Questions tagged [power]

The time rate of change of energy

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1answer
404 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 ...
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1answer
6k 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 ...
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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 ...
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1answer
43 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 ...
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5answers
209 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 ...
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1answer
55 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?
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1answer
755 views

Minimum Power Required

Here is a question, I found two methods to solve it, differing in the answers and I'm not sure why a method is incorrect. Question: A fire hose of cross section area ...
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1answer
361 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 ...
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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 ...
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1answer
238 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 ...
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0answers
393 views

What is the physics behind the vibrating feeling you get when you touched an inducted metal surface?

When I was last working on the highway, I stopped to help a motorist parked under high tension power lines. I'm driving a large Ford Transit van, fairly box shaped. My van was parked 90 degrees (...
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2answers
95 views

Electric power and resistance dependance

According to the equations, $$P=VI =I^2R\,\text{ and voltage } V=IR$$ it seems clear that when the resistance is lower by fixing the voltage at constant, the current is therefore, higher, generating ...
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85 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 ...
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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 ...
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130 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 ...
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0answers
178 views

Estimating the battery capacity using current power consumption and battery percentage

I want to estimate the current maximum capacity (in kWh) having the current power consumption (in kWh) and the state of charge of the battery (in %) available in a time series. I do not have a full ...
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3answers
98 views

Solar plants and energy conservation

I'd like to apologise if this question is stupid or if it was asked ten thousand times - I haven't found an answer to it. Quite possibly due to mediocre English or abominable knowledge of physics. So,...
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0answers
539 views

Calculate signal strength received by an Ultrasonic Transducer [closed]

Below is an image showing transducer A transmitting a signal with an amplitude of 3000mV, I am trying to calculate the amplitude of the signal received by transducer B in mV, I understand that there ...
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1answer
846 views

Eddy Currents: Relationships between thickness and deceleration

This is probably a question with a very simple answer, but I want to make sure before I write too much. I'm doing an investigation on Electromagnetic fields, in particular eddy currents and how they ...
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0answers
97 views

Energy stored in a battery?

I want to calculate the energy lost from a battery to heat, and the energy stored in a battery. Are these formula's correct? $ E_b$ = $I$$V$ x $t$ $ E_H$ = $I$$R$$^2$ x $t$ $E_b$ = energy stored in ...
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2answers
1k views

Numerical: Work Power Energy [closed]

A particle moves along a straight line. A force acts on the particle which produces a constant power. It starts with initial velocity 3 m/s and after moving a distance 252 m its velocity is 6 m/s. ...
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1answer
248 views

Does a 27 hp engine output the same amount of energy as lifting a 1 ton stone block almost 3 meters per second?

I’m trying to get a sense of how much energy a 27 horsepower engine outputs. 27 hp $=$ 20 133 watts (joules/second). Potential energy can be calculated as $E = mgh$ where $g = ~9.8\ m/s^2$ on earth. ...
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7k views

Finding power with Drag Force equation

The mass of the car is 1500 kg. The shape of the body is such that its aerodynamic drag coefficient is $C_D=0.330$ and the frontal area is $2.50 m^2$. Assuming that the drag force is proportional to $...
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1answer
2k views

Total power consumption of electric device

For some electric devices, like a fan or air conditioner, I read about their power consumption in watts on their specification guide. Does it tell about the power at normal or full speed? or Does the ...
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2answers
414 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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5answers
5k 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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686 views

How do electrical devices suck electricity?

If the electric potential is 220V and some device needs 1500watts then how does it suck exactly that amount of electrical energy from outlet?
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4answers
2k views

Will Earth Hour do damage to power supply system?

There is always a debate around Earth Hour every year, and the opposite side of Earth Hour usually claims that The (sudden) decrease and increase of the power usage in the start and end of Earth ...
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3answers
2k views

How does Ohm's law apply to superconductors?

As radio amateurs we've all learned the various relationships of power, voltage, current and resistance as expressed in Ohm's Law. My question relates to the following simple circuit showing an ideal ...
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2answers
36k views

Confusion about P=VI and V=IR

If we look at $P=VI$, we see that if the current doubles then the potential difference is halved but this doesn't seem to make sense according to $V=IR$. If we look at that equation, since the ...
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3answers
267 views

If energy of a wave is proportional to frequency, why are higher-powered lightbulbs brighter?

