ...
2
votes
2answers
62 views

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 $A$...