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Questions tagged [energy]

Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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Magnetic field inside a conductor

We know there is an energy associated with magnetic fields is $$\frac{1}{2}\mu B^2$$ Consider a magnetic field perpendicular to a conductor (any metal). Shouldn't it theoretically be possible to make ...
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Is the work-energy theorem only valid for conservative forces? [duplicate]

I have learnt that it states "The work done on a body is equal to the change in kinetic energy of the body" which we derived by considering a position-dependent force. However, if mechanical energy is ...
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283 views

Energy of the particles in the particle accelerator

Recently I came across something and I was surprised. I always thought that huge amount of energy is required to accelerate particles in the accelerator in the particle physics.But looks like no. The ...
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30 views

Conceptual doubt regarding energy loss of a system

Suppose I have an ideal spring with a block of mass M attached to it at one end and the other end of the spring on a wall. The spring is executing SHM. I now gently place a small mass m on the block M ...
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24 views

What are the key concepts of solar energy? [closed]

My goal is to compile a list of key concepts for learning solar energy. I aim to self-teach and eventually be able to experiment with building solar-powered products. What items would be part of the ...
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2answers
146 views

Energy measurement from position eigenstate

Given that the eigenstates of the position operator can be written as $\delta(x-x')$, and suppose we measure a particle in an infinite potential with walls at $x=0$ and $x=L$. I measure the particle ...
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60 views

Kinetic Energy - dependence of mass

There are 2 equations to kinetic energy: $\frac{1}{2} mv^2$ and $\frac{3}{2} kT$. By the first equation kinetic energy depends on mass, while by the second one it doesn't. How comes? So does it ...
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1answer
79 views

An invariant for transformations of Lorentz

Exist a physical demonstration why $$E^2- p^2c^2 =m^2c^4=E'^2- p'^2c^2 $$ is an invariant for transformations of Lorentz? N.B.: $m$ is mass; $E$ is the energy and $p$ is momentum in the frame $\...
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How to calculate velocity of an object at the bottom of a circular hill if there is an energy loss due to friction?

Object slides down a circular hill and loses some of its initial mechanical energy due to friction. As an object slides down, friction increases and force in direction of motion decreases until the ...
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1answer
69 views

Physical significance of free energy in canonical ensemble?

From macroscopic thermodynamics I understand that the free energy equals the total energy of the system minus the energy it would have cost a thermal reservoir to create it. So any energy in "excess" ...
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0answers
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Meaning of zero temperature in terms of $1/T=\partial S/\partial E$

Suppose we have a $NVE$ system of a pure crystal. From the definition of the temperature in macroscopic thermodynamics, $$\frac{1}{T}=\left. \frac{\partial S}{\partial E}\right |_{V,N}$$ . Then ...
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Strong empirical falsification of quantum mechanics based on vacuum energy density

It is well known that the observed energy density of the vacuum is many orders of magnitude less than the value calculated by quantum field theory. Published values range between 60 and 120 orders of ...
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1answer
33 views

What is energy band gap?

Explanations for graphene's high electrical conductivity often discuss energy band gap. What is energy band gap and how does it relate to the conductivity of a material?
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How does $dS \ge 0$ imply a system cannot receive more energy than work supplied?

I am reading a book that asserts the following: The second law arises from an equation of the form $$dS = \mu\,dE + \mu\,dW \tag{1}$$ where $dS$ is the change in entropy, $dE$ the change ...
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1answer
87 views

Same energies are transferred to 2 objects. But they gain different amounts of energy. Where is the mistake?

