Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Hannah-Louise on 26/02/2009 10:30:03

Title: Why can't energy be destroyed?
Post by: Hannah-Louise on 26/02/2009 10:30:03

Hannah-Louise asked the Naked Scientists:
   
I'm doing my GCSEs right now and I listen to your science audio on the bbc bitesize website (http://www.bbc.co.uk/schools/gcsebitesize/audio/science/).

I have been learning about physics and i got confused and my teacher wont explain why and I was wondering if you could help me, here is my question: Why can energy be lost and transferred but it can't be destroyed or created?

What do you think?

Title: Re: Why can't energy be destroyed?
Post by: lancenti on 26/02/2009 13:24:51

I honestly don't know the answer, but I suppose you could think of it the same way as matter - you can break it up into little bits and pieces but you can't make it cease to exist. Of course, this is a terrible analogy especially since matter can be converted into energy and vice versa but I think it's one way of understanding.

I suppose it's like the saying "You can't get something from nothing" and it's opposite, "You can't get nothing from something".

Title: Re: Why can't energy be destroyed?
Post by: guest10038 on 26/02/2009 14:25:23

I  think  if  we  want  to  destroy   energy,for this  purpose,we  have  to  apply  force,which  is  also  energy.When  we  give  energy  to  energy  how  it  can  be  destroyed?(I  think  so)

Title: Re: Why can't energy be destroyed?
Post by: lightarrow on 26/02/2009 14:58:04

Quote from: Hannah-Louise on 26/02/2009 10:30:03

Hannah-Louise asked the Naked Scientists:
I have been learning about physics and i got confused and my teacher wont explain why and I was wondering if you could help me, here is my question: Why can energy be lost and transferred but it can't be destroyed or created?

In mechanics energy conservation comes from the homogeneity of time: given the postulate that every instant of time is equivalent, then energy conservation is proved mathematically: you write energy H as: H = Σ_ip_iq_i' - L(q_i,p_i,t) then you evaluate dH/dt using the Hamilton's equations:

q_i' = ∂H/∂p_i
p_i' = -∂H/∂q_i

(the ' means time derivative)

and the fact that, if time is homogeneous, then ∂L/∂t is = 0; the result is zero, so energy is conserved, so it cannot be destroyed or created.

Title: Re: Why can't energy be destroyed?
Post by: JP on 26/02/2009 18:43:48

To expand upon what Lightarrow said, in there's a general rule (Noether's theorem) that if physics is the same at every point (in some variable), that some quantity is conserved.  Energy conservation comes from the fact that physics is the same at every point in time.  Momentum conservation comes from the fact that physics is the same at every point in space. 

http://en.wikipedia.org/wiki/Noether's_theorem