Welcome to blog help for PHY 122! Here's how it works.
For each homework question and lab report we will make a post, this will probably contain a few tips on what the problems are about and how to solve them. If you are stuck on something then instead of emailing us directly you should post a comment in reply to the relevant post. We will try to guide you through tough points and help you understand the problems and the concepts behind them.
Sunday, September 5, 2010
Chapter 16, Questions 1, 2
Remember that this homework assignment is due on Wednesday, September 15th at 5:00 PM. This thread is to discuss the first two questions, which come from SV8 section 16.1
2 comments:
Muntazim
said...
This question is relatively simple (I got the correct answer). However, I have a conceptual question regarding this problem: why is the work done by the battery charger positive? Based on my reasoning, I would say that the potential energy of the charge is increasing (delta PE is positive) as it is moved from the negative to the positive terminal. After gaining potential energy, the charge spontaneously moves in the circuit, converting its PE to KE and going from high to low PE.
Since delta PE is positive, the work done by the charger should be negative, since W= - delta PE.
I may be thinking of the situation incorrectly/be missing something.
The battery charger moves charge "uphill" where the energy is stored in the battery. If the battery is connected to a load, charge flows out of its + terminal, loses its potential energy doing whatever work it does, and then returns to the -- terminal. The battery charger pushes charge the opposite direction - taking it out of the - terminal, increasing its potential energy enough to get it to flow into the + terminal. To look at it a different way - it is the battery's delta PE that is positive, and something had to do the work to increase that PE, namely the charger. Does that clarify?
2 comments:
This question is relatively simple (I got the correct answer). However, I have a conceptual question regarding this problem: why is the work done by the battery charger positive? Based on my reasoning, I would say that the potential energy of the charge is increasing (delta PE is positive) as it is moved from the negative to the positive terminal. After gaining potential energy, the charge spontaneously moves in the circuit, converting its PE to KE and going from high to low PE.
Since delta PE is positive, the work done by the charger should be negative, since W= - delta PE.
I may be thinking of the situation incorrectly/be missing something.
The battery charger moves charge "uphill" where the energy is stored in the battery. If the battery is connected to a load, charge flows out of its + terminal, loses its potential energy doing whatever work it does, and then returns to the -- terminal. The battery charger pushes charge the opposite direction - taking it out of the - terminal, increasing its potential energy enough to get it to flow into the + terminal.
To look at it a different way - it is the battery's delta PE that is positive, and something had to do the work to increase that PE, namely the charger.
Does that clarify?
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