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# Questions tagged [power]

The time rate of change of energy

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### 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 ...
137 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 ...
104 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?
8k views

### 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 ...
258 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 ...
32 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 - ...
68 views

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 ...
51 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 ...
325 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). ...
555 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.
103 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 ...
411 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 ...
106 views

$\newcommand{\F}{\mathbf{F}}$$\newcommand{\r}{\mathbf{r}}$$\newcommand{\v}{\mathbf{v}}$$\newcommand{\d}{ \frac{ d #1 }{ d #2 }} There’s a well-known known formula for the power P exerted on a ... 0answers 2k views ### How to calculate theorectical engine max. torque from max. power and RPM? I'm trying to come up with a formula to estimate an engine's maximum torque (and at which RPM) given other values and some assumptions. I'm approximating an engine's torque curve as 2 linear ... 1answer 106 views ### Can a fusion bomb make electricity? [closed] Would blast energy remove a single piece of pre-cut earth above an underground explosive? Could you then break apart the piece of earth and lower it back into the cavity as a way of generating power? ... 2answers 80 views ### Electromagnetic plane wave interaction with metal plate Given a plane wave with \overrightarrow E field parallel to z axis and a square metal plate in xy plane. Suppose we know the power of the incident wave and the size of the plate, how can we ... 1answer 307 views ### The physics of a dog sled — how much power or energy over time? I'm looking for some equations that govern the physics of a dog sled. Let's say we have 8 dogs pulling a sled. Each dog provides a force of F, the tractive effort. The sled weighs W and there's a ... 2answers 106 views ### Electric power and resistance dependance According to the equations,$$P=VI =I^2R\,\text{ and voltage } V=IR$$it seems clear that when the resistance is lower by fixing the voltage at constant, the current is therefore, higher, generating ... 1answer 45 views ### “Reference dependence of fuel gauges” in classical mechanics The power exercised by a force on an object is dependent on the velocity of the object and hence the frame of reference. I am wondering wherein the error lies with arguments such as the ones presented ... 1answer 50 views ### Watts and Newtons revisited At the junkyard, an electromagnet holds a 1 ton car freely in the air (no acceleration exc. gravity). Assuming 100% power efficiency, how many Watt electrical power are needed? I know that the magnet ... 2answers 163 views ### Why isn't work a logical quantity? [duplicate] Please read till the end, before you answer. I'll start off with the examples to get to the point. Drop a block from 1 meter and it will have speed v when it reaches the ground, with total work ... 2answers 91 views ### Why does having a high input impedance and low output impedance cause minimal voltage drop when you connect two circuits? Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to ... 0answers 94 views ### Wind Power in a Wind Tunnel Two questions in one day! What? Okay, so the power equation for air flow is as follows:$$ P = 1/2 \cdot \rho \cdot A \cdot V^3 $$In a wind tunnel due to the Bernoulli principle as the area (A) ... 3answers 789 views ### 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 ... 4answers 1k views ### 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 ... 2answers 51 views ### 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 ... 1answer 764 views ### 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 ... 2answers 1k 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 ... 2answers 286 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 ... 2answers 1k views ### 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 ... 1answer 91 views ### 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 ... 1answer 4k 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). ... 1answer 750 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 ... 2answers 214 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 ... 2answers 153 views ### 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 ... 2answers 212 views ### 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 ... 2answers 71 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? 2answers 113 views ### How can we show Power = \mathbf{F}\cdot \mathbf{v}? [duplicate] How can we say that$$\text{Power} = \mathbf{F}\cdot \mathbf{v}$$We know that small work done by a force \mathbf{F} to displace an object by '\mathbf{x}' is$$W = \mathbf{F}\cdot \mathbf{x}$$... 5answers 441 views ### Why does a rocket engine have increasing power as the speed of the rocket increases? [duplicate] Sorry to repeat this question, but I got no definite answer last time, so I am trying to state the question more clearly. Power = force x distance / time = force x velocity. So if we apply a small ... 1answer 336 views ### Infinitesimal work I am a newbie in Physics (Senior on highschool) and our teacher wrote in a proof$$\dfrac{dK}{dt}=\dfrac{dW}{dt},$$where K is the Kinetic energy of a body and W is the Work. So now that I am ... 1answer 23 views ### Experiment to determine cyclette power I have a cyclette with 8 resistance levels set magnetically using a knob (a magnet is moved near the rotating disk thereby increasing its rotating resistance). The distance from the pedal to the ... 4answers 401 views ### infinite power from constant force? P = W/t = Fd/t = Fv So, if we keep exerting a small constant force and the velocity keeps increasing to near infinite, we should be able to get near-infinite power from the small constant force. We ... 4answers 63 views ### Why does only one of the DC circuit power equations work for me here? I know that$$ P=IV=I^2R=\frac{V^2}{R}$$I solved for the current using the loop rule$$I=.47$$but to solve for the Power output of the 16V battery I could only find the power by finding the ... 1answer 207 views ### Why is my solar panel more efficient as the temperature increases, rather than less efficient? I have recently performed an experiment in which I placed a solar panel inside a closed box, together with a heater and a lamp (60W). I increased the temperature using a heater from 20ºC to 50ºC. The ... 1answer 259 views ### What is the energy output of Solar thermal collector? I have a solar thermal collector. Its LCD screen shows an output of about 5KWH at the end of the day. How to interpret this number? Is this electrical KW (kilo watt)? If yes, isn't too low for a 1.... 1answer 75 views ### Calculating the power output of car gives two different answers [closed] Here is the problem: A 500 kg car accelerates from rest to 100 m/s over a distance of 400m with the average frictional force of 1200N. If it took the car 7.3 seconds to do this, what is the power ... 2answers 260 views ### what causes a decrease in velocity? Let's assume that a car is travelling at a constant velocity (V) on a horizontal flat plane , hence the force of friction is balanced by the force applied by the engine. The power which the engine ... 3answers 201 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 ( ...