Define electric charge, electron, proton, and the coulomb: Question bank

This page hunts the misconceptions that the parent note Define electric charge, electron, proton, and the coulomb deliberately sets up: the direction of current versus the particle that moves, the size of a coulomb, and the quantised nature of charge.

True or false — justify

Two labels appear a lot below, so pin them down first:

True or false — and say why.

An object with zero net charge contains no electric charge.

False. It contains equal amounts of positive and negative charge that cancel in the sum; a copper penny holds ~

1 0^{24}

electrons and just as many protons, netting zero.

A proton and an electron have exactly opposite charges.

True. Same magnitude

e

, opposite sign:

+ e

and

- e

. This exact equality is why neutral atoms exist.

The charge

- e

is smaller than the charge

+ e

because of the minus sign.

False. The minus is a direction on the charge number line, not a size. Both have magnitude

e

;

∣ - e ∣ = ∣ + e ∣

Charge is a made-of-something substance we could crush to see its parts.

False. Charge is a primitive property of matter (like mass), not a material. We describe what it does, never what it is built from.

One coulomb is a tiny amount of charge.

False. One coulomb is

6.242 \times 1 0^{18}

elementary charges — enormous. It only feels small in circuits because it flows fast (1 amp = 1 C per second).

You can isolate a free charge of

0.5 e

False. Free charge is quantised — only whole-number multiples of

e

exist as free particles. Half an electron's charge has never been observed free.

In a metal wire, protons drift to carry the current.

False. Protons are locked in nuclei. The mobile carriers are the loosely held outer electrons.

Removing one electron from a neutral atom gives it a net charge of exactly

+ e

True. The protons'

+ e

that the missing electron used to cancel is now unbalanced, leaving net

+ 1.602 \times 1 0^{- 19}

Adding an electron to a neutral atom makes it a positive ion.

False. Adding a negative charge makes it a negative ion (

- e

). Losing an electron makes it positive.

Like charges attract and opposites repel.

False. It is the reverse: like charges repel, opposites attract. Two electrons push apart; an electron and a proton pull together.

Spot the error

Each statement hides one wrong idea. Name it and fix it.

"Conventional current shows positive charges physically moving from + to − in the wire."

The error is treating a drawing convention as a physical claim. In metals the actual movers are electrons going − to +; conventional current is just the agreed direction label, chosen before electrons were known.

"Since

Q = N e

, a charge of

3.2 \times 1 0^{- 19}

C means

N \approx 2

, so

2.0

electrons — but

4.8 \times 1 0^{- 19}

C would be

3.0

, so any decimal is fine."

The error is concluding "any decimal is fine."

N

must be a whole number;

3.2 \times 1 0^{- 19} / e = 2

and

4.8 \times 1 0^{- 19} / e = 3

both land on integers precisely because charge is quantised. A value like

2.4 e

cannot occur for free charge.

"The coulomb is defined by picking a random bucket size, so

6.242 \times 1 0^{18}

is a coincidence."

The error is calling it a coincidence.

6.242 \times 1 0^{18} = 1/ e

exactly — it is the definition of the coulomb read backwards, not an accident.

"An atom that loses an electron gains a proton to become positive."

The error is inventing a new proton. The proton count never changes here; the atom becomes positive because it lost a negative electron, leaving the existing protons uncancelled.

"Electrons carry

+ e

and protons carry

- e

Signs are swapped. Electrons carry

- e

(negative), protons carry

+ e

(positive).

"1 coulomb per second is a strange made-up rate; current isn't related to charge."

The error is denying the link. Current is charge-per-time: 1 ampere = 1 coulomb every second — see Electric Current and the Ampere.

Why questions

Why did nature need only two labels, + and −, and not three or more?

Experiments show only two force behaviours — some pairs push apart, some pull together. One number line with two directions (+ and −) captures every observed case, so no third label was ever required.

Why do we count charge in "buckets" (coulombs) instead of in single electrons?

One electron's charge,

1.602 \times 1 0^{- 19}

C, is absurdly tiny. Everyday charges involve billions of billions of them, so a large unit (the coulomb,

\approx 6.24 \times 1 0^{18}

electrons) keeps the numbers manageable.

Why is bulk matter — a table, a wire — usually neutral?

Each atom has equal protons and electrons, so every atom's charge cancels to zero, and the whole object's net charge sums to zero unless charges are deliberately added or removed.

Why do electrons, not protons, move to make current in a wire?

Protons sit locked inside heavy nuclei; the outer electrons in metals are loosely held and free to drift under a push (voltage) — see Voltage and Potential Difference and Conductors and Insulators.

Why is "charge is additive" the fact that turns counting into multiplication?

Because putting

N

identical

e

-sized charges together gives a total of

e + e + \dots = N e

. Repeated addition of the same amount is multiplication, which is why

Q = N e

Why is the sign of a charge attached separately from its magnitude

e

Magnitude answers "how much" (always

e

for one particle); sign answers "which of the two kinds." They are independent questions, so we store them as two facts: size

e

and direction

\pm

Why does calling charge "quantised" matter for hardware?

It guarantees charge always comes in exact whole pieces, so bits, currents, and stored charge are countable and reproducible — no fuzzy fractional carriers to worry about.

Edge cases

What is the net charge of a completely neutral atom, and does that mean it has no charges?

Net charge is exactly

0

, but it holds equal positive and negative charge that cancel — plenty of charge, zero sum.

What is

N

(number of elementary charges) for a net charge of exactly

0

N = 0

in the net sense, but the object may still contain enormous equal counts of

+ e

and

- e

Q = N e

uses the net

N

Can

Q = N e

ever give a non-integer

N

for real free charge?

No. If your arithmetic yields a fractional

N

, either the charge value is wrong or you are not dealing with free elementary charges — quantisation forbids it.

What happens to

Q = N e

if you double the number of electrons?

Q

doubles too — the relation is strictly proportional, so twice the count means twice the total charge.

Is there a smallest possible non-zero free charge?

Yes: one elementary charge,

e = 1.602 \times 1 0^{- 19}

C. Nothing smaller exists as a free, isolated charge.

Two objects each with net charge zero — can they still exert electric forces on each other?

Yes, weakly. Even with zero net charge, their internal

+

and

-

can shift and align, producing small attractions — but with no net charge there is no simple large repulsion or attraction like Coulomb's Law gives for charged bodies.

An ion has net charge

+ 2 e

. How many electrons did the originally neutral atom lose?

Two electrons. Each lost electron leaves one uncancelled proton, so

+ 2 e

means two missing electrons — see Atomic Structure.

If a wire carries 1 C of charge past a point in 1 second, is that "a lot" or "a little" charge?

It is a lot of charge (

6.24 \times 1 0^{18}

electrons), delivered quickly. The largeness hides because it passes in just one second (1 ampere).

Connections

Electric Current and the Ampere — where "charge per second" turns these counts into current.
Voltage and Potential Difference — the push that makes the free electrons drift.
Conductors and Insulators — whether electrons are free to move at all.
Coulomb's Law — the force between charged objects.
Atomic Structure — where the protons and electrons live.