Situation #1. In space, an object is moving with a constant velocity in an inertial reference frame. The object is connected to a winch (winch “A”) with a rope. Winch “A” is in front of the object, ...
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7answers
111 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, ...
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1answer
27 views

Energy of light

If a beam of light is travelling in full vacuum in same medium ,then , the energy of the light beam will decrease or not while moving through space ? The wavelength of the beam of light will change ...
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37 views

Questions about the energy density and momentum density in linear media

Precisely which energies are accounted for in the energy density of a linear medium given by $$u = \frac{1}{2}\left(\epsilon E^2 + \frac{1}{\mu}B^2\right)?$$ For example, is there a kinetic energy ...
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32 views

Potential Energy and Antimatter anihallation

Apologies if I've misunderstood anything fundamental, but the following has been on my mind for a while: Imagine the following scenario: Take two matter objects of identical mass etc. Now one is ...
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4answers
171 views

Where is the mistake in this reasoning? [closed]

2 identical objects are moving with different constant velocities. Then, in turn same forces act on them for some period. Both times, the same amount of energy is used to produce those forces. As the ...
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1answer
31 views

What is the change in the form of energy that is shown by the change in direction of the compass needle when an electric current is passed beneath it?

If the magnetic field generated by the electric current does not move the compass needle, no work is done, thus no energy is exchanged. If the magnetic field generated by the electric current does ...
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1answer
42 views

Energy in central force orbit

Given a mass $m$ moving under the central force $$F(r) = -k/r^n$$ and a circular orbit passing through the force center, I want to show $n$ must equal 5. I ultimately arrive at the expression $$E = \...
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2answers
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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 ...
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7answers
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Why is the total work done on an object its change in kinetic energy, but not change in total energy? [closed]

We have just started learning so explain simply. We don't know about conservative or non conservative forces. We are just learning basics.
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0answers
69 views

We just need a solar panelled SPAIN to power the world? [closed]

A stiffer, more formal (form, schmform!) way to ask this is: Is a solar panel field the size of Spain enough to cover the worlds energy needs? (This assumes that everything in the world runs on ...
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5answers
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Why does energy conservation give me the wrong answer in this inelastic collision problem?

Suppose there are two objects. The first object is stationary and has the next parameters: v1=0 m/s, m1= 4kg. The second object is heading towards the first one with speed v2= 8m/s and mass m2= 60 kg. ...
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2answers
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What is exact definition of potential energy if all forms of energy except kinetic comes under potential energy? [duplicate]

I need to know the exact meaning of potential energy. I AM confused what it actually means.They say all types of energy comes under 2 types kinetic and potential so what is potential energy then? My ...
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4answers
97 views

Why does energy flow downhill/entropy increase?

I'm not sure if this is asked anywhere else before, but so far my search isn't fruitful. I'm in my second bachelor of physics and mathematics, and currently taking a thermodynamics course. We just ...
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0answers
35 views

Query about working out the Specific heat ratio of a gas

The specific heat ratio for $\rm CO_2$ at room temperature is $1.28$ according to my tables. Since $C_V= \left.\frac{\partial U}{\partial T}\right|_V$ and $C_P=\left.\frac{\partial U}{\partial T}\...
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1answer
38 views

Pellet collides with hanging block, what is the velocity of pellet? Two different methods of answering with different answers which is correct? [closed]

There are two different methods to solve this question and both of them make sense to me yet they give different answers: Method 1: The energy of the system at the start and at the end must be the ...
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2answers
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If matter comes from energy, does this mean that energy has weight?

If the matter in the Universe formed from the energy of the Big Bang, and matter has weight, does this mean that energy has weight? Moreover, as the Universe expands over time, does its overall ...
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1answer
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How can a hammer of very less weight apply a large force when hit from a height?

Consider a hammer of weight 20N . When it is at on the table the only force it can apply on the table is it's weight. so it applies 20N when kept on the table . But when it is raised and dropped, the ...
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2answers
51 views

Why is the work done by friction zero when body is rolling? [duplicate]

Why is the work done by friction zero when body is rolling? Does it mean no energy is dissipated by friction.
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1answer
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Why use Work-Energy Theorem here?

