Unit 5: Current, Resistance, and Conduction
What actually moves inside a wire, how slowly it moves, and how a material's own atomic jostling turns into the number you call resistance.
7 topics · 25 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Electric Current
- How it worksHow does electric current get defined as the rate at which charge flows past a point in a wire?
- Common mistakeWhy doesn't current get used up as it travels around a circuit and back into the battery it came from?
- Worked exampleFind the current in a wire that carries 6 coulombs of charge past a point every 2 seconds.
- Going furtherWhat happens to a loop's current if the number of charge carriers flowing past each second is doubled?
- How it worksHow does conventional current direction relate to the actual direction electrons drift inside a metal wire?
Your turn1 of 2
Find the current in a wire that carries 35 coulombs of charge past a point every 13 seconds.Drift Velocity and the Microscopic Model of Conduction
- How it worksHow does the tiny drift velocity of electrons combine with their huge number to produce a real current?
- Worked exampleFind the drift velocity in a copper wire of area 2 times 10 to the -6 square meters carrying 3 amps, with 8.5 times 10 to the 28 free electrons per cubic meter.
- Common mistakeWhy isn't a light switch's near-instant response proof that individual electrons themselves move that fast?
- Going furtherWhat happens to the drift velocity in a wire if its cross-sectional area is cut in half at the same current?
- Ties togetherHow does a wire's drift velocity formula connect back to the plain definition of current as charge per time?
Your turn1 of 2
Find the drift velocity in a copper wire of cross-sectional area 7 times 10−6 square meters carrying 13 amps, given a free-electron density of 9 times 1028 per cubic meter. Use e = 1.602x10−19 C.Resistivity and Resistance
- How it worksHow do a material's resistivity, a wire's length, and its cross-sectional area combine into its resistance?
- Worked exampleFind the resistance of a 2 meter copper wire of area 1 times 10 to the -6 square meters and resistivity 1.68 times 10 to the -8 ohm-meters.
- Common mistakeWhy isn't a material's resistivity the exact same thing as the resistance of a particular wire made from it?
Your turn1 of 2
Find the resistance of a 14 meter copper wire of cross-sectional area 6 times 10−6 square meters and resistivity 1.68 times 10−8 ohm meters.Ohm's Law
- How it worksHow does Ohm's law relate the voltage, current, and resistance of an ordinary ohmic material?
- Worked exampleFind the current through a 220 ohm resistor connected across a 9 volt battery.
- Common mistakeWhy doesn't every material in a circuit obey Ohm's law the same clean way an ordinary resistor does?
Your turn1 of 2
Find the current through a 351 ohm resistor connected across a 17 volt battery.How Resistivity Changes with Temperature
- How it worksHow does heating a metal make its atoms jostle enough to scatter drifting electrons more often?
- Worked exampleFind the resistance at 80 degrees C of a wire with 10 ohms at 20 degrees C and a temperature coefficient of 0.004 per degree C.
- Going furtherWhat happens to a metal's resistivity as its temperature keeps dropping toward absolute zero?
Your turn1 of 2
Find the resistance at 114 degrees C of a wire with 34 ohms at 20 degrees C and a temperature coefficient of 8 times 0.001 per degree C.Electrical Energy and Power
- How it worksHow do voltage and current combine to give the rate a circuit element converts electrical to other energy?
- Worked exampleFind the power dissipated by a 60 ohm resistor carrying a current of 0.5 amps.
- Ties togetherHow does the power dissipated in a resistor connect back to drifting electrons colliding with jostling atoms?
Your turn1 of 2
Find the power dissipated by a 71 ohm resistor carrying a current of 7 amps.Superconductors
- How it worksHow does cooling certain materials below a critical temperature make their resistance drop to exactly zero?
- Common mistakeWhy isn't a superconductor just an unusually good ordinary conductor with a very low resistance?
- Going furtherWhat happens to a superconducting material's behavior the moment its temperature rises back above critical?
Your turn1 of 2
A material has extremely low but nonzero resistance. Is this the same thing as being a superconductor?