Questions tagged [power]
The time rate of change of energy
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How to calculate average power? [on hold]
With a baseband signal with a total period, $T$, of 4, and an amplitude $x(t)$ of 3, I have to find the peak to average power ratio. I know how to find this using the average power but I don't know ...
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1answer
33 views
Brightness of lamps
The brightness of the lamp (power) depends on two factors:
The voltage and the current which flow through it.
In Figure(A) as the variable reaistance increases its reaistance, the current which flow ...
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1answer
38 views
Force exerted by a motor, velocity, and friction
Some motor make an object move on a surface very slowly. One can write at steady state : $F_{motor}-F_{friction}=0$. The friction is supposed to come from the surface so : $F_{motor}-\mu_s m g=0$ with ...
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0answers
16 views
Calculating power by using effective mass
For a multi-body mechanical system, how do you calculate power if you know the effective mass of the whole system and the velocity of the system at the point where effective mass is defined and ...
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1answer
29 views
How can I find the power given only the torque?
I have a system where I am trying to find the power generated by a DC electric generator. However, I am not very familiar with generators, so I am having difficulty determining the rotational velocity....
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1answer
30 views
Formula for Power from Kinetic Energy
Work Done = $Fd$
Power = $Fv$
If the (net) Work Done = Change in Kinetic Energy, and the object starts from rest:
Work Done = $\frac{1}{2} mv^2$
Power = $\frac{1}{2} m av$
Power = $\frac{1}{2} ...
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1answer
28 views
Mechanical and electrical power of discharging a capacitor
I want to find the power of completely discharging a capacitor with capacitance $C$ during a time interval $\Delta t$.
Using the mechanical definition of power as the rate of change of energy $W$
...
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1answer
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How to estimate energy of piezo ignighter discharge?
Question for fun. All we know piezo ignighters, these small friends, like following one:
They are of quite same size and parameters with rough approximations: same spring, same piezo, same hit force, ...
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42 views
Constant power, vehicle velocity and drag forces
I wanted to be able to understand how velocity changes while its applied a constant power.
I know power can be calculated as this:
$$P_t= (F_{rol} + F_{ar} + F_g)*v$$
where,
$F_{rol}$ is rolling ...
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9answers
6k views
The water analogy seems to imply that power = current. Why is this incorrect?
Many people think of the water analogy to try to explain how electromagnetic energy is delivered to a device in a circuit. Using that analogy, in a DC circuit, one could imagine the power-consuming ...
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3answers
31 views
Internal resistance problem
A 12V battery has an internal resistance of $2.0\Omega$. A load of variable resistance is connected across the battery and adjusted to have resistance equal to that of the internal resistance of the ...
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1answer
10 views
Power - why do we use the driving force over the resultant force?
I'm an A-Level student, studying Further Maths. We've chosen the Further Mechanics as our optional module of the course.
Further Mechanics, unfortunately, is not making a lot of sense to me, because ...
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3answers
85 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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5answers
2k 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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8answers
3k views
Understanding voltage and power in the fluid analogy for DC circuits
I am trying to understand electric circuits (ie voltage, current, power, and resistance). For the most part, everything makes perfect sense, but for some reason I do not feel as if I understand the ...
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0answers
24 views
How much heat energy is generated by the Sun and Moon's gravitational action on the Earth?
How much heat energy is generated by the Sun and Moon's gravitational action on the Earth?
It would seem fairly obvious that any spinning body orbiting a massive object will convert its own motion ...
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1answer
17 views
How can I calculate the power required to make an air turbine spin at a constant rpm?
Say I have an air turbine with diameter $d$, number of blades $N$, and moment of inertia $\frac {d^{2}*M_{Blade}^{**1}}{4}$ (at the tip of the blade).
How do I calculate the power required to spin ...
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1answer
46 views
Calculating Laser Focal Power & Focused Beam Waist
If I focus a 1 watt, 405nm laser with a 25mm fl lens what is the power at the waist. I see calculators online but none that make sense. Rather than just telling me can you put your equations you use ...
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1answer
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Calculating power with force as a function of time [closed]
While doing an AP question in Physics C today I answered the question differently from the professor but I'm not sure what part of my reasoning is incorrect. The question gives the position of a block ...
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1answer
30 views
Heating effect of fast neutrons in Fast Breeder Reactors
Do fast neutrons play effective(meaningful) role in heating up the water in Fast Breeder Reactors? If so, how much do they?
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2answers
47 views
Power by normal force
Would the power created by the normal force always be zero? Since normal force is always perpendicular to the object?
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2answers
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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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2answers
47 views
Converting Wattage to Temperature [closed]
Let us say you have an 8 watt source or heater, what is the highest temperature it can attain or be converted to?
I was asking in the context of transformer losses. At no-load, there is core loss.....
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0answers
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Why didn't my Capactive Wireless Power Transfer work?
Today I had the delight of setting up a bipolar capactive wireless power transfer (CWPT) circuit for a school experiment. Disappointingly, it did not work.
It was set up just like in the diagram ...
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1answer
42 views
How to calculate the power required by a fountain pump?
How can I calculate the power required by a fountain pump, if just the efficiency of the pump, the height of the fountain and the diameter of the water jet is given?
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1answer
73 views
Significance of the ion Hall parameter
For an ionized Helium gas, for example, MHD Hall generators rely on the mobility ratios between electrons and ions to function and produce a Hall current. As far as I can tell, intuitively, increasing ...
