Questions tagged [power]
The time rate of change of energy
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Instantaneous Power transmitted across a wave
As the power transmitted across a point in a string wave is given as $u \omega^2 A^2 v \cos^2{ \omega(t- \tfrac{x}{v})}$ and average power transmitted is given as $\tfrac{1}{2}u\,\omega^2A^2v$ ...
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2answers
37 views
Why do current carrying wires with smaller area heat up more than those with larger area?
Consider two wires of length $L$ and resistivity $\rho$, and consider a fixed voltage source with voltage $V$. The first wire has cross sectional area $A_1$ and the second wire has cross sectional ...
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15 views
Determine power of the engine [closed]
Upon a sloping roller and from a negligible height, sand is placed with a rhythm of $\frac{dm}{dt}$. Thanks to an engine that provides a power $W$, the speed of the roller is kept constant ($v_0$). ...
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1answer
19 views
What is the meaning of power delivered by the battery after $t$ seconds?
I don't get it when a question asks power delivered by a battery after t seconds, are they talking about the total energy delivered by the battery in time t seconds divided by the time, or are they ...
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1answer
24 views
Derivation of the Gaussian Beam Pattern
I am currently reading this book by Mireille Levy. It says (on page 40) that the Gaussian beam pattern with half-power beamwidth $\beta$ is gien by:
$$B(\theta)=A\text{exp}\left(-2\text{log}2\frac{\...
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29 views
Power required to reach X velocity in X time with drag
I wrote a program that adds force to a car like so:
Engine Force = Power / Velocity
Drag Force = -Velocity²
Net Force = Engine Force - Drag Force = Power / Velocity - Velocity²
I'd like to ...
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1answer
19 views
Is combustion engine power defined by speed and torque?
Of a combustion engine, the speed, torque, and power is measured at the crankshaft.
For a measured pair of values for speed $N$ and torque $M$, can there be more than one value for power $P$? In other ...
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2answers
32 views
Rate of charging the capacitor
Why does the rate at which a capacitor in an RC circuit charges, only depend on $R$ and $C$? We know that for power $$P = \frac{V_{\text{emf}}^2}{R} e^{-t/RC}=\frac{QI}{C} $$
so why are we ignoring ...
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1answer
43 views
If a light bulb is connected to different voltages, is there a single method to calculate the utilized power?
If a bulb is marked at 100W and 200V, we know that the bulb utilises a power of 100W when connected to a 200V potential difference. So, can we say that the bulb, when connected to twice the potential ...
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16 views
Linear relationship between air resistance and power consumption?
Suppose that the air resistance a car encounters is independent of its speed. When the car travels at $15\mathrm{~m/s}$, its engine delivers $20\mathrm{~hp}$ to its wheels. (a) What is the power ...
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22 views
Derivation of power transmitted in a wave [closed]
In the derivation of power transmitted by a travelling wave, we consider only the force on the left side of the element which has been assumed. The force on the right side or the net force are not ...
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33 views
In a tungsten filament bulb can be approximated as $P=a(R-b)^4$. Where does the parameter $b$ come from?
I did an experiment measuring the voltage drop across and current passing through a tungsten bulb at increasing input voltage. I then calculated the resistance and power from this data, which when ...
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1answer
16 views
Calculating solar radiation toward earth
This problem is from the book by Arthur Beiser, Concepts of Modern Physics, example 1.7.
Solar energy reaches the earth at the rate of about $1.4$ kW per square meter of surface perpendicular
to ...
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4answers
820 views
Why do (kilo)watt-hours double-count units per time?
A watt is defined as 1 joule per second and is therefore a measurement of energy transfer over a period of time.
A kilowatt-hour is a measurement of energy equal to one kilowatt sustained over a ...
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2answers
24 views
How loud really are ultrasound transducers?
There are a plethora of mini ultrasound transducers available. Many of which have a Sound Level SPL > 110dB. This seems extremely "loud". If you were to listen to one of these transducers, would they ...
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2answers
41 views
Is work-energy theorem valid for Power = force * velocity?
Work energy theorem states "work is said to be done when there is change in Kinetic energy",
i.e. $W = ∆KE$
but as we know, $P=\vec F \cdot \vec v$, where a constant force $\vec F$ maintains a ...
