Questions tagged [power]
The time rate of change of energy
607
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16 views
Physics for resistors in parallel which resisto is the warmest
for all resistors in parallel, which of the three became warmest when a voltage is applied? Explain your reasoning. Rhigh 9930 ohms, Rm 501 ohms, and Rlow is 17 ohms
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1answer
18 views
What are simple comparisons of energy draws? [on hold]
I have recently attended a Bitcoin mining conference. Some of the attendees were specialists in energy with their business being oriented around building data centres or catering to the needs of data ...
0
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0answers
34 views
What happens to the brightness of bulbs if there is a “bridge” with a bulb connecting 2 branches in a parallel circuit? [on hold]
The title might be poorly worded but here is a picture:
Bulb A & C is the same, bulbs B & D is the same and bulb E is added to the circuit as shown. What would happen to the current flow/...
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0answers
19 views
Active Power for Systems with Several Machines
Starting from the following definitions I am trying to figure out how to find the active power of a system with several machines.
Where $Y_{ij}$ is the bus admittance matrix, $I$ is the injected ...
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2answers
35 views
Confusion in usage of formulae for power dissipated in a circuit
When should we use $\frac {V^2}{R}$ and when to use $I^2R$. Because many times they ask about the power without giving details about series and parallel. I am really confused
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0answers
6 views
Power absorption - ferromagnetic resonance
Magnetic energy is given by: $$ E(t) = \mathbf{M}(t)\cdot \mathbf{H}(t)$$
Hence, the magnetic power becomes:
$$P(t) = \mathbf{M}(t) \cdot \frac{\mathbf{H}(t)}{dt} + \mathbf{H}(t) \cdot \frac{\mathbf{...
0
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0answers
24 views
How can i calculate the friction force in this problem? [closed]
Question:
if the engine of a 1400kg car uses 25kw to maintain an average speed of 17m/s, calculate how much friction is acting on the car.
my try:
p = $\frac{w}{t}$, w = ΔKE, KE = $\frac{1}{2}mv^2$...
0
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1answer
28 views
Power of conveyor belt required to accelerate a mass
Assume the following scenario:
A conveyor belt is driven at velocity v by a motor. Sand drops vertically onto the conveyor belt at a rate of $m$ kg/s. What is the power required to move the belt at ...
0
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1answer
29 views
Interpretation of real and imaginary parts of the Poynting vector in a vacuum
It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it.
Then, its real part is the active power, while its imaginary ...
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6answers
75 views
Potential difference and current
According to Ohm's law: $V=IR$. Therefore $V$ is directly proportional to $I$. Furthermore, $V=W/q$ and $q=IT$. Thus $V=W/IT$. Therefore $V$ is inversely proportional to $I$.
How can $V$ be both ...
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1answer
81 views
Power in different reference frames
I would like to reopen the question asked in this post because I am not quite satisfied with the accepted answer.
Imagine observer A stationary (in world reference frame) and observer B moving with ...
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1answer
29 views
How Much Power Is Required to Hover?
Suppose I have a spaceship that weighs 1,000 kilograms. I take it to the surface of the planet with a gravitational acceleration of 10 meters per second-squared. The planet has no atmosphere and I'm ...
0
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0answers
8 views
How would you find the time taken for a steady current for a superconductor?
For part (c) I need to find the time taken for the solenoid to gain maximum magnetic field strength. However, since it uses superconducting wires, the total resistance is zero, so how would I be able ...
0
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2answers
30 views
Contradicting relation between total power consumed and power consumed by each individual resistor in series and parallel configuration
In a given series configuration of n resistances of equal resistance r, I derived the relation bewteen total power and individual power consumption as follows:
P(Total) = I^2 * R
R= nr (equivalent ...
2
votes
1answer
54 views
Is a Microwave oven without a faraday cage a health hazard?
Microwave ovens have something of a Faraday-cage/shield to keep from leaking (emitting) EM radiation.
What would happen if the shield or the door was removed? No doubt it would be less efficient as ...
0
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0answers
29 views
Power voltage relation
I have a question concerning the relation between power and voltage.
We have a lamp with power of 36W when used with 12V. With what power does the lamp work if we connect it to a 6V battery?
First,...
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2answers
47 views
Power through an unknown element
I've been stumped by a pretty simple question about power through an element in a circuit for a few days, and I was hoping someone could clear it up.
In the first diagram, the way I think of it is ...
0
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0answers
16 views
Fan and venturi effect
A simple calculation shows the power $P$ you can get from wind at speed $v$ passing through an area $A$ is (in Watts)
$$P = 1.23\,v^3 \frac{A}{2}.$$
Using the above equation and taking the Venturi ...
0
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1answer
42 views
Behavior of force, velocity and acceleration when the power output becomes constant
This might look like a homework question, but really isn't. I am teaching myself some basic physics and am shamefully stuck in one place. I have designed the problem below to help myself understand ...
1
vote
1answer
42 views
Calculate Retinal irradiance (W/cm² to log photons/cm²/s)
I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system.
Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a ...
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3answers
100 views
Basic question on Ohm's law
I have what I assume is a very basic question on Ohm's law. Let's say that we have $n$ equal light bulbs in a series with a battery. We know that $$ U - IR - IR - \cdots -IR= U - nIR = 0.$$ Solving ...
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1answer
28 views
Height and Velocity vertically falling body
Feynman wrote in his lecture
From Newton’s Second Law we can easily tell how the object moves, and it is easy to find out how the velocity varies with time, namely, that it increases proportionally ...
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2answers
42 views
Calculating the power for acceleration of an elevator [duplicate]
An elevator does need some kind of acceleration when it starts to rise up so there has to be s force acting on it. But it surely does not accelerate the whole time so after some distance or time it ...
0
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1answer
16 views
Radiation EIRP impact on organisms
I read some documentation about how harmless the non ionizing radiation can be and it looks like the arguments are around the frequency(hence the energy) of the wave. In the sense that a particle with ...
1
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3answers
179 views
Conversion between Power and Flow rate of Fan taking into consideration the geometry of the fan
I need to find the relation between two properties of a fan, that is between power and flow-rate. I expect such a formula to depend on the pressure and on the geometry of the fan.
I have found in ...
0
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1answer
38 views
Energy in radar equation
In the book Skolnik, Introduction to Radar Systems, ed. 2, pag. 52 (sec. Transmitter Power) the radar equation in its simplest form is initially cited:
$$R_{\max}=\left[\frac{P_t G A_e \sigma }{(4 \...
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2answers
47 views
Why can't we use air pressure to recharge an electric cars battery
If we use energy to accelerate a vehicle, a lot of that energy is lost to drag and friction, especially in an electric vehicle. Why can't we harvest some of that energy using an alternator and some ...
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2answers
79 views
How to derive $dW = dE$? [closed]
From the definition of power:
$$P \ = \ \frac{dW}{dt} \ = \ \frac{dE}{dt}$$
Multiplying both sides by $dt$, we get:
$$dW = dE$$
$1.$ What does this imply?
$2.$ Are there any other (perhaps more ...
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1answer
57 views
What does this mean? World consume 14 trillion watts of power [closed]
I am not very clear about this statement. Does that mean in every second we are consuming that much energy?
3
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1answer
62 views
Why does current arrange itself in such a way as to minimize power loss in resistors connected in parallel?
In problem 2 in Problem Set 6, it is said:
"Electricity prefers to flow in the way that minimizes energy loss to resistance."
Using Lagrange multipliers I was able to show that assuming the above ...
1
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3answers
58 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 ...
1
vote
1answer
72 views
Expression for net power consumed when resistances are connected in series in electric circuit
To find the expression for net power consumed, I did this :-
$$
\text{Suppose some resistances } R_1, R_2, R_3, ... \text{ are connected in series in an electric circuit.} \\
\text{Let R be the ...
