I keep getting “Series and parallel resistors” questions wrong in Current Electricity. What's the trap?
adding parallel resistances directly instead of their reciprocals.
The usual trap: adding parallel resistances directly instead of their reciprocals. How to avoid it: parallel resistance is always less than the smallest branch. Go back to the definition — in series the current is the same and resistances add; in parallel the voltage is the same and 1/R values add.
Key point
Series and parallel resistors
Common mistake
adding parallel resistances directly instead of their reciprocals.
Memory tip
parallel resistance is always less than the smallest branch.
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