IB Physics SLTopic 4 — Electric & Magnetic FieldsPaper 1 & 2q = Ne~8 min read
Electric Charge
The spark off a doorknob, a balloon clinging to your hair, a whole lightning bolt — every one of them traces back to a single property of matter called charge. Get charge straight now and the rest of electricity, from Coulomb’s law to circuits, has something solid to stand on.
📘 What you need to know
Charge is the property of matter responsible for the electric force. Its unit is the coulomb (C), and it’s a scalar
There are two kinds: protons are positive, electrons are negative, neutrons are neutral. An object is neutral when its positives and negatives balance
Charge is quantised — it only comes in whole-number multiples of the elementary chargee = 1.60 × 10⁻¹⁹ C, so q = Ne
An object charges up only by gaining or losing electrons — the light, mobile ones. Positive charge means electrons were lost, never that “positives were added”
Opposite charges attract; like charges repel — the everyday “opposites attract”
Charge is conserved: in an isolated system the total charge stays constant — it can be transferred but never created or destroyed
When two identical conductors touch, they share the total charge equally, then keep their half after separating
What Is Charge?
Charge is the thing that makes the electric force happen — the deep reason two objects can pull on or push away from each other without touching. We measure it in coulombs (C). One coulomb is a big amount: it’s the charge delivered by a current of one amp flowing for one second. Unlike force or velocity, charge has no direction; it’s a scalar, just a signed number.
Everything is built from atoms, and atoms carry charge in two flavours:
In a normal atom the number of protons and electrons is equal, so the pluses and minuses cancel and the object as a whole is neutral. Disturb that balance and the object becomes charged.
Charge Comes in Lumps: Quantisation
Here’s a fact that surprises people: you can’t have just any amount of charge. Every proton carries exactly the same tiny positive charge, every electron exactly the same negative one, and that smallest unit is the elementary charge, e = 1.60 × 10⁻¹⁹ C. Any charged object is really a pile of these identical units, so its charge is always a whole number of them:
Charge is quantisedq = N × e
Where q is the total charge (C), N is a whole number of extra electrons or protons, and e = 1.60 × 10⁻¹⁹ C is the elementary charge (in your data booklet). We say charge is quantised — it comes in indivisible lumps, like being able to pay only in whole pennies, never half a penny.
object gains electrons
extra negatives
becomes negative
object loses electrons
now short of negatives
becomes positive
Notice what moves in both rows: electrons. Protons are locked in the nucleus. An object never gets positive by “collecting positive charge” — it gets positive by losing electrons. That distinction wins marks.
Attract or Repel?
Put two charges near each other and they feel a force. Which way depends only on their signs:
Opposite charges (one +, one −) attract — they’re pulled together
Like charges (both +, or both −) repel — they’re pushed apart
Like charges push apart, opposite charges pull together. The force arrows sit on each charge and point the way that charge is pushed. The rule to remember: opposites attract.
Charge Is Conserved
Charge follows a bookkeeping rule as strict as the one for energy — the law of conservation of charge:
The total charge in an isolated system stays constant.
Charge can be shuffled from one object to another, but it’s never conjured up or wiped out. A great example is two identical metal spheres: touch them together and their combined charge simply spreads out evenly, so each ends up with the average. Separate them and each keeps its half — the total is exactly what you started with.
Identical spheres share the total charge equally on contact. Nothing is created or lost: the +8 μC simply splits into +4 μC on each. That “share the average” trick is a favourite exam scenario.
Charge conservation isn’t just for spheres — it polices particle physics too. In beta decay a neutron (charge 0) turns into a proton (+1), and an electron (−1) plus an antineutrino (0) appear alongside it, so the books still balance at 0 on both sides.
🧭 Solving charge problems
Sharing between identical conductors? Add up all the charges, then divide by how many objects — each ends on that average
Check conservation — the total charge after must equal the total before; sign and all
Counting electrons? Use N = q ÷ e with e = 1.60 × 10⁻¹⁹ C — a huge N is normal
Mind the signs — gaining electrons makes charge more negative; losing them makes it more positive
Watch the prefixes — μC = ×10⁻⁶ C, nC = ×10⁻⁹ C; convert before dividing by e
Quick recap: charge is a scalar measured in coulombs, comes in ± lumps of e = 1.60 × 10⁻¹⁹ C so q = Ne, only electrons move, opposites attract and likes repel, and the total charge in an isolated system is always conserved.
WE 1
Three identical metal spheres carry charges of A = +9.0 μC, B = −3.0 μC and C = +6.0 μC. (a) Two of them are touched together and separated, leaving each of that pair at +7.5 μC. Which two spheres were they? (b) Instead, all three are touched together at once and then separated. Find the final charge on each.
Part (a) — identical spheres share the average
For an average of +7.5 μC, the two charges must total +15 μC
+9.0 + (+6.0) = +15 μC → average = +15 ÷ 2 = +7.5 μCspheres A and CPart (b) — average of all three(+9.0 − 3.0 + 6.0) ÷ 3 = +12 ÷ 3+4.0 μC on eachTotal before = +12 μC, total after = 3 × (+4.0) = +12 μC. Charge is conserved.
WE 2
A plastic rod is rubbed with a cloth and ends up with a charge of −6.4 μC. (a) State whether the rod gained or lost electrons. (b) Calculate how many electrons were transferred. (Take e = 1.60 × 10⁻¹⁹ C.)
Part (a) — sign tells the story
The rod is negative, so it must have gained electronsThe cloth lost those same electrons and is left equally positive.Part (b) — quantisation: N = q ÷ eN = (6.4 × 10⁻⁶) ÷ (1.60 × 10⁻¹⁹)N = 4.0 × 10¹³ electronsForty trillion electrons for a few microcoulombs — that’s how tiny e is.
💡 Top tips
Only electrons move. Explain any charging in terms of electrons gained or lost — never invent “moving positive charge”
Positive = electrons lost. If one object goes negative, another must go equally positive — charge is conserved as a pair
e lives in the data booklet. You don’t memorise 1.60 × 10⁻¹⁹ C, but you must know charge only comes in whole multiples of it
Identical conductors share equally on contact — take the average. (Different-sized conductors don’t split it evenly, so watch the wording)
⚠ Common mistakes
Saying an object “gains positive charge” — it becomes positive by losing electrons, since protons can’t move
Forgetting the signs when averaging shared charge (e.g. treating −3.0 μC as +3.0 μC)
Thinking charge can be any value — it’s quantised in lumps of e, so q is always a whole number of them
Dropping the prefix: using 6.4 instead of 6.4 × 10⁻⁶ C when counting electrons, giving a nonsense answer
Charge sorted — now we can measure the smallest lump of it directly. Up next: Millikan’s Oil Drop Experiment, the beautifully simple set-up that balanced tiny charged oil drops in mid-air to pin down the value of e.
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