Read RL Circuits: Read Time Constant from the Curve
An RL current rises as I = E/R times [1 - exp(-tR/L)]. The final current sets E/R; the curve's time scale sets L/R.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Start with the condition
Compare the initial slope E/L, asymptote E/R and time constant L/R of both curves before deciding which parameter changed.
Build the governing relation
Changing more than one parameter can preserve one visual feature while changing another, so use all three curve features.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Reading only the final current cannot distinguish a proportional change in E and R from no change at all.
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