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2answers
408 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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0answers
52 views
Power Dissipation in Internal Resistance of a cell in a circuit with varying resistance
In the circuit , the battery has an e.m.f. of 3.00V and an internal resistance r. R is a variable resistor. The resistance of the ammeter is negligible and the voltmeter has an infinite resistance.
...
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1answer
26 views
Capacitor, stored energy versus reactive power
When an AC voltage U is put onto a loss less capacitor C the capacitor will require a certain reactive power Q (current 90 degress out of phase with the voltage):
$Q=UI = \omega C U^2$ in VAr (Only ...
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3answers
55 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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1answer
42 views
How can only 2 phases of a 3 phase power system be used to power a load?
When looking at 2 of the 3 phases on a graph, there's a point where they're both positive or both negative. How does one of the phases act as a return path?
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2answers
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Does the depth that the accelerator pedal is depressed correspond more closely to the force or the power that the engine delivers to a car?
Consider a car that only moves directly forward without turning (so that we can model its trajectory as motion in one dimension), and assume we can neglect both air resistance and the frictional ...
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2answers
34 views
Relationship between speed and power in wind/hurricanes
I was looking at hurricanes today when question crossed my mind.
Cat 1 Hurricanes are the ones going up to 95mph. Cat 5 goes above 157 mph.
That says a lot, but not all.
In motors or engines, RPM ...
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2answers
25 views
Power factor in Transamision
What is normally the power factor in the transmssion.? Is there some standard or guidelines for that.?
Also, the low factor is not desirebale as it leads to more power loss. That makes sense. They ...
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1answer
35 views
Black body radiation net power emitted
I have a question related to a definition (intuitive understanding). It is stated that :
The human body radiates energy as infrared light. The net power
radiated is the difference between the ...
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1answer
72 views
Is the net power of an open system always equal to zero?
Are the following statements, line of reasoning all correct? I'm particularly interested in the last bullet.
For a closed system energy is conserved and constant.
For an open system energy is not ...
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1answer
57 views
Energy consumption from acceleration
I have an interesting problem.
I have a machine which has to lift 35 kg (this is the total mass of the object and the lifting arm). The lifting is done by a servo motor.
Now I want to calculate the ...
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1answer
49 views
Can a tunnel through a planet be used as a power generator?
Assuming that it were possible to do so, if a tunnel from one side of a planet to the other were lined with copper wire, could dropping magnets down the hole be used to generate power?
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1answer
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Can emissivity value be 4?
We know I=p/A and I=uc which I is radiation density , P is total radiation power , A is surface area and u is energy density. By putting A in the other side we will have p=ucA since energy density is ...
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1answer
82 views
Tidal Power Gen Multiplier? [closed]
Regarding tidal power generation, the following scheme uses an outer tank and an inner tank that floats on the water of the inner tank. You fill the inner tank first (while it's at high-tide level) ...
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1answer
67 views
Why is beam forming more energy efficient than a straight up omnidirectional signal?
Someone tried to explain the process of beam forming to me, and how beam forming is far more efficient than an omnidirectional signal of similar amplitude.
This seemed obvious at the time: the power ...
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0answers
23 views
Photon Flux Density of a 1 dimensional Top Hat by 1 dimensional Gaussian Profile
I am working with a laser right now that takes a 4 mm input beam and creates a 60 um Top Hat by 10 um Gaussian when viewed through a 60 mm lens. The laser beam carries 180 mW of power and the ...
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1answer
50 views
When and how do we use the formula Power = force X velocity?
I'm unable to understand the solution given in this book mine... Why are we adding the resistive force? In fact, i would really appreciate if someone explains the question to me, I'm really struggling ...
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2answers
57 views
Power in 1st Gear
The max power an engine puts out is what determines the max speed a car travel at right, obviously it's more nuanced than that because there are losses in the drive train, and the shape of the car ...
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1answer
98 views
Power in series
I knew that power of two resistors in series is equal to $$\frac{P_1P_2}{P_1+P_2}$$ where $P_1$ and $P_2$ are the individual powers of the two resistors.
I tried to prove it using $P=I^{2}R$ and I ...
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1answer
44 views
A Pump is spraying water to form a fountain
This is a three part question. I have answered two of them.
“A pump is spraying water into the air to form a fountain. The water emerges vertically from a jet of cross-sectional area 2.0*10^-4 m^2 ...
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3answers
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Does density of light power (w/m²) increase after passing a convex lens ?
Does density of light power (w/m²) increase after passing a convex lens ? and if so i need the equation that represent that increase
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1answer
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predicting light loss in fiber optic strand
Background:
I'm working with fiber optics and I'd like to try to calculate the change of perceived brightness of visible light due to signal attenuation in a segment of fiber optic cable, based on ...
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0answers
53 views
Melting Aluminium powder with laser - Calculations
I'm trying to figure out how to calculate the necessary power of a laser for melting a thin layer of aluminium powder. I found this formula, applied to the stainless steel as example :
W∙(1-R) = cm (...
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2answers
82 views
Propulsive Power, Thrust and Velocity
If my rocket requires thrust $T$ to maintain a velocity $v$, the propulsive power can be calculated using
$$P=Tv$$
Assuming the same friction (so the required thrust to maintain the velocity doesn't ...