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1answer
41 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 ...
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What is meant by transmission voltage?
I had a question in class that went :
$120$kW of power is generated at a power plant and is then transmitted. The resistance in the transmission lines is $0.4 \Omega$. Calculate the power lost as ...
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1answer
32 views
According to Lenz’s law, does actual number of turns in a transformer matter, or just turns ratio? If so, why?
I ‘m trying to make a transformer and I understand the basic concept of Lenz’s law, and I understand how the ratios work. I get that if you have a transformer with 100 turns on the primary and 10 on ...
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21 views
Energy transmitted through a point in Wave motion
If the energy transmitted through a point is $uw^2A^2v \cos^2w(t-x/v)$ and energy transmitted for a wavelength is half of that,how come when u apply the formula to find the energy of a wavelength it ...
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57 views
How does the area under a $F$-$v$ graph relate to power?
The Force-velocity graph for linear motion with constant acceleration from 0 to 15 m/s, followed by linear motion with constant power from 15 to 60 m/s is shown below
I can then draw the Power-...
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22 views
How does increasing the battery voltage affect the power delivered to a light globe in a single light globe circuit?
I am doing a prac in my physics class and I would like to know how does increasing the battery voltage affect the power delivered to a light globe in a single light globe circuit?
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1answer
19 views
Is energy density of two $P$ W lamps the same for one $2P$ W lamp?
Do two lamps of power $P$ produce the same energy output as single lamp of power $2P$, assuming ideal environment? E.g. lamps do not occlude each other, distance between lamps is negligible, no ...
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62 views
Rate of energy dissipation in a wire carrying current cut in half [closed]
In my physics textbook, I have a sample problem stated as follows:
You are given a length of Nichrome heating wire; it has a resistance R of 72 Ω. At what rate is energy dissipated in each of the ...
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Is there a graphical summary of which modulation technique to use in communication systems using Visible Light Communication techniques?
I am currently looking at various papers written (some listed at the bottom) that discuss how laser pulses can be modulated to send encoded messages to a receiver for communication systems. The more I ...
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1answer
42 views
Why is work measured linearly with the distance [closed]
If I run and push a car, the force considered is the one felt in my arms not in my legs?
The distance the one marked on the field? If the energy outputs is constant and linear with time, why isn't ...
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28 views
What happens to radians in this calculation? [duplicate]
I rewrite N as kg m s^-2 and try to get Pmax, which is in Watts to kg m^2 s^-3 but when I do so I am left with an rad^2.
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1answer
33 views
Simulating a fan speed given power applied to it over time [closed]
I am driving a fan using a micro-controller. I want to test the micro-controller code without having fans physically attached to it so I thought I would model one programmatically.
I want to model a ...
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1answer
56 views
Is the term “Power” meaningful? [closed]
As far as my intuition of Force goes, I believe it is a kind of 'field' (that was not the most precise of ways to describe it!)
Like say, a force of 5 N acts on a 1 kg block. It produces an ...
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36 views
Radiant Intensity
Radiant intensity is power per unit area and area being perpendicular to propagation of energy. Moreover, intensity varies inversely with square of distance. My question is if a beam of light is ...
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1answer
52 views
The power generated from an hydroelectric plant is taken constant?
Using the energy equation for a flowing fluid, by doing a lot of assumptions on the flow behavior, (incompressible fluid, stationary flow, ideal flow...) and applying it to the inlet and outlet of a ...
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1answer
55 views
How same power is distributed in different voltages in power lines?
I have seen when a resistor is connected to a battery, it carries the same voltage across the resistor, if the resistance is changed the current changes but voltage remains the same making the battery ...
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3answers
160 views
Is power equal to $\vec{F}\cdot\vec{v}$ where $\vec{F}$ is force and $\vec{v}$ is velocity? [duplicate]
Can you please tell me when $P=\vec{F}\cdot\vec{v}$ is valid?
Let us consider $\vec{F}$ as to be a function of time $t$. We know that $P=\frac{dW}{dt}$, where $W$ stands for the work done.
$$P=\frac{...
