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Tricky V I EMF Graph Need help please
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Tricky V I EMF Graph Need help please
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Manager123
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Tricky V I EMF Graph Need help please
«
on:
07/06/2013 09:34:15 »
Im really confused with this question, newbielink:https://www.dropbox.com/sh/v6ydqbfrg20whtb/ZWR-r9ZOSH?lst#f:q17.JPG
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the way i did it was like this newbielink:http://www.s-cool.co.uk/a-level/assets/learn_its/alevel/physics/Resistance/internal-resistance-emf-and-potential-difference/Finding%20the%20internal%20resistance.gif
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However the voltmeter is over the battery, so does this mean that its the opposite and a positive gradient
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evan_au
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Re: Tricky V I EMF Graph Need help please
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Reply #1 on:
08/06/2013 02:12:40 »
The answer just gives you one quadrant to graph the results, so it presumes that the voltmeter V and current meter A are connected to give positive readings.
The graph you give looks right:
When there is zero current, the voltage measured is ε Volts
When the variable resistor=0, the voltage will be zero, and the current will be at a maximum = ε/r Amps.
The equation of the line is V=ε-rI
But the gradient is "-r", not "r", giving a positive resistance
As a "sanity check", this looks right because, with realistic passive components:
Resistors are ≥ 0 Ohms
The battery output power is finite:
If you put too much load on it, the power output (VxA) drops to zero because V=0
If you don't put any load on it, the power output (VxA) is zero because I=0
There is some maximum output power in the middle, where the variable resistor is the same as r
The idea of having a circuit where the slope of the line is positive means that as you increase the load, the output voltage also increases
So if you reduced the load to zero, the battery output power would be infinite!
This is quite unlike ordinary batteries!
PS: This assumes "ideal" measuring instruments: A Voltmeter with infinite impedance, and an Ammeter with zero impedance.
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syhprum
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Re: Tricky V I EMF Graph Need help please
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Reply #2 on:
08/06/2013 11:11:54 »
Ohms law is of course a poor representation of how things behave in the real world if Manager123 measured the ratio of current to voltage in his lamps he would find that it varies by a factor of ten to one depending upon how much current is flowing.
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syhprum
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Re: Tricky V I EMF Graph Need help please
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08/06/2013 11:11:54 »
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