Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Manager123 on 07/06/2013 09:34:15
-
Im really confused with this question, https://www.dropbox.com/sh/v6ydqbfrg20whtb/ZWR-r9ZOSH?lst#f:q17.JPG
the way i did it was like this 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
However the voltmeter is over the battery, so does this mean that its the opposite and a positive gradient ???
-
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.
-
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.