Questions tagged [power]

The time rate of change of energy

Filter by
Sorted by
Tagged with
2
votes
1answer
450 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 ...
2
votes
1answer
254 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 ...
2
votes
2answers
334 views

Why does discharging a capacitor give a higher power output than using a battery?

Let's say you charge a capacitor with a 10V battery. The capacitor will have a potential difference (pd) of 10V once it is fully charged. If you then connect a light bulb to the capacitor, the pd ...
2
votes
2answers
471 views

Does a rocket engine apply more power as the rocket's speed increases? [duplicate]

To clarify my level of knowledge, I'm a high school student all the way through AP Physics C: Mechanics. So, let's imagine that there is a rocket travelling through the vacuum of space (ignoring ...
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 ...
2
votes
1answer
16k views

Why is power dissipated in a circuit maximum when external resistance is equal to the internal resistance in the circuit? [closed]

[![question 24][1]][1]Suppose in a circuit the battery has emf 6V and internal resistance 3ohm. It is connected to an external resistance = R ohm. According to my ...
2
votes
2answers
899 views

Why dont you take derivative of force in definition of power ? P=F.v

The derivative of work is $\bf F\cdot v .$ $$P(t)= \frac{\mathrm dW}{\mathrm dt}= \mathbf{F\cdot v}=-\frac{\mathrm dU}{\mathrm dt}\;.$$ But why not $$\frac{\mathrm{d}\mathbf{F}}{\mathrm{d}t}\cdot \...
2
votes
1answer
345 views

Why are GPS transmitting signals made so weak?

Twice this year, my car's GPS receiver got totally lost. It seemed the cause of this was heavy cloud, (because I can't think of any other reason), and the GPS stayed in "Recalculating" mode, until ...
2
votes
2answers
200 views

Non-linear relationship between power supplied and perceived brightness in a filament lamp?

I was expecting the relationship between these two to be linear, but after conducting an experiment my data fits almost exactly on the curve, $y = 2800.2 x^{2.0173}$, where $y$ is the brightness in ...
2
votes
2answers
112 views

Why are large batteries described by their power?

Reading about renewable energy sources, I came across the idea to use huge batteries as a temporary storage to be able to deal with the fluctuations that e.g. solar panels might produce. The most ...
2
votes
2answers
2k views

Measuring energy when no work is done — comparing push-ups to planks

tl;dr Is there any meaningful (physical) way to compare the energy expended in the exercise of doing $x$ pushups in $t_1$ seconds, vs the exercise of doing the plank for $t_2$ seconds? I'm confused ...
2
votes
2answers
778 views

Power as a Function of Distance

I am interested in trying to understand the meaning of the integral of power with respect to distance. In the case of force, we have the two formulae: $\int F \, \operatorname{d}\!t = \Delta p$ and $\...
2
votes
2answers
276 views

Can wireless power transfer harm organic life by anyway?

Now, humanity can beam 50-60 watts power through 10 centimeter-thick concrete block. Can such wireless power transfer harm us by anyways?
2
votes
4answers
1k views

Do Alkaline batteries, $\mathrm{NiMH}$, rechargeable alkaline, Lithium $\mathrm{AA}$, all have similar MAX POWER?

So in physics, one smart teacher told me, $$ V = IR $$ or $$ I = \frac{V}{R} $$ but it is not always true, because $P = VI$ and each power supplying device (such as battery) has a "maximum power" it ...
2
votes
1answer
186 views

How to estimate energy of piezo igniter discharge?

Question for fun. We all know piezo igniters, these small friends, like following one: All of them are probably quite similar: same spring, same piezo, same hit force, and so on. Also I have some ...
2
votes
2answers
65 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 $A$...
2
votes
4answers
1k views

$ P = \frac {V^2}{R} $ derivation confusion [duplicate]

I have always had a confusion of why we use $ P = V I $ or $ P = I^2 R $ and not $ P = \frac {V^2}{R} $ for relating to power loss due to heat in high tension lines. I know there are a lot of ...
2
votes
2answers
116 views

Shorting a Superconducting Coil with A/C

When a superconducting coil carrying a direct current is shorted, it continues to carry the current without loss (approximately), assuming it stays cooled and superconducting. What would happen if ...
2
votes
1answer
63 views

Radioisotope beta decay generator

Why are there no electrical generators utilising the electron/s of beta decay from a radioisotope for generating a working current? For example, how much radioisotope would I need to generate 1A or ...
2
votes
3answers
14k views

Calculate acceleration from power [closed]

I am currently programming a simple computer space ship simulation. This is just for fun to train my programming, 3D graphics and a little bit of physics skills. What I want is the user to pick a ...
2
votes
1answer
201 views

What exactly is three phase power? [closed]

What is three phase power? How does it differ from normal AC current?
2
votes
1answer
371 views

Could geothermal energy extraction in Yellowstone significantly diminish the threat of a supervolcano?

I recently read an article about a team in Iceland that had managed to drill directly into magma coming up from the earth's mantle, and then use the resulting bore-hole to generate steam to power ...
2
votes
2answers
2k views

Human power on treadmill

On an elliptical treadmill a regular person can easily burn 1000 calories in one hour (treadmill reports calories burnt). This translates into: $$(1\times 10^3\mathrm{cal/hr}\times 4.2\times10^3\...
2
votes
1answer
10k views

Calculating the mechanical power of a water pump

Say I want to pump water from one container to another. The water levels are 3 meters apart, and I want to pump 10 litres per hour. I figure the mechanical power necessary, assuming no losses, is: $$ ...
2
votes
2answers
13k views

Light bulbs, Wattage meaning? [closed]

Light bulbs, Wattage meaning? Two incandescent bulbs (120 V, 25 Watt) and (120 V, 500 Watt) connected to the same batteries. Which one shines brighter? And why?
2
votes
2answers
94 views

For a real battery, why is two identical bulbs in parrallel slightly more dim than just one in series

Image for reference: $$\begin{align} &r = 0.8\,\Omega\\ &\mathscr{E} = 1.5\,\mathrm V\\ &A = B = 6\,\Omega. \end{align}$$ The current when the switch is open, the only other resistor is ...
2
votes
1answer
273 views

Significance of the ion Hall parameter

For an ionized Helium gas, for example, MHD Hall generators rely on the mobility ratios between electrons and ions to function and produce a Hall current. As far as I can tell, intuitively, increasing ...
2
votes
1answer
218 views

Is the net power of an open system always equal to zero?

Are the following statements, line of reasoning all correct? I'm particularly interested in the last bullet. For a closed system energy is conserved and constant. For an open system energy is not ...
2
votes
2answers
802 views

Is there an RMS value for power delivered to an inductor?

My textbook defines power delivered to an inductor as: $$P= V_{L\rm\ peak}I_{\rm peak} \cos ( \omega t) \sin( \omega t)$$ where $\omega$ is angular frequency. but makes no mention of $P_{RMS}$. It ...
2
votes
1answer
1k views

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 ...
2
votes
1answer
635 views

Is there a difference between joules of energy per coulomb and volts?

There is a question in my textbook: A current of $7\ \mathrm{A}$ goes through the heating element in a hair-drier. The voltage of the heating element is $240\ \mathrm{V}$. b) How many joules of ...
2
votes
2answers
6k 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 ...
2
votes
1answer
8k 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 ...
2
votes
1answer
41 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
1answer
251 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
56 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 ...
2
votes
5answers
217 views

No need Power to fly ? Paradox [duplicate]

I would like to know how much power I need for magnetic levitation. $$P = Fv.$$ $F = F_g \approx \text{mass} \times 10$, okay. But $v = 0$! I can't believe I need no power to keep an object in the ...
2
votes
1answer
56 views

Cooling induction current generation

We know that conducting materials can be heated by electromagnetic induction. Is it possible to generate current using a cooling process?
2
votes
1answer
381 views

How to understand the thermal radiation?

I am studying the thermal radiation (Stefan–Boltzmann law) by myself $$P = \epsilon \sigma A T^4$$ here $\epsilon$ is the emissivity, $\sigma$ is Stefan-Boltzmann constant, $A$ is the surface area ...
2
votes
1answer
84 views

Sound intensity and space/things in the way

Suppose you have two stationary people, A and B, who are equally effective in terms of hearing ability, and A emits a sound that is heard by B at a certain intensity. If they remain stationary, and B ...
2
votes
1answer
243 views

What are the properties and impediments of a liquid air fueled engine?

I recently came across a very interesting article that suggested the possibility of using liquified gases like air, nitrogen, or oxygen as a power source for cars. It appears that this company is ...
2
votes
3answers
47 views

Velocity reference point when calculating power [closed]

As shown in the picture, a person (80 kg) is on a platform (20 kg) that's suspended by a rope which is on a pulley. As they pull on the other side of the rope, they reach a constant velocity of 0.4 m/...
2
votes
0answers
34 views

PMDC Motor Characteristic Graph for Given Parameters

TL;DR Need to make a graph which showcases all parameters of a PMDC motor against the torque input. I am trying create a graph similar to as shown below. However, I am confused as to how the torque is ...
2
votes
0answers
71 views

Complex Poynting vector and energy loss

Consider continuum medium (no boundaries) and the electromagnetic wave Poynting vector $$ \mathbf{S} = \mathbf{E}\times \mathbf{H}^*$$ with $^*$ the complex conjugate. When there is a phase difference ...
2
votes
0answers
37 views

Analytically understanding wireless power transfer

https://youtu.be/M8aZoM9p43k This YouTube link shows a experiment I quickly made. Two resonant lc circuits that use magnetic fields to wirelessly light up a led. I manually found the best frequency ...
2
votes
1answer
38 views

Energy transfer between oscillators [closed]

Suppose I have two mechanical oscillators $a(t), b(t)$, coupled through the interaction $V_\text{int} = \mu^2 a(t) b(t)$. Is there a simple way to express the rate of energy transfer from $a$ to $b$ ...
2
votes
1answer
60 views

Why does Joule's Law of Heating defined by $H=i^2R$ only? [duplicate]

Joule's Law Of Heating states that for a conductor Heat loss is given by $$H=i^2Rt$$ Couldn't we substitute $$V=iR$$ and show that $H=(V^2/R)t$ ? If this is so, when transmitting power across large ...
2
votes
0answers
101 views

Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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
0answers
169 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 ...

1 2 3
4
5
15