IB Physics HLTopic 4 — Force FieldsPaper 1 & 2Friction · Induction · Contact~16 min read
Static Charge
Millikan’s drops arrived at the plates already charged, straight out of the nozzle. Nobody put charge on them — friction did. That same quiet effect makes a balloon cling to a wall, makes your hair stand up, snaps at your fingers on a car door, and, if you are careless with an aeroplane fuel line, sets the whole thing on fire. There are exactly three ways to charge an object, and in every single one of them, the only thing that actually moves is an electron.
📘 What you need to know
Charge can be transferred three ways: by friction, by electrostatic induction, and by contact
Friction: rub two insulators and electrons move from one to the other. One ends up positive, the other equally negative
Earthing (grounding) connects a body electrically to the Earth. A charged body that is earthed discharges until it is at 0 V
Induction: a nearby charged object separates the charge in a material without touching it
An object charged by induction ends up with the opposite sign to the rod
An object charged by contact ends up with the same sign as the rod
A very large p.d. makes the field strong enough to cause the breakdown of air, and a spark jumps
Sparks are dangerous: electrocution and ignition of flammable gases. A bonding line to Earth removes the risk
Objects become positive by losing electrons, never by “gaining positive charge”. Protons do not move
Three ways to charge something
They differ in one respect only: whether the two objects touch, and what that does to the final sign.
Method
Do they touch?
Final charge on the object
Friction
Yes — rubbed together
Opposite to the other material
Induction
No — never
Opposite to the charging rod
Contact
Yes — brought together
Same as the charging rod
Learn that last column and half of this topic is done. Induction gives you the opposite sign, contact gives you the same sign. If you can say why — and you will be able to, in about four minutes — you can answer almost any static electricity question they throw at you.
Charging by friction
Rub two insulators together and electrons are dragged from the surface of one onto the surface of the other. Which way they go depends on the two materials, not on you.
Take the classic: a plastic rod and a dry cloth. Rub them together and electrons are transferred from the rod to the cloth.
The rod loses electrons and is left positive. The cloth gains exactly those electrons and is left equally negative. Add the two and you get zero, just as you started.
Say this out loud until it is boring: the rod became positive because it lost electrons. Not because it gained protons. Not because positive charge flowed onto it. Protons are locked in the nucleus and cannot go anywhere. The examiner is specifically hunting for the word electrons in your answer.
And this is not just a solids-and-schoolrooms effect. Any two substances rubbing past each other will do it — including a liquid flowing through a pipe, which is exactly where the aeroplane story below begins.
Earthing
To earth (or ground) a body is to connect it electrically to the Earth. The Earth is so vast that it can absorb or supply any number of electrons without noticeably changing its own charge — it is a bottomless reservoir.
So if a charged body is earthed, electrons flow until there is nothing left to push them, and the body ends up at a potential of 0 V. It has discharged.
Electrical appliances are kept safely at 0 V by an earthed conductor, usually a copper wire
Current always takes the path of lower resistance
Copper has a far lower resistance than a person, so any build-up of charge flows to Earth through the wire, not through you
Charging by induction
Electrostatic induction — definitionthe separation of charge caused by a nearby charged object, without any physical contact
Bring a charged object near a material and the electrons inside it shuffle — towards the surface, or away from it. The charges are redistributed. Nothing has been added or taken away, but one side of the material now has an excess of negative charge and the other an excess of positive.
This is electrostatic induction. It has nothing whatever to do with electromagnetic induction, which is about changing magnetic fields and comes much later. Same word, different universe. Do not let them collide in your head.
Why a comb picks up paper
Charge a comb by running it through your hair, then hold it over some torn-up paper. The paper is neutral — and yet it leaps up. Here is why:
The comb is negative. It repels the electrons in the paper away from the top surface
So the top of the paper is left positive, and the bottom is left negative
The positive top is attracted up to the comb; the negative bottom is repelled down
But the top is closer, so the attraction is the bigger force
The result is a net upward force, and the paper flies
Read that fourth bullet again, because it is the whole answer. Both forces exist. Neither is switched off. The attraction simply wins on distance, because the electric force gets weaker the further apart the charges are — which is the very thing Coulomb’s law will make precise on the next page.
