Questions tagged [power]

The time rate of change of energy

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48 views

How to determine the best fit battery for a battery powered application? [closed]

This is an attempt to get some scientific information about how to best use batteries (I'm going to refer to both single cells or assemblies of cells as batteries for the sake of this question) in ...
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138 views

Question about Power lost in a wire given voltage and current

The question is as follows: A high-voltage transmission line carries $1000 \mathrm{\,A}$ at $700 \mathrm{\,kV}$ for a distance of $100 \mathrm{\,miles}$. If the resistance per length of the wire is $0....
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37 views

How is it that a single transmission line can be modeled by a two port network?

I am studying electric power distribution. I've read in Kirtley's Electric Power Principles (2010) that a transmission line can be modeled by the 'pi' model. Which is a two port network with series ...
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1k views

What does the Force in the equation P=FV represent?

So this is the question in context. A car of mass 1200 kg tows a trailer of mass 300 kg along a straight horizontal road. The resistance to motion is modeled as being a constant force of magnitude ...
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How to calculate the power produced by a single cylinder petrol (car) engine at a certain rpm? [closed]

I want to know: What are the main parameters that make power/torque generated by engines different and what makes some engines more efficient? (except friction and things like that!) How do I ...
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712 views

Measuring voltage output of a Wimshurst machine

I see that there are a few variables that effect the voltage output of the Wimshurst machine. For one the size of the disk, the amount of conductive strips on the disk, and obviously the spark gap ...
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1answer
155 views

What are some good analogies about electrical power? [closed]

What are some good analogies about electrical power? I understand the mathematical part of $P=VI$, but I would like to somehow visualize it in a practical electric circuit.
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227 views

Building a CO2 laser power supply [closed]

So i bought a 15,000 volt franceformer for $160. This is used to power my CO2 laser that is also homemade. The power supply is the most expensive part by a landslide so I decided to ask around. I ...
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Can power (in the context of work and energy) ever be negative?

Consider this question which HAS definitely been asked but i ask you now to question your first inclination to just 'do the question' and explain the thought process. Restrictions: assume you only ...
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1answer
26 views

How do microwave transformers avoid power loss through the skin effect?

I was reading about the skin effect and how the effect increases for higher frequencies. I also read that Litz wires can be used to counteract the effect, but usually only up 1 Mhz frequency. Is ...
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617 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: $$ E_1 = \int F_T(t) .v(t) .dt $$ ...
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Transmission line loss [closed]

Let us suppose that a transmission line is carrying power to a resistor $R$ The resistor $R$ requires a power of $P=IV$. Applying the most simple case where a transmission line is supplying a current $...
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187 views

Why we say that electrical energy is power over time p x t? [closed]

How electrical energy can be power over time when electrical energy is actually potential energy? from a lot of sources (wiki ,study.com..) comes the information that electric energy is potential or ...
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121 views

Why we need Watts in electricity if power is done by the Amps?

If: I = V / R And a light bulb needs X amount of electrons flowing per second (I=amps) to create light. Why do we use watts on the first place if all the light bulb cares is the number of ...
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166 views

Power and work of tension force of a rotating body [closed]

Suppose that we have a body rotating on a massless string with a uniform velocity. What can we say about the work done by this tension. Moreover, what is the power related to this work. I do not have ...
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106 views

Why do power stations use steam to drive turbines?

Wouldn't it make more sense to use a liquid with a lower specific heat capacity/latent heat of vaporisation so that less energy was used up in converting the water to steam?
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What is the relationship between power and energy? [duplicate]

What is the relationship between power and energy? Of course, this is a very basic and fundamental question, but it's one that many people learning physics struggle with. I'm asking this to have a ...
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277 views

Work, Energy, and Power- Hooke's Law & graphing

The purpose of the work, energy, and power lab is to demonstrate the validity of Hookes Law over a limited range of displacements, measure the spring constant of several springs, and determine the ...
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36 views

How to devise a microwave power sensor?

I am fairly aware of the general thermistor/ diode/ thermocouple methods of sensing microwave power. Currently my requirements are simpler: A device that senses only microwave power incident on it - ...
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71 views

Question about power

Say we have a waterfall and we know that 10kg of water falls over the ledge per second. We are also given that the acceleration due to gravity is 10m/s^2, and the distance to the ground is 5m. We are ...
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2answers
55 views

Energy Creation Using Water, Generators, and a Pump

So a friend and I are arguing over an infinite energy idea that he had about using gravity, that is an almost infinite source of energy. Water to use to push a generator by dropping and hitting the ...
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1answer
403 views

How to convert RF absorption rate (SAR) values to electric field values?

I'm currently reading a bunch of journal articles that describe biological effects of electromagnetic fields. They often refer to the field in terms of specific absorption rate (SAR) values (W/kg). ...
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607 views

Why power is proportional to resistance in $I^2R$ and inversely proportional in $V^2/R$?

In one relation electrical power is directly proportional to resistance while in other it is inversely promotional.
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107 views

What are the natural forces that slow the neutron down?

In a nuclear bomb, I've noticed that slow neutrons don't affect plutonium 239, if we're using this kind of atoms (plutonium 239), we need fast neutrons to make a sustainable chain reaction. Also I've ...
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423 views

Power consumption of a qubit?

