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Physics G482 question 3cii- january 2012 ocr

Im wondering how a cell's chemical energy can just suddenly decrease. Doesn't it get constantly used up over the hours to zero?
Original post by runny4
Im wondering how a cell's chemical energy can just suddenly decrease. Doesn't it get constantly used up over the hours to zero?


Cells aren't perfect power sources, so while the graph shows a constant output for 9 hours (or w/e), when the cell reaches the end of its life the power diminishes, but not all at once. The perfect (ideal) cell would have constant power output for its whole life and then fail completely at the end of its life.
Reply 2
Original post by Duskstar
Cells aren't perfect power sources, so while the graph shows a constant output for 9 hours (or w/e), when the cell reaches the end of its life the power diminishes, but not all at once. The perfect (ideal) cell would have constant power output for its whole life and then fail completely at the end of its life.


yeah but im saying doesn't the chemical energy from the cell be constantly conveted into electrical so the cell's emf decreases as this energy is transferred instead of dropping down at the end?
Original post by runny4
yeah but im saying doesn't the chemical energy from the cell be constantly conveted into electrical so the cell's emf decreases as this energy is transferred instead of dropping down at the end?


The graph is current supplied by the cell against time (emf against time would be similar). The cell tries to keep the emf constant but fails towards the end of its life, hence the graph. What does constantly decrease is the chemical potential energy of the cell, but that has nothing to do with the emf, and isn't on the graph.
Reply 4
Original post by Duskstar
The graph is current supplied by the cell against time (emf against time would be similar). The cell tries to keep the emf constant but fails towards the end of its life, hence the graph. What does constantly decrease is the chemical potential energy of the cell, but that has nothing to do with the emf, and isn't on the graph.


what does chemical potential energy have to do with then?
Original post by runny4
what does chemical potential energy have to do with then?


That's just how much energy the battery has in Joules.
Reply 6
Original post by Duskstar
That's just how much energy the battery has in Joules.

well v=w/q so if w is chemical energy then it does have something to do with emf
Original post by runny4
well v=w/q so if w is chemical energy then it does have something to do with emf


Not really. The battery has a value that is it's chemical potential energy, and that's just a number that represents how many Joules of energy the battery has stored in a chemical form.

V = W/Q means voltage = work done per unit charge, but work done per unit charge isn't equal to the chemical potential energy of the cell per unit charge, else the cell would drain after providing all it's energy to one coulomb. For a 100% efficient cell, you could work out the the chemical potential energy stored in it by draining it through a resistor, where E = Pt and P=VI, but notice how W therefore is definitely not equal to E.

I really can't think of a better way to explain it. The chemical potential energy is simply the energy that the cell can supply in total - the amount it has 'stored' in it, and for that equation W is the work done per coulomb, which has nothing to do with the chemical potential energy. It would be a certain fraction of it, but that would be different for every battery, also.

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