Giving a sphere a permanent charge
Induction on its own only separates charge. Take the rod away and everything shuffles back. To make the charge stay, you have to let some electrons escape — or arrive — while the rod is still holding them in place. That means earthing it, and it means doing four things in a strict order.
The rod is positive, so it pulls electrons up from the Earth onto the sphere. The sphere is left negative — the opposite sign to the rod — and the rod’s own charge never changed.
Charged rod brought near
charges separate
Earth it — electrons flow
remove earth, then the rod
Opposite charge, trapped
Step 3 before step 4. Always. While the rod is still there it is holding those electrons on the near side of the sphere — break the earth connection now and they are stranded, with nowhere to go. Pull the rod away first and the electrons, no longer held, simply stroll back down the wire to Earth. You would end up with a perfectly neutral sphere and a very confused expression.
Charging by contact
The third route is the blunt one: let the two objects touch. Electrons flow across from one to the other, reducing the potential difference between them, and when you pull them apart the excess charge stays behind.
It happens most readily between a charged insulator and an earthed conductor, when:
there is a large potential difference between the two objects, and
the insulator prevents the charge leaking away into a neighbouring object first
You have felt this. Walk across a nylon carpet, reach for a metal door handle, and the charge you have built up jumps across as a small shock. That is charge transfer by contact.
Contact lets electrons flow off the sphere onto the positive rod, leaving the sphere positive. Induction pulls electrons up from Earth onto the sphere, leaving it negative. Same rod. Opposite outcomes.
Dangers of static electricity
Let a potential difference between two objects grow large enough and the electric field in the gap becomes ferocious. Air, normally a perfectly good insulator, gives up: its molecules are torn into ions, the gap turns conducting, and charge discharges through the air as a spark. This is the breakdown of air, and in dry air it takes a field of roughly 3 × 106 V m−1.
Two things can go badly wrong:
Electrocution — lightning is exactly this effect, on a spectacular scale
Ignition — a single spark near a flammable gas or liquid starts a fire or an explosion
Refuelling an aeroplane
Fuel rushing through a pipe rubs against the pipe walls. That is friction, and it charges the fuel and the tank. Now you have a large charged object sitting next to an aircraft full of vapour. One spark is all it takes.
The fix is a bonding line: a wire connecting the fuel tank to the Earth. Charge flows away to ground as fast as it builds up, the tank stays near 0 V, no potential difference accumulates, and no spark can form.
⚡ Answering a static electricity question
Name the method. Did they rub, hover, or touch? Friction, induction, contact.
Say which way the electrons went. Onto the object (→ negative) or off it (→ positive).
Never move protons. Positive means “lost electrons”. Write those two words.
Induction? Final sign is opposite the rod. Contact? Final sign is the same.
Earthing? Say the body discharges to 0 V, and that Earth supplies or absorbs the electrons.
Counting electrons?N = Q/e, converting nC and µC to coulombs first.
WE 1
Describe how a negatively charged rod can be used to give an insulated metal sphere a permanent positive charge, without the rod ever touching it. Explain what happens at each stage.
Step 1 — bring the rod near, but do not touch
The rod is negative, so it repels the electrons in the sphere.
They are pushed to the far side, leaving the near side positive.
Step 2 — earth the sphere
The repelled electrons now have somewhere to go.
They flow off the sphere, down to Earth.
Step 3 — remove the earth connection, rod still in place
The sphere is now short of electrons, and cannot get them back.
Step 4 — finally, remove the rod
The remaining positive charge spreads out evenly over the sphere.
the sphere is left permanently positiveThe sign is opposite to the rod, which is the signature of induction. And notice the rod’s own charge is unchanged throughout — it never touched anything, so it never gave or took a single electron.
