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Questions tagged [power]

The time rate of change of energy

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0answers
89 views

Finding force exerted on a turbine blade by water flow

I am trying to find the maximum theoretical rotational power of a turbine from the following quantities: $\omega$, $r$, $\gamma$, $I$, and $t$. This is in an experimental setting, not an assignment,...
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22 views

How to calculate minimum optical power required from datasheet of camera sensor?

How can I determine from the datasheet of a camera sensor the minimum optical power required to see a spot on the image? I have 2 lasers with a diameter of 0.35 mm with respectively a power of 12nW/...
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102 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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59 views

Is it possible to create usable light from magma?

Say you drilled a hole down to the magma layer. Hot lava emits light. From what I've seen lava is red(?) hot. Then if you put some fibre optic cables down into the magma layer maybe protected at the ...
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1answer
33 views

How deep would you have to put a steel pole to generate enough electricity to power one LED using geothermal energy? [closed]

I was thinking about how you might grow plants inside without sunlight powered only by LEDs. But to get the energy for the LEDs, one might drill a hole down into the Earth's magma and put a steel pole....
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40 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 ...
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3answers
68 views

Could pressure differentials be used to produce power?

There is potential energy between something like a vacuum in a container and a gas on the outside of it, or vice-versa. Could this energy be harvested in a vacuum, or close to vacuum situation like on ...
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1answer
41 views

Which law is true at the transformator?

According to Faradays law The changing flux will generate voltage in the secondary winding/circuit, so if the N2 = 2*N1, then the U2 =2*U1, and according to conservation of energy in that case I1 = 2*...
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2answers
593 views

How to determine which bulb will be the brightest in the series and parallel connection?

I stumbled upon these sentences while studying and this seems to be horribly confusing. resistance is proportional to the inverse of the power of the bulb in series connection and so the bulb with ...
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20 views

How to calculate energy (KWh) from Temperature (T),Air quality (Co2), and illumination (lux)?

I have data of temperature, air quality, and illumination.I want to calculate energy consumption (KWh) from these parameters.My data is shown in the image . Also, examples are also shown in image 2 .
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87 views

Why am I getting two answers from the same question? [closed]

In this problem, if I use the formula for power as $\frac{F\cdot s}{t}$, then the answer is 37.5 W. But if I use the formula $F\cdot v$, then the answer is 75 W. How is it possible that the man has ...
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92 views

How step-up transformers help in transmission of electrical energy over long distances?

i have read just read this blog: http://www.blueraja.com/blog/194/do-transformers-obey-ohms-law below blockquotes is taken from the blog above: We connect a 1 volt AC generator to a 1 ohm ...
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1answer
43 views

Path to ground / Source in AC circuit

I'm a 1st year EE student and i'm trying to get my head around earthing systems at the moment and could do with some help. The first thing that i'm struggling to understand is how current establishes ...
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1answer
27 views

Energy conversions due to movement of car up the slope

We know if a car is travelling at constant speed up the hill then the power output of engine is equal to change in gravitational potential energy per unit time . (Assuming friction negligible). What ...
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2answers
58 views

Power in a simple electrical circuit [closed]

In this picture, as we know power due to heating of the resistance is $V^2/R$ . Now, if we add a parallel resistance to the circuit, the power dissipated through this resistor is also $V^2/R$ ...
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1answer
118 views

Power absorbed by electron in plane electromagnetic wave

How can the power (in Watts) absorbed by the electron be calculated, knowing the incident electric field amplitude $ E_0 $, wavelength $ \lambda $, and the electron momentum relaxation time $ \tau $ ...
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2answers
33 views

Electrical vs Heating Insulation

Let's say we have a 120V cable and a 600V cable, this means that the 600V needs more electrical insulation to prevent the insulator from exceeding its dielectric strength. But in the other case, the ...
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1answer
57 views

Is the force from an engine constant as a vehicle reaches top speed?

I've come across a textbook question and I've got myself confused by it although it seems like it should be simple. The van accelerates with a (supposedly) constant resistance force up to its top ...
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2answers
470 views

Why High Voltage Power Lines need more Insulation than a low one?

"A 400 kV cable requires less insulation than a 240V cable." This was regarded as False in one of my question papers, but why? Doesn't a higher voltage means, lower current(P=V. I) and a lower ...
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22 views

Power dissipated through bulb decreases to a steady value

The instant at which bulb is switched on, its power is maximum, then it decreases to a steady value. This can be explained by the increasing resistance due to heating of the filament. (P = V*2*/R). ...
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1answer
392 views

Brightness of lamps

The brightness of the lamp (power) depends on two factors: The voltage and the current which flow through it. In Figure(A) as the variable reaistance increases its reaistance, the current which flow ...
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1answer
107 views

Force exerted by a motor, velocity, and friction

Some motor make an object move on a surface very slowly. One can write at steady state : $F_{motor}-F_{friction}=0$. The friction is supposed to come from the surface so : $F_{motor}-\mu_s m g=0$ with ...
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1answer
34 views

How can I find the power given only the torque?

I have a system where I am trying to find the power generated by a DC electric generator. However, I am not very familiar with generators, so I am having difficulty determining the rotational velocity....
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1answer
37 views

Formula for Power from Kinetic Energy

Work Done = $Fd$ Power = $Fv$ If the (net) Work Done = Change in Kinetic Energy, and the object starts from rest: Work Done = $\frac{1}{2} mv^2$ Power = $\frac{1}{2} m av$ Power = $\frac{1}{2} ...
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31 views

Mechanical and electrical power of discharging a capacitor

I want to find the power of completely discharging a capacitor with capacitance $C$ during a time interval $\Delta t$. Using the mechanical definition of power as the rate of change of energy $W$ ...
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1answer
84 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 ...
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9answers
7k views

The water analogy seems to imply that power = current. Why is this incorrect?

