If electrons in a copper wire drift at roughly 10⁻⁴ m/s, why does a light switch appear to work instantaneously?
AElectrons near the bulb travel faster to compensate for those farther away.
BThe electric field propagates through the wire at near light speed, setting all free electrons in motion nearly simultaneously.
CConventional current flows at light speed in the opposite direction.
DOnly a small number of fast electrons carry all the energy.
Current is not the speed of individual electrons — it is the rate of charge flow driven by an electric field. The field itself propagates at close to the speed of light, so all free electrons throughout the circuit begin drifting almost at once. The slow drift velocity only tells you how far each electron moves, not how quickly the signal reaches the bulb.
Question 2 True / False
Doubling the cross-sectional area of a wire (keeping length and material constant) will double the wire's resistance.
TTrue
FFalse
Answer: False
Resistance is R = ρL/A. Doubling the cross-sectional area A divides the resistance by two, not doubles it. A wider wire provides more parallel paths for charge flow, reducing opposition. This is directly analogous to widening a pipe to reduce flow resistance.
Question 3 Short Answer
What is the distinction between resistivity and resistance, and why does it matter in practice?
Think about your answer, then reveal below.
Model answer: Resistivity (ρ) is an intrinsic material property — it depends only on the substance and its temperature, not the shape of the conductor. Resistance (R = ρL/A) depends on both the material and the geometry. The distinction matters because you choose a material based on its resistivity, then engineer the geometry (length and area) to achieve the desired resistance for a specific application.
Mixing up the two leads to errors like claiming 'copper has low resistance' — copper has low resistivity, but a kilometer of thin copper wire can still have substantial resistance. Resistivity is the fundamental quantity; resistance is derived from it.