WE 2
An insulated metal sphere is charged by induction using a rod of charge +6.4 nC. The sphere ends up with a charge of −4.8 nC. (a) State where the electrons on the sphere came from. (b) Calculate how many electrons were transferred. (c) State, with a reason, the charge on the rod at the end.
(a) Step 1 — where the electrons came from
The positive rod attracts electrons, and the sphere was earthed.
they flowed up from the Earth onto the sphere(b) Step 2 — convert, then use N = Q / eQ = 4.8 nC = 4.8 × 10⁻⁹ CN = (4.8 × 10⁻⁹) / (1.60 × 10⁻¹⁹)N = 3.0 × 10¹⁰ electrons(c) Step 3 — what happened to the rod?
The rod never touched the sphere, so no charge left or joined it.
the rod is still at +6.4 nCPart (c) is the trap. The rod’s charge is a red herring — it appears in the question purely to see whether you understand that induction transfers no charge to or from the rod. The +6.4 nC is never used in any calculation.
WE 3
(a) A negatively charged comb is held above small pieces of neutral paper, and the paper is attracted upwards. Explain this. (b) When an aeroplane is refuelled, a wire called a bonding line connects the fuel tank to the Earth. Explain why this is necessary.
(a) why neutral paper is attracted
The comb repels electrons away from the top of the paper.
The top becomes positive; the bottom becomes negative — charge is induced.
The positive top is attracted up; the negative bottom is repelled down.
The top is closer to the comb, so the attraction is larger.
a resultant upward force lifts the paper(b) why a bonding line is needed
Fuel rubbing through the pipe charges the tank by friction.
A large p.d. builds up, and the field can cause breakdown of the air.
The resulting spark could ignite the fuel vapour.
The bonding line lets the charge flow safely to Earth.
the tank stays at 0 V, so no spark can formIn (a) you must mention both forces and then say why one wins. “The paper is attracted because it is neutral” scores nothing. In (b), the marks are for friction → p.d. → spark → ignition, and then the earth path.
💡 Top tips
Induction → opposite sign. Contact → same sign. Learn it as a pair.
In induction, remove the earth before the rod. Reverse it and you get nothing.
The charging rod in an induction question never changes its charge. It never touched anything.
Always explain charging with the word electrons, and say whether they were gained or lost.
Earthing means the body ends up at 0 V. Say the number.
For a neutral object being attracted: say both forces act, and the nearer one is bigger.
“Breakdown of air” is the phrase for a spark. Pair it with “large p.d.” and “strong field”.
⚠ Common mistakes
Saying an object “gained positive charge”. It lost electrons. Protons never move
Confusing electrostatic induction with electromagnetic induction. Completely different topics
Removing the rod before the earth. The electrons just flow back and the sphere ends up neutral
Thinking the rod loses charge during induction. It never touches, so it cannot
Giving induction the same sign as the rod. Induction gives the opposite sign; only contact matches
Saying neutral paper is lifted because “only attraction acts”. Both forces act — attraction is just closer
Forgetting to convert nC to coulombs before dividing by e
Quick recap: Charge is transferred three ways. Friction rubs electrons from one insulator onto another, leaving them equal and opposite. Induction separates charge without contact; earth the object, remove the earth then the rod, and it keeps the opposite sign. Contact lets electrons flow directly, leaving the same sign as the rod. Earthing discharges a body to 0 V. And a big enough p.d. causes the breakdown of air and a spark — which is why fuel tanks get a bonding line. In every case, the only thing that ever moves is an electron.
We have leaned on one idea again and again without ever writing it down: that the electric force gets weaker with distance. It is why the near side of the paper wins, why the comb must be held close, and why the spark jumps a small gap and not a large one. It turns out this force obeys a law of beautiful precision — an inverse square law, the identical shape to Newton’s law of gravitation. Next page: Coulomb’s Law.
Induction and contact getting mixed up?
Book a free meeting and we’ll nail the four-step induction sequence, the sign rules and the “why is neutral paper attracted” answer.