I keep hearing “A quantum computer will consume so much power that it'll need a dedicated power plant.” I'm trying to find literature to back this up. I would love to get an estimate for the ...
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117 views

What assumptions would I have to make to rigorously prove power equals force dotted with velocity, $P=\mathbf{F}\cdot\mathbf{v}$?

$\newcommand{\F}{\mathbf{F}}$$\newcommand{\r}{\mathbf{r}}$$\newcommand{\v}{\mathbf{v}}$$\newcommand{\d}[2]{ \frac{ d #1 }{ d #2 }}$ There’s a well-known known formula for the power $P$ exerted on a ...
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How to calculate theorectical engine max. torque from max. power and RPM?

I'm trying to come up with a formula to estimate an engine's maximum torque (and at which RPM) given other values and some assumptions. I'm approximating an engine's torque curve as 2 linear ...
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115 views

Can a fusion bomb make electricity? [closed]

Would blast energy remove a single piece of pre-cut earth above an underground explosive? Could you then break apart the piece of earth and lower it back into the cavity as a way of generating power? ...
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88 views

Electromagnetic plane wave interaction with metal plate

Given a plane wave with $\overrightarrow E$ field parallel to $z$ axis and a square metal plate in $xy$ plane. Suppose we know the power of the incident wave and the size of the plate, how can we ...
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343 views

The physics of a dog sled — how much power or energy over time?

I'm looking for some equations that govern the physics of a dog sled. Let's say we have 8 dogs pulling a sled. Each dog provides a force of F, the tractive effort. The sled weighs W and there's a ...
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111 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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48 views

“Reference dependence of fuel gauges” in classical mechanics

The power exercised by a force on an object is dependent on the velocity of the object and hence the frame of reference. I am wondering wherein the error lies with arguments such as the ones presented ...
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1answer
54 views

Watts and Newtons revisited

At the junkyard, an electromagnet holds a 1 ton car freely in the air (no acceleration exc. gravity). Assuming 100% power efficiency, how many Watt electrical power are needed? I know that the magnet ...
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163 views

Why isn't work a logical quantity? [duplicate]

Please read till the end, before you answer. I'll start off with the examples to get to the point. Drop a block from 1 meter and it will have speed v when it reaches the ground, with total work ...
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96 views

Why does having a high input impedance and low output impedance cause minimal voltage drop when you connect two circuits?

Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to ...
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107 views

Wind Power in a Wind Tunnel

Two questions in one day! What? Okay, so the power equation for air flow is as follows: $$ P = 1/2 \cdot \rho \cdot A \cdot V^3 $$ In a wind tunnel due to the Bernoulli principle as the area ($A$) ...
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Question about the relationship between resistive drag, dissipated power, and velocity

This is from a high school physics exam for 16 year olds that I did two years ago. I still don't understand it. The power $P$ required to move an object through a medium at constant speed depends ...
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Units in the power equation

I have a question with the formula power = energy / time. I know it's a simple formula, but when substituting into this formula, Does the energy have to be in joules or watts (no kJ or something ...
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Power of an engine

Suppose a car of mass $m$ starts out with an acceleration $a$ in the positive direction (say +ve x-axis), against a constant Resistive force $R$. At the instant it's velocity is $v$, what is power of ...
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Converting lux to watts with known wavelength

I was wondering if there was a possible way to convert lux to watts accurately if the wavelength is known. I know that for the sun there is an approximate conversion of 0.0079W/m$^2$ per Lux using ...
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2k views

How do you find instantaneous power?

I was wondering is there a way to find instantaneous power when you are not given an equation relating work with time but only the numerical values of quantities like force, mass of body, acceleration ...
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327 views

Acceleration with zero initial power?

Power = Force x velocity At the initial instant, the velocity = 0, so the power is zero, even though there may be force, but zero power is taken from that force. So how can the object even start ...
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How much power in a lightning strike?

The Biblical story of 1 Kings 18:20-40 relates how a lightning strike ignited a bulls carcass, as well as a wood pyre, both of which had been thoroughly wetted with abundant water. Is there enough ...
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How can energy of petrol be compared to electricity?

When driving my car, which is powered by an electrical engine, I'm consuming between 10 and 17 kWh per 100 km (measured at the charging outlet). The car I had before consumed between 5 and 8 litres ...
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5k views

Relationship of Pump Power to Flow Rate

I read two different things about the relationship between power (P) of a pump and flow rate (Q). The most common thing that I found was simply that Q is directly proportional to P (power-of-pump). ...
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854 views

How does the voltage source provide energy in a circuit?

I understand that a voltage source drives the current to flow from the terminal with lower potential to the terminal with higher potential. However, if the electrons are gaining potential energy then ...
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279 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 ...
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Relative height or height from ground?

Ok so I encountered this question -> A 10 H.P. motor pumps out water from a well of depth 20m and fills a water tank of volume 22380 litres at a height of 10m from the ground. The running ...
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Why does Work Done Appear to be More than 100% Energy in a System? [closed]

Energy is equivalent to the work done on a motor. The work done on the motor is multiplied by more than 100% by recycling the energy the motor uses. Why then does the energy in the system appear to be ...
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74 views

Power in electrical circuits

Why does $P=i^2R$ apply only to transfer of electrical potential energy to thermal energy in a device with resistance while $P=iV$ apply to electrical energy transfers of all kinds?

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