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1answer
53 views
Why does not work done increase when power is increased? [closed]
Suppose, a man pushes a wooden block through a distance of 2 m in 2 seconds and an other man pushes the same block to the same distance in 1 second. According to my assumption, the first man must have ...
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1answer
26 views
Harmonic driving of a non dissipatice circuit (LC for example): does the energy really sometimes goes back in the generator?
Let's assume you have a non dissipative circuit represented by a box: it involves only capacitance and inductance for example.
You drive this box via a sinusoidal voltage generator.
I call $U(t)$ ...
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1answer
32 views
How much solar power in kWH is there in the stratosphere during the day? [closed]
The stratosphere is above much of the atmosphere. People have talked about collecting solar energy there and beaming it back to Earth. How much power are we talking about?
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Power meter zero-offset calibration
I have to measure the output power of a laser diode. To this aim, I am going to use an integrating sphere connected to a photodetector and a power meter.
Concerning the photodetector, electrical ...
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2answers
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power = force times velocity of object, what if the object is “abstract” such as “contact patch” of a tire?
Consider the case of an accelerating motorcycle, the engine converts potential energy into mechanical energy which is used to generate a Newton third law pair of forces at the rear tire's contact ...
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1answer
86 views
What does work really mean deeply? I think the definition of work is not convincing
Suppose we have two objects 1kg mass. The one of them moves by constant velocity 3m/s and then we apply to it force equals 1N so the done work is 3.5 joule. the other object is at rest and then apply ...
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1answer
44 views
Electric reactive power? Is it real or is the design accepted for calculation? [closed]
Do you agree that the reactive power definition is an acceptance for calculations? I disagree with this view. Then; How the electromagnet force or the power stored by the capacitors can be explained.
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How does a car gain kinetic energy?
I understand that the engine delivers power to the wheels and friction from the ground causes the wheels to roll. However, given the power (work per time) at the wheels, how does that energy become ...
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1answer
70 views
Energy power relation [closed]
If we have an energy equation like
$E=A\sin(wt-kx)$
If we differentiate it w.r.t. time, we will get power.
If we again differentiate the last equation, it will be termed as power per unit time.
But ...
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2answers
47 views
Multiple postive solutions for resistor value selected to have a given power
Textbook Example: find the load resistance $R_L$ such that the power at the load is $500\times10^6~\text{watt}$.
By Voltage Divider, voltage of load is:
$$V_L = \frac{R_T}{R_L + R_T}V_s\tag{1}$$
...
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Why doesn't NASA try using high-explosives to fuel rockets?
For a smoothly burning liquid-vapor mixture, I understand that burning hydrogen and oxygen is the best possible solution, not just for now but even theoretically. (?)
But I read about ideas, that ...
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2answers
73 views
Isn't the equation for power more complicated? [closed]
I’ve been told that work is equal to $Fs$ and power is work done per second $dW/dt$ so looking at the equation, isn't power more like this? $\frac{dF}{dt}s + F\frac{ds}{dt}$
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21 views
Why are rotors like these on a cup anemometer not effective for wind turbines?
Intuitively, radial rotors (like these you would have on a wind speed meter) equipped with cups/cones/sails look like they should be very efficient at converting air flow into energy, given that they ...
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1answer
34 views
Problem: find power required to keep metal attached to electromagnet
Suppose I need to find out the efficiency of electromagnet (solenoid) mounted on the table.
I energize it so that $1\text{lb}$ steel object gets attached to it. I energize it with DC current from a ...
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4answers
114 views
Where does the power delivered to car's wheels go?
Okay, so power is work/time. Most cases, when power is provided to something, energy is gained as kinetic energy or lost to friction.
But in a car, the engine puts power ( torque x rpm/5252) to ...
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1answer
50 views
Calculating Rocket Engine Power from Force and Burntime
I am struggling to relate power to force. The example that I am working on in my head is a model rocket. This model rocket (sans engine) has a mass of 50g. Its engine has a mass of 10g (for the ...
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1answer
112 views
Why is the steady speed neglected when calculating work or power? [closed]
A driver notices that her 1080-kg car, when in neutral, slows down from 95 km/hr to 65 km/hr in about 7.0s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car ...