E = hf. Got it. P = dE/dt, right? Not seeing an amplitude anywhere in the formula, so shouldn't the power rating on the lightbulb only be driven by the color? Is it a problem with the way the ...
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3answers
176 views

Conversion between Power and Flow rate of Fan taking into consideration the geometry of the fan

I need to find the relation between two properties of a fan, that is between power and flow-rate. I expect such a formula to depend on the pressure and on the geometry of the fan. I have found in ...
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1answer
368 views

why countries can't make nuclear bombs? [closed]

Somehow when I google about the nuclear bombs I find a lot of books and resources that seem to explain everything about how those bombs are made. But sometimes I often hear that countries that want to ...
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2answers
494 views

Why doesn't a phone charge faster, rather than slower when it is in use

In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). So,...
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2answers
108 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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1answer
61 views

Is there a way to express stream power for an entire watershed?

Fluvial geomorphologists describe stream power, $\Omega$, as the 'rate of energy dissipation against the bed and banks of a river per unit downstream length'. It is expressed as a function of water ...
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2answers
517 views

Couldn't high gear ratio replace need for high torque engines?

I've noticed that there is a correlation between displacement and torque, so trucks, SUVs etc. have large displacement engines to produce lots of torque to move heavy loads. Sports cars have lower ...
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2answers
2k views

Electric power for current density

The electric power produced by a current $I\in\mathbb{R}^+$ and a voltage $V\in\mathbb{R}^+$ is $$ P = IV. $$ Now the current is given as an (alternating) current density $J(\mathbf{x},t)=\Im(e^{i\...
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3answers
106 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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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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3answers
191 views

What does the equation $P=\vec{F}\cdot \vec{v}$ really mean?

Let's say that we have a car travelling at a constant velocity (V) on a flat plane , let's assume that it's applied force or the force applied from its engine ( Fapp ) balances the frictional force ( ...
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1answer
975 views

Relation between power and energy

If I have an expression for the 'rated generator power' of a wind turbine, call it $P(t)$ how is the energy, that the wind turbine generator generate, related to the power $P(t)$? So: $$E(t)=\...
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2answers
48 views

Different interpretations of the power in a resistor law?

On this KhanAcademy article, under the Power in a resistor heading, it states three equations, all of which can be derived from $V=iR$ and $P=iV.$ They are: $P=Vi$ $P=i^2R$ $P=\frac{V^2}{R}$ Then ...
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4answers
2k views

Relationship among heat generated, power, current and voltage

Can you help me understand the relationship between current, voltage and heat generated? I tried two sets of heated gloves. One uses a battery which generates 7.4 volts x 2 amperes (14.8 watts of ...
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2answers
933 views

Human Muscle Energy - Increases or Decreases by doing work?

When Human does some work, he gets tired. I will think he looses some energy. If he really loses energy by doing work how does he gain more strength to do more work later. His muscles become strong ...
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1answer
596 views

The inverse square law of sound through solids?

We all know about the inverse square law of sound. In short the power of the wave will get evenly spread on an ever increasing spherical expansion and this will dissipate the power of the wave at a ...
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3answers
872 views

Should the term Watt's Law be used for $P = IV$?

I'm revising some electrical curriculum for a technical training program. In the curriculum students have to calculate values using Ohm's law and the equation $$\text{Power = Current * Voltage}$$ ...
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2answers
668 views

Calculate power $P=V \times I$

I have an adapter which mentions like this... Input: 100-240V~1.8A 50-60hZ So how much power does it really consume? I just knew ...
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3answers
9k views

How do I calculate the power consumed by a lightbulb?

I'm studying a lightbulb and its variable resistance, given by the expression: $R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$. In this case, $R$ is not given by Ohm's ...
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3answers
52 views

Why a Box moving at Constant Velocity isn't doing Work?

This was a question from an A-Level past paper and the question is pretty straightforward. I was in a hurry so I couldn't think much on this problem and chose $C$ as my answer, checked it on the ...