Discussion in 'Homework Help' started by farhanshahid2009, Apr 10, 2010.

1. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
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i cannot get how energy is lost by electron in a circuit. e.g. if you connect both terminals of battery with connecting wire (short circuit) all energy of electron is lost in the connecting wire. but if in same circuit you add a resistor almost all energy is lost in resistor and very little in wires.but theoretically same amount of energy should be lost in wire as in previous case because it is offering same resistance.

OR

when you add another resistor(say B) to a circuit which already contains a resistor(say A) the energy loss by an electron through A decreases even though it is offering same resistance as before.

2. ### retched AAC Fanatic!

Dec 5, 2009
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There is a voltage drop in the wire after the resistor. Therefore there is less current flowing through the wire than there was before the resistor.

3. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
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Sorry Retched i dont get it. some more detail please

4. ### retched AAC Fanatic!

Dec 5, 2009
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There is actually two things.. The energy is 'lost' through heat in the resistor. Since energy cannot be made or lost, just converted, the resistor converts some of the electrons to heat so they are not available on the other side of the wire.

If you have a hose (wire) and kink the hose in the middle (resistor) the amount of water that comes out of the end is much less then without the kink.

Ok as for the change in resistance with voltage..

The resistance of a material changes with the amount of current flowing through it. So, when you added the resistor to the wire connecting the battery terminals, you slowed the current, dropping the resistance of the wire in comparison.

5. ### Ghar Active Member

Mar 8, 2010
655
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This sentence doesn't sit with me at all.
Resistance changing with current is not part of the basic mechanism, nor do I think the word dropping is appropriate.

A resistor offers significantly more resistance than a wire.
With just wire, you have small resistance and very rapid energy loss.
A wire might be 0.01 ohms.
A resistor is commonly 1000 ohms or more.
Because of that difference you can ignore the wire.
With the resistor, energy loss is much slower than before and the energy loss is split between the wire and resistor based on the resistance value. Since the resistor is many orders of magnitude larger it will take many orders of magnitude more energy than the wire.

6. ### retched AAC Fanatic!

Dec 5, 2009
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In the case of wanting to know why the resistance of the wire drops:
You have to also take into consideration the change in voltage and current in the wire when the resistor is added.

As for using the word "dropping"

I thought it better than the word "ignore" as in:
The reason the resistance in the wire is NOT the same before and after the resistor is the voltage and current change due to the resistor. The resistance is dropping, as in LESS than before.

7. ### Ghar Active Member

Mar 8, 2010
655
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I really don't know what you're describing.
Just what resistance is changing and why?

8. ### retched AAC Fanatic!

Dec 5, 2009
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I was trying to answer the OPs question.

He wanted to know why the wires resistance did not appear to be the same after adding the resistor.

Look at part two of the first post.

He seems confused on the WHY part of the changes. I thought I was offering some insight on the topic.

9. ### Ghar Active Member

Mar 8, 2010
655
72
As far as I can tell he's confused about the change in energy loss with constant resistance, and I just don't see where resistance would be changing. We are adding resistances but they don't change.

10. ### retched AAC Fanatic!

Dec 5, 2009
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I think where I went haywire was the suggestion of loosing electrons via heat.

If you are measuring electron loss as heat in a one wire device, hooked to a battery and (pulling this out of the air) the wire is 25degC then you add a resistor and the wire drops to 22degC and the resistor is 44degC. The thermal loss increases in the resistor while decreasing in the wire. This APPEARS to be less resistance in the wire after the resistor is attached.

Why this happens is because of the change in current flow in the components. The short circuit described in the first post would cause the wire to heat considerably, causing the resistance to increase in the wire. By adding the resistor, you are slowing the electron flow through the wire, reducing resistance in the wire while increasing resistance in the circuit.

Did I forget to stop drinking last night or something?

If I am way off here stop me before I really make a fool out of myself.

11. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
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let me put the Q another way :
say a circuit has a resistor of 50 ohm. now all the voltage (say 20V) is lost in the resistor.
voltage = energy per columb of electrons . so all the energy a columb of electrons possess is lost in the resistor. Now if i replace the resistor with a 25 ohm resistor 20v is still lost in the resistor. but what should happen is that less energy (or voltage) should be lost as the resistor is providing less resistance or hinderence.

12. ### retched AAC Fanatic!

Dec 5, 2009
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It will all be lost, but It will take longer.

13. ### Ghar Active Member

Mar 8, 2010
655
72
Now we agree, with 'appears' being the key word.

Well it is past 1am now... don't need much drinking for the same effect

Edit:

The 25 ohm will lose energy faster than the 50 ohm.
The rate of columbs going through the resistor will be doubled because of the lower resistance.
Voltage remains the same.

14. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
6
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what ive learned at Alevels is that each electron has to lose all its energy (that has been provided by battery) before it enters the positive terminal. Why is this independent of the resistance in the circuit and is it really correct in the first place.

15. ### retched AAC Fanatic!

Dec 5, 2009
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Using the water analogy, If you have a 5 gallon bucket of water and you pour it through a 1 inch diameter hose, it may take two minutes to empty the bucket.. If you pour it through a 2 inch diameter hose, it will only take one minute to empty the bucket.

Now the hose diameter is the resistance. The energy doesn't come out of the battery at the same speed always. If you have a large resistor, it comes out of the battery slowly. if you have little resistance (big hose) it comes out quicker.

16. ### Ghar Active Member

Mar 8, 2010
655
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Voltage is not energy and individual electrons do not carry energy.
The energy is in the entire system of electrons in all the wires and the electromagnetic fields.

The rule is that going around the loop the sum of the voltage rises must equal the sum of the voltage drops. Voltage is a property of the system, not any particular electron.
Electrons in the wire are all at the same speed, they don't "lose" anything in the resistor.
The energy transfer happens through the system of electrons and electromagnetic fields.

17. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
6
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but isnt voltage joule per columb
edit: by the way i get the time thing its just the voltage issue

18. ### retched AAC Fanatic!

Dec 5, 2009
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It sure isn't easy to be eloquent all the time. Listen to that Ghar fella, he knows whats hes talking about.

19. ### Ghar Active Member

Mar 8, 2010
655
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It is.

It's how many joules are required to move, or how many joules you'll gain from moving, a coulomb of charge through the path where you defined your voltage.
The electron doesn't have a voltage, the system does.

An electron sitting still at 0 V and an electron sitting at 5 V are both doing the same thing, there's no difference.
What will happen though, is that electron at 0 V will be attracted towards the spot at 5 V.

With voltage you can define your zero wherever you want, so you can make it whatever you want.
With 5 V and 10 V and you still have the exact same situation, just as if you had 1000 V and 1005 V.

I should correct one thing... individual electrons have some energy, but the vast majority is in the combined system. It's the interactions that make things happen.

20. ### farhanshahid2009 Thread Starter New Member

Apr 10, 2010
6
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What kind of interactions. i thought voltage was lost due to collisions of electrons with atoms of coducting path.