13
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5answers
3k views
Watts vs. volts amperes
What I understand:
In simple DC circuits, this is a product of the current and voltage, such that 1 watt = 1 ampere x 1 volt
I understand that a watt is a unit of power (change in energy per unit ...
0
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7answers
85 views
Initial speed is zero and so is power?
If I want to accelerate something from standstill to max speed, with a constant force (acceleration and mass don't change), the equation P = F * v would say that in the beginning we use 0 W power. How ...
0
votes
1answer
39 views
Power and intensity of a sound wave
So in the case of a point source of a sound wave in a sphere, I'm wondering why the Intensity decreases with $r^2$ because if i setup the formula the Power always increases with $r^2$. So these two ...
0
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0answers
40 views
Can the mains voltage affect laser intensity?
First of all in very new to the photonics field and i am only venturing into it because I have to troubleshoot an experiment and the issue seems to be the optics block.
So i am using an instrument ...
1
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3answers
81 views
When high voltage leads to low current in power transmission system how its not applicable for household voltages?
When high voltage leads to low current in power transmission system using transformers,how increase in voltage in household circuits from 220 volts makes the current increase damaging the appliances,
...
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2answers
71 views
Power in Series and Parallel circuits
My textbooks states that the brightness of a light bulb that is connected in a circuit is determined by its power. That is, a light bulb connected in series would be brighter if it has a higher ...
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0answers
107 views
power consumption of vacuum pump
I am using the vacuum pump from KNF Neuberger Inc to create a reduced pressure environment for the dryer. I want to calculate the energy consumption of this pump. The pump is removing nitrogen gas ...
0
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0answers
15 views
Normalizing Efficiencies with different motor power ratings
I have obtained data of the efficiencies of a few models of electric vehicles. We would like to compare them, but we have noted that the different motor power ratings may have an effect on the ...
0
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0answers
25 views
Emissivity not accounted for in Equilibrium Temperatures of Planets
I'm trying to understand how to calculate the Equilibrium Temperatures of Planets.
This is the beginning of my working for it...
However, whenever I look at external sources, (like this Princeton ...
1
vote
0answers
48 views
Can neutronic emissions be used as a power source?
The free neutron will degrade to a proton electron and anti neutrino in about 15 minutes. Assuming you could slow a beam of free neutrons sufficiently to keep the size of the reactor manageable: Is ...
0
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0answers
21 views
Angular Velocity of a wake
I am trying to use Blade Element Momentum Theory to run some power calculations for a turbine I built, however, I'm having trouble with one value in particular.
I derived the following equation for ...
2
votes
3answers
127 views
What happens to $P = F × v$ for a car moving on a frictionless surface and in vacuum?
I'm considering a scenario in which a car with an engine capable of producing maximum power of 200 hp is moving on a frictionless surface and in vacuum. Since no work is lost due to friction or air ...
2
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7answers
153 views
Why does incline on treadmill burn calories faster?
So I was at the gym today, running on the treadmill, when the question hit me:
If I run with the treadmill on incline, I burn more calories. Since I run at the same speed, kinetic energy is constant, ...
4
votes
2answers
222 views
Did Usain Bolt really sustain 11 horsepower of exerted power during the 100 m sprint?
I've just come across a WP article which says: "The Jamaican sprinter Usain Bolt produced a maximum of 3.5 hp (2.6 kW) 0.89 seconds into his 9.58 second 100-metre (109.4 yd) dash"
Out of curiosity, I ...
0
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1answer
137 views
How to calculate total power generated?
I have a circuit with a piezoelectric transducer which converts mechanical energy to electrical. The ceramic is connected to a 1 Megaohm resistor and a voltmeter which reads voltage.
If I want to ...
0
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0answers
78 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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0answers
19 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/...
0
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2answers
85 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 ...
