1.1.4 · D5Electricity & Charge Basics
Question bank — Define current (flow of charge) and the ampere
True or false — justify
A current of 2 A means 2 coulombs pass a point every second.
True — one ampere is defined as one coulomb per second (see Electric charge and the coulomb), so 2 A is exactly 2 C/s.
Doubling the drift speed of the electrons always doubles the current.
True if , and stay fixed, because is linear in — but in a real wire raising the field also barely changes these, so the honest answer is "yes, all else equal."
A thicker wire carrying the same current has faster-moving electrons.
False — from , a larger area at fixed means a smaller ; the wider road lets the same flow crawl more slowly.
Current is a vector because it has a direction along the wire.
False — current has a direction but does not add like a vector; it is a scalar that obeys the junction rule (what flows in flows out), so it is called a scalar despite having a sense of flow.
The ampere is a derived unit built from the coulomb.
False — it is the other way around: the ampere is the SI base quantity and the coulomb is defined from it as .
Zero net current means no charges are moving inside the wire.
False — electrons are always jostling thermally at high speed; zero net current just means no net drift in one direction, so charge crossing left equals charge crossing right.
If current is steady, the instantaneous current equals the average .
True — for a constant flow the slope of the charge–time line is the same everywhere and equals total charge over total time.
A wire with more current is always hotter to the touch.
False — heating depends on current and resistance (, see Ohm's Law and resistance); a thick superconducting-like path can carry huge current with little heat.
Spot the error
"Since electrons carry the charge, conventional current must flow the same way electrons drift."
The error: conventional current points the way positive charge would move, which is opposite to electron drift; the convention was fixed before electrons were known.
"The bulb lights instantly, so electrons must travel from switch to bulb at nearly light speed."
The error: electrons drift at fractions of a mm/s; it is the electric field / signal that propagates near light speed, nudging the whole column of electrons at once (see Drift velocity).
" gives the charge for any current, so I'll use it here."
The error: holds only for constant current; for a changing current you must use , the area under the – graph.
"An insulator has no charges, so no current can ever flow through it."
The error: insulators have charges, but they are bound and cannot drift freely (see Conductors and insulators); given enough voltage they can even break down and conduct.
"Current gets 'used up' as it passes through a bulb, so less comes out than goes in."
The error: charge is conserved — the current leaving a single component equals the current entering it; energy is used up (converted to heat/light), not charge.
"Because , using as a negative number for electrons makes the current negative."
The error: in the magnitude formula is the size of a carrier's charge; the sign only sets the direction, and conventional current is still counted as positive in the direction positive charge would move.
"1 A = 1 C means the ampere and coulomb are the same unit."
The error: , not ; an ampere is a coulomb per second, so it carries a hidden division by time.
Why questions
Why divide charge by time instead of, say, multiplying them, to measure "flow strength"?
Because flow means "how much per unit time" — the same charge spread over longer time is a weaker flow, so time belongs in the denominator.
Why is drift speed so tiny yet a device responds instantly?
The pipe is already full of charges everywhere; pushing at one end shoves the whole column immediately via the field, like water spurting from a tap the instant you turn it even if any one molecule barely moves.
Why did physicists keep conventional current even after finding electrons are the real movers?
All circuit theory, formulas and instruments were already built around the positive-flow convention, so flipping it would rewrite everything for no physical gain; both pictures describe identical physics.
Why is current chosen as an SI base unit rather than charge?
Current can be measured very precisely via the force between wires (a mechanical, reproducible experiment), making it the more practical fundamental standard from which charge is derived.
Why does the same current give a bigger drift speed in a copper wire than in a thick copper bus-bar?
Because — with and fixed for copper, the thinner wire's smaller forces the electrons to drift faster to deliver the same .
Why can two different wires carry the same current with wildly different numbers of electrons moving?
Because current is a product — a wire with more carriers per volume () or wider area () can match another's current at a lower drift speed, so counts differ while agrees.
Why does voltage matter for current if current is just charge per second?
Voltage (see Voltage (potential difference)) is the push that makes charge drift; without a potential difference the electrons jostle randomly with zero net flow, so no current.
Edge cases
What is the current if charge crosses left-to-right at exactly the same rate as right-to-left?
Zero net current — the two flows cancel, even though charges are moving; current counts the net rate across the surface.
What happens to at the instant a switch is first closed and is still building up?
The current rises from near zero as the drift speed grows; the formula still holds at every instant, giving a small while is small before it settles to steady state.
If is flat (constant charge, not increasing) over an interval, what is the current there?
Zero — current is the slope , and a flat charge–time graph has slope zero, meaning no charge is crossing during that interval.
For an alternating current that reverses direction, what does "1 A" describe over a full cycle?
The net charge over a full cycle can be zero, so "1 A" for AC refers to an effective (RMS) value, not a simple — the instantaneous swings positive and negative.
Can current exist with essentially zero drift speed?
Only trivially — if then ; any nonzero net current requires some net drift, however tiny.
In a wire of zero cross-sectional area, what does predict?
gives — a wire of no width carries no charge, which matches physical sense: there is no channel for carriers to cross.
What is the current in a perfect insulator with a voltage across it (before breakdown)?
Essentially zero — the bound charges cannot drift, so no net charge crosses per second despite the push (contrast with a conductor in Conductors and insulators).
Recall One-line summary of every trap
Current is net charge per second (), the ampere is its base unit (charge is derived from it), electrons drift slowly opposite to conventional current, needs steady current, and is a product — so any of can change it.