A doubt in the Law of conservation of energy

[Nivvedan]

Lets suppose I have a fan attached to a motor in complete vacuum. The electrical energy supplied to the motor is converted into the kinetic energy of the fan. Now I replace the motor with another one which rotates in the opposite direction. I keep the switch on till the blades slow down and stop and switch it off before it begins to rotate in the opposite direction.

Lets now compare the initial and the final states. The kinetic energy of the fan is the same before and after the event. But I have spent some electrical energy. Has the enegy been lost?

If it is lost, then the law of conservation of energy does not hold. I know I am wrong but I don't know where. Because Physics SHOULD work.

Please help me.

[Country Boy]

Lets suppose I have a fan attached to a motor in complete vacuum. The electrical energy supplied to the motor is converted into the kinetic energy of the fan. Now I replace the motor with another one which rotates in the opposite direction. I keep the switch on till the blades slow down and stop and switch it off before it begins to rotate in the opposite direction.

Lets now compare the initial and the final states. The kinetic energy of the fan is the same before and after the event. But I have spent some electrical energy. Has the enegy been lost?

If it is lost, then the law of conservation of energy does not hold. I know I am wrong but I don't know where. Because Physics SHOULD work.

Please help me.

The law of conservation of energy does not say that MECHANICAL energy is conserved. Here you have lost energy through the resistance of the wires and friction in the parts, without which you could not have turned the blades of the fan. In net, the temperature of the fan will go up.

[insane_alien]

there will also be some lost through EM radiation from the motor.

[Mr Skeptic]

The motor will act as a generator, converting some of its kinetic energy into electricity. In any case, the end result is mostly heat and a tiny bit of EM energy.

[thedarkshade]

The kinetic energy of the fan is the same before and after the event. But I have spent some electrical energy. Has the enegy been lost?

If it is lost, then the law of conservation of energy does not hold.

In thermodynamics a very basic thing to know is that when you transfer energy to a system, not the entire transfered energy is now part of the system. Quiet a lot of that transfered energy is used in the forces that resist to this energy transfer. In other words, some energy is "lost" (not actually lost, it's used), while we transfer it to the other system.

[MrMongoose]

The motor will act as a generator, converting some of its kinetic energy into electricity. In any case, the end result is mostly heat and a tiny bit of EM energy.

 

and in the simplest ideal case where there are no losses at all, then ALL the kinetic energy will be converted back to electrical energy.

[Nivvedan]

Now suppose I try to stop a moving ball. When I stop the ball, the ball loses its kinetic energy and I spend some of my energy. And it appears the energy is lost. Because it is not converted to any other form. Please Explain.

[swansont]

Now suppose I try to stop a moving ball. When I stop the ball, the ball loses its kinetic energy and I spend some of my energy. And it appears the energy is lost. Because it is not converted to any other form. Please Explain.

 

Yes, it is. You just aren't looking at all the channels. Heat, sound, deformation, pressure change (if it's an inflated ball), etc. All are processes by which an object can lose energy in a collision.

[Eric 5]

Lets suppose I have a fan attached to a motor in complete vacuum. The electrical energy supplied to the motor is converted into the kinetic energy of the fan. Now I replace the motor with another one which rotates in the opposite direction. I keep the switch on till the blades slow down and stop and switch it off before it begins to rotate in the opposite direction.

Lets now compare the initial and the final states. The kinetic energy of the fan is the same before and after the event. But I have spent some electrical energy. Has the enegy been lost?

If it is lost, then the law of conservation of energy does not hold. I know I am wrong but I don't know where. Because Physics SHOULD work.

Please help me.

 

 

 

If it is in a vacuum how do you suppose this experiment is going to work.

[MrMongoose]

If its in a vacuum how do you suppose this experiment is not going to work?

 

If it doesn't have a question mark how do you suppose a question can work?

[swansont]

People put fans in vacuums all the time. They're called turbomolecular pumps.

[carrotstien]

Kinetic energy is something which is actually relative. If I am moving along a straight road at 1m/s and i have a mass of 60kg, what is my kinetic energy? Could you say that it is 60kg * (1m/s)^2, what about that velocity, to some viewers I could be seen moving at .5c in one way or .6c the other way or not moving at all.

About the ball:

You are doing work to the ball by slowing it down, this work is equal to F dot X which ends up to be -FX. So you are doing negative work. If you count the kinetic energy of the ball before hand add (-FX) you will get 0 which explains why you see the ball stop.

 

The fan situation is similiar to if you were to take a ball and wiggle it back and forth. Each time you accelerate the ball, you apply a force over a distance so you are doing work. This work could be negative or positive depending on the direction, but that is just bookkeeping. The only reason you get tired is because your arm isn't 100% effecient. Imagine if your arm was, or if the ball was bouncing on a perfect spring with no air resistance attached to an unmovable object (lets say the earth, since the balls affect on the earth is negligable). The ball will bouce forever. So energy is not being lost at any time. It get converted between kinetic energy, potential energy of the spring and potential energy of gravity. The sum of all those energies is always equal to the energy put into the system to begin with.

 

This is easier to think about then the motor and the fan because the potential energy is different. There, even if you were to use superconducting wire, you would still need to do work to accelerate the fan and decelerate the fan due to the lorentz force and the eddie currents that arise. Because the work is negative when u slow it down and positive when u speed it up, it all adds up to 0.

I would imagine that if u did use perfect wires than the battery would not do any work to decelerate the fan, because the fan would send a voltage back through the battery causing the total voltage around the circuit to be 0. If you used a perfect rechargable battery, or even a very large capacitor instead, than you would have the same case as with the perfect spring and ball.