My confusion is in reference to the solution of this problem, whose problem statement and its solution I restate here: Problem: Two blocks of masses $m_1$ and $m_2$ are placed on a rough ...
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0answers
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Change in temperature of dielectric when capacitor is charged and discharged

Suppose I have a capacitor with a dielectric between its plates. Intially the temperature of dielectric is $T_{-1}$. The capacitor is fully charged and the temperature of dielectric is noted.. say, $...
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2answers
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Will a wheel keep rolling in perfect vacuum? [duplicate]

I have a query which I cannot resolve by finding enough arguments. A perfectly round wheel that also doesn't have any contractions due to gravity is placed in a perfect vacuum, so no drag is present. ...
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1answer
35 views

How to find acoustical output of a horn?

If I had a horn or cone that could be written as a function $f(x)$ rotated around the x-axis, how would I calculate the output, knowing the input (intensity, frequency, everything)? I want a way to ...
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1answer
76 views

How much kinetic energy is there in the observable universe at any given time?

I know that it fluctuates, but, what would be a good estimate for how much kinetic energy is in the universe right now? I know that things cancel it out, but, how much is there?
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Do black holes violate the conservation of mass?

This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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4answers
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Conceptual question regarding velocity in projectile motion vs. up a ramp

I have a question regarding conservation of energy in regards to two different scenarios: In the first case I was told that a ball with an initial velocity, V, was launched at an angle, $\theta$ and ...
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2answers
45 views

Energy Density with a Linear Dielectric Inconsistency

The energy density of an electromagnetic field with a linear dielectric is often expressed as $0.5 E \cdot D$. It is also known that energy can be found by $ 0.5\int_{V} \rho V dV $. Using the latter, ...
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1answer
50 views

Energy balance of this machine is positive. What is wrong? [duplicate]

Machine on this picture is inspired by video: https://www.youtube.com/watch?v=rxFXsoqbfrk There must be error in the computation of energy balance or there is another negative energy. Can you help ...
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1answer
63 views

can we use the power of ice to float to make free electricity?

ice and water have a volume to weight ratio of 12/11 so 11 tons of ice will support 1 ton of weight 1 ton of ice takes 1000 watts or 12000 b.t.u. of energy to be produced question is , at what ...
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2answers
39 views

Why is the mechanical energy lost whenever an object undergoes plastic deformation?

I have read here that "Mechanical energy is lost whenever an object undergoes plastic deformation." Why is that? I mean the object would be changing its shape, and while it's changing its shape it ...
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2answers
73 views

Potential Energy equation is wrong as $mg$ force acting upwards (by us) and gravitational force ($-mg$) get cancelled and no movement is possible [duplicate]

In physics, the potential energy of an object is mgh. We say that the potential energy of an object is equal to the work done on it to make it reach that height. So the work done = mg h. Mg is the ...
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1answer
40 views

Question about quanta and $E=h\nu$

It is my understanding that Planck's major breakthrough was that energy comes in the forms of 'packets' called quanta, each of which has the energy of $6.62e-34$ of one Joule. By looking at Planck's ...
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Human vs. machine energy expenditures

I've read that a typical human-powered bicycle power generator generates around 100W. For the same amount of effort, the bike travels probably around 20k/h on a road. Does that mean it requires 100Wh ...
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1answer
37 views

Manual jack up of Maxwell-Boltzmann scale parameter?

I first noticed this phenomenon when i was a child, about 4 decades ago. I needed a piece of wire but could not find a pair of pliers with which to cut the wire. So i used a technique i once saw my ...
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1answer
42 views

Energy balance in a thermodynamic system

I think I've missed this point in class but I was wondering how to determine for example if I have a turbine, the Energy balance would look something like this: $W$ as work $h_1$ as intake and $h_2$ ...
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1answer
27 views

Minimum energy of proton converting into a neutron

I'm given the isotope sodium 22 (which is positron emitter) and I'm stuck with calculating the minimum energy that a proton needs to convert into a neutron and a positron. It got to do with $E=MC^2$ ...