The Naked Scientists

The Naked Scientists Forum

Author Topic: Explain how the discrepancies vary with the current and why?  (Read 1406 times)

Offline bomblyn

  • First timers
  • *
  • Posts: 1
    • View Profile
My teachers keeps on telling me that internal resistance is constant and I don't understand what it means.
Please HELP!!!


 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Trust me, once you've been married ten years.... the meaning will become abundantly clear!
« Last Edit: 05/03/2014 00:05:44 by Aemilius »
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4101
  • Thanked: 245 times
    • View Profile
I assume that this was a science class dealing with electricity and batteries?

Perhaps you did an experiment where you took a 1.5V battery, connected it to a variable external resistance, and measured the current and/or voltage across the external resistance (sometimes called the load resistance)?

The question would then be to estimate the internal resistance of the battery.

There are a lot of variables in this experiment, but if you assume that the internal resistance is a constant resistance, then it becomes much easier to find an answer.

You can simplify the electrical behaviour of a battery to being an "ideal" voltage source in series with a constant resistor.
  • Measuring the voltage of the battery when there is no load tells you what the internal voltage is (because there is effectively no current flowing through the internal resistance).
  • Measuring the voltage of the battery when there is an external resistance allows you to estimate the internal resistance. The internal resistance forms a voltage divider with the external resistance.
The equation is here: http://en.wikipedia.org/wiki/Internal_resistance#Batteries

(If you took several measurements with different settings of the external resistance, you can plot them on a graph of current vs voltage, and try to draw a line through the points.)
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4101
  • Thanked: 245 times
    • View Profile
PS: When the question asks about "discrepancies", are they talking about deviations from the simple "ideal voltage source+series resistance" model, described above?

Some other factors that come into play are the battery running down as the experiment progresses, changes in the resistance and voltage as the battery heats up, and (if you test it with an external AC voltage source), the response time of the chemical reactions which power the battery.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums