The Narrow Pipe Problem
🔌 The Narrow Pipe Problem
Ohm's Law states that the voltage across a component is directly proportional to the current through it, provided temperature stays constant: V = I × R (Voltage in volts = Current in amps × Resistance in ohms). Rearranged: R = V ÷ I (to find resistance) and I = V ÷ R (to find current). A component is ohmic if it obeys
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🧠 Memory Aid
VIR Triangle — "Very Important Relationship":
Write V at the top, I and R at the bottom. Cover the letter you want to find: V on top alone = I × R; I on bottom-left = V ÷ R; R on bottom-right = V ÷ I.
Non-ohmic components — "FDT Changes":
Filament lamp, Diode, Thermistor — these three all have changing resistance. Everything else (fixed resistors) is ohmic.
Thermistor vs LDR — "Hot is Less, Bright is Less":
Hotter thermistor = less resistance. Brighter LDR = less resistance. Both sensors work the same way — the thing they detect reduces their resistance.
Wire resistance factors — "LAMT":
Length (more = more R), Area (more = less R), Material, Temperature.
Now try it yourself
Quiz · Question 1 of 16
Which of the following best describes electrical resistance?
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This topic in real past papers
Every real exam question we've found on resistance & ohm's law, with a full worked answer.