Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bannister, Paul on 30/09/2011 21:01:02

Title: What happens to the electricity in a disconnected cable?
Post by: Bannister, Paul on 30/09/2011 21:01:02

Bannister, Paul  asked the Naked Scientists:
   
If I stop using a hedge trimmer and go and unplug it at the wall what happens to electricity in the lead (that I've paid for) and if it's a long extension lead is the cost of that lost electricity significant?

Thanks

Paul Bannister

What do you think?

Title: What happens to the electricity in a disconnected cable?
Post by: CliffordK on 30/09/2011 21:25:59

You can think of a conductor as a material that has the ability to have electrons displaced.
Voltage is the pressure to displace the electrons.
Current is the actual displacement of the electrons.

So, when the wire is unplugged, the "free electrons" would stay in the wire, but would be neutral, with no particular force to move them.

Think of it like the hose at a gas station pump.  The hose remains full.  The meter will be at one end of the hose, and a valve at the other end of the hose.  The system always calculates volume delivered with the assumption of a constant volume inside the hose.

In the case of the wire, the electrons stay with their atoms so they retain a neutral charge.

With electricity, most residential electricity is AC (Alternating current), so 50 or 60 times a second, the direction of the current is actually reversed.

Title: What happens to the electricity in a disconnected cable?
Post by: Johann Mahne on 03/10/2011 10:05:39

Quote

and if it's a long extension lead is the cost of that lost electricity significant?

Paul,Clifford has give the correct answers but I think your question was meant to be about power losses through long cables?
  This would not be an issue with a small hedge trimmer, but..
If you have a really long extension lead and you are cutting long grass with your mower, you could develop a voltage over the resistance of the wire.
  If your lead resistance is say 2 ohms ( which is not unsual for a really long lead) and you are drawing 15 amps then you would develop a power of i²r. This would then be 450 watts.
  Your mower would be quoted as a power of 3.3 kw, but you would lose the 450 watts as wasted power which you would have to pay for.
 

Title: What happens to the electricity in a disconnected cable?
Post by: Geezer on 04/10/2011 00:13:35

Johann,

I think the question was about the energy stored in the cable rather than the power disspated during operation. However, you are quite right about power loss in cables, except that, for the numbers you gave, the power loss in the cable is only 349W.

(I'm assuming the voltage is 220V)