Many people think of the water analogy to try to explain how electromagnetic energy is delivered to a device in a circuit. Using that analogy, in a DC circuit, one could imagine the power-consuming ...
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3answers
142 views

Internal resistance problem

A 12V battery has an internal resistance of $2.0\Omega$. A load of variable resistance is connected across the battery and adjusted to have resistance equal to that of the internal resistance of the ...
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1answer
48 views

Power - why do we use the driving force over the resultant force?

I'm an A-Level student, studying Further Maths. We've chosen the Further Mechanics as our optional module of the course. Further Mechanics, unfortunately, is not making a lot of sense to me, because ...
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117 views

If mass is doubled, how much more power is needed to sustain the same speed?

My problem involves an object completely immersed in a fluid, moving at some speed, using a certain amount of power to sustain that speed. It could be an airplane or a torpedo, among other things. (...
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5answers
8k views

Why does a 60W bulb glow brighter than a 100W bulb in a series?

In my physics class I have this problem that shows two lightbulbs, one 60W and one 100W in series, connected to a 120V battery. The problems are: Which bulb is brighter? (A: 60W) Calculate the ...
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Understanding voltage and power in the fluid analogy for DC circuits

I am trying to understand electric circuits (ie voltage, current, power, and resistance). For the most part, everything makes perfect sense, but for some reason I do not feel as if I understand the ...
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29 views

How much heat energy is generated by the Sun and Moon's gravitational action on the Earth?

How much heat energy is generated by the Sun and Moon's gravitational action on the Earth? It would seem fairly obvious that any spinning body orbiting a massive object will convert its own motion ...
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1answer
20 views

How can I calculate the power required to make an air turbine spin at a constant rpm?

Say I have an air turbine with diameter $d$, number of blades $N$, and moment of inertia $\frac {d^{2}*M_{Blade}^{**1}}{4}$ (at the tip of the blade). How do I calculate the power required to spin ...
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2answers
94 views

Calculating Laser Focal Power & Focused Beam Waist

If I focus a 1 watt, 405nm laser with a 25mm fl lens what is the power at the waist. I see calculators online but none that make sense. Rather than just telling me can you put your equations you use ...
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1answer
91 views

Calculating power with force as a function of time [closed]

While doing an AP question in Physics C today I answered the question differently from the professor but I'm not sure what part of my reasoning is incorrect. A $100\ \rm{kg}$ block is being pulled ...
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1answer
37 views

Heating effect of fast neutrons in Fast Breeder Reactors

Do fast neutrons play effective(meaningful) role in heating up the water in Fast Breeder Reactors? If so, how much do they?
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62 views

Power by normal force

Would the power created by the normal force always be zero? Since normal force is always perpendicular to the object?
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2answers
110 views

Does gravity matter when calculating work done horizontally?

My teacher gave the following question: A 212lb running back can complete the 40yd dash in 4.53s. What time would a 198lb running back have to post in order to be more powerful? My teacher says that ...
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2answers
269 views

Converting Wattage to Temperature [closed]

Let us say you have an 8 watt source or heater, what is the highest temperature it can attain or be converted to? I was asking in the context of transformer losses. At no-load, there is core loss.....
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Why didn't my Capactive Wireless Power Transfer work?

Today I had the delight of setting up a bipolar capactive wireless power transfer (CWPT) circuit for a school experiment. Disappointingly, it did not work. It was set up just like in the diagram ...
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1answer
229 views

How to calculate the power required by a fountain pump?

How can I calculate the power required by a fountain pump, if just the efficiency of the pump, the height of the fountain and the diameter of the water jet is given?
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1answer
146 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 ...
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2answers
418 views

Why doesn't using more appliances at home decrease the electricity bill? [closed]

I know that at home the electric circuits are parallel, and this explains why if one appliance (eg bulb) fails, everything else continues to work, but if more devices are added in parallel to each ...
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1answer
41 views

Capacitor, stored energy versus reactive power

When an AC voltage U is put onto a loss less capacitor C the capacitor will require a certain reactive power Q (current 90 degress out of phase with the voltage): $Q=UI = \omega C U^2$ in VAr (Only ...
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3answers
61 views

Dependency of temperature acquired by a resistor on power

A nichrome heating element across 230V supply consumes 1.5kW of power and heats up to 750*C. A tungsten bulb across the same supply operates at a much higher temperature of 1600*C in order to emit ...
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1answer
119 views

How can only 2 phases of a 3 phase power system be used to power a load?

When looking at 2 of the 3 phases on a graph, there's a point where they're both positive or both negative. How does one of the phases act as a return path?
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2answers
340 views

Does the depth that the accelerator pedal is depressed correspond more closely to the force or the power that the engine delivers to a car?

Consider a car that only moves directly forward without turning (so that we can model its trajectory as motion in one dimension), and assume we can neglect both air resistance and the frictional ...
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2answers
64 views

Relationship between speed and power in wind/hurricanes

I was looking at hurricanes today when question crossed my mind. Cat 1 Hurricanes are the ones going up to 95mph. Cat 5 goes above 157 mph. That says a lot, but not all. In motors or engines, RPM ...
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2answers
32 views

Power factor in Transamision

What is normally the power factor in the transmssion.? Is there some standard or guidelines for that.? Also, the low factor is not desirebale as it leads to more power loss. That makes sense. They ...