IB Physics SLTopic 4 — Electric & Magnetic FieldsPaper 1 & 2Only electrons move~8 min read
Static Charge
Rub a balloon on your hair and it clings to the wall; drag your feet on a carpet and the doorknob bites back. That’s static charge — electrons quietly hopping from one surface to another and getting stranded. Here’s how objects pick up charge, how we drain it away safely, and when a harmless crackle turns into a real hazard.
📘 What you need to know
Objects become charged when electrons move between them — by friction, induction, or contact
Only electrons move. An object goes negative by gaining electrons and positive by losing them — protons never move
Friction: rub two insulators; one gains electrons (−), the other loses them (+). The pair end up equal and opposite
Induction: a nearby charged object separates the charge in a conductor without touching it; earthing then leaves the conductor with the opposite sign
Contact: a charged object touches a conductor and shares charge, leaving it the same sign
Earthing connects an object to the ground so excess charge flows away and it sits at 0 V
A big enough build-up can spark across an air gap — dangerous near fuel; a bonding/earth wire drains it safely
Three Ways to Charge Something Up
All static charging comes down to moving electrons around. There are three routes, and the exam loves to test whether you can tell them apart:
Friction — rub two objects together so electrons rub off one onto the other
Induction — hold a charged object near a conductor (no touching) to push its electrons around
Contact — touch a charged object to a conductor so charge flows across
Through all three, keep one rule fixed in your head: protons stay put; only electrons move.
Charging by Friction
When two insulators are rubbed together, friction drags electrons off one surface and onto the other. The object that gains electrons becomes negative; the one that loses them is left positive. It’s not limited to solids either — even a liquid flowing through a pipe can charge up this way.
Rubbing hands electrons from one surface to the other. The one that loses electrons is left positive, the one that gains them negative — and the two charges are always equal and opposite.
rub two insulators
electrons transfer
one − (gained e⁻)
and
one + (lost e⁻)
Charging by Induction
Electrostatic induction is the separation of charge in a conductor caused by a nearby charged object — with no contact at all. (Don’t confuse it with electromagnetic induction, which is a completely different topic.) The charged object pushes or pulls the conductor’s free electrons, so one side ends up with an excess of one charge and the other side the opposite. Team induction up with earthing and you can leave a conductor permanently charged.
Charging a sphere by induction: a negative rod separates the charge, earthing lets the repelled electrons escape, and removing the earth then the rod leaves the sphere positive — the opposite sign to the rod, without ever touching it.
Charging by Contact
If a charged object actually touches a conductor, charge simply flows across until the two settle down. Because charge moves directly from one to the other, the conductor is left with the same sign as the charging object. Touch a negatively charged rod to a metal sphere and electrons spread onto the sphere, leaving it negative too. (The everyday “shock” off a doorknob is charge transferring by contact from your charged-up body to the earthed metal.)
charged rod touches conductor
electrons flow across
conductor ends the SAME sign
Earthing & Staying Safe
Earthing (grounding) means connecting an object to the Earth by a conductor. Any excess charge then drains away until the object sits at a potential of 0 V. It’s a safety must-have: appliances are wired to an earthed copper conductor, and because copper has a far lower resistance than a person, any stray charge runs to ground through the wire rather than through you.
Static becomes genuinely dangerous when the build-up gets large. A big enough potential difference makes the electric field strong enough to break down the air, and charge leaps across as a spark (a discharge). That can mean electrocution — think lightning — or, near flammable vapour, a spark that ignites a fire or explosion. Refuelling an aircraft is the classic risk: fuel rushing through pipes charges up by friction, so a bonding line to earth is attached to bleed that charge safely away before it can spark.
large charge build-up
huge p.d. → air breaks down
spark / discharge
bonding wire to earth
charge drained safely
🧭 Explaining a charging process
Name the method — friction (rub), induction (near, no touch), or contact (touch)
Talk electrons only — say which way they move and why (attracted or repelled)
State each object’s final sign — gained electrons → negative, lost electrons → positive
Check conservation — if one object goes negative, its partner (or the earth) takes the opposite
For induction, get the order right — earth it, remove the earth, then remove the rod
Quick recap: objects charge by friction, induction or contact, and only electrons ever move. Induction leaves the opposite sign to the charging object; contact leaves the same sign. Earthing drains charge to 0 V, and a bonding wire stops a dangerous spark near fuel.
WE 1
A polythene rod is rubbed with a wool duster and becomes negatively charged. (a) Explain, in terms of electrons, how the rod became negatively charged. (b) State and explain the charge left on the duster.
Part (a) — the rod
Rubbing transfers electrons from the duster onto the rod
The rod gains electrons, and electrons are negative, so
the rod becomes negativePart (b) — the duster
The duster lost those same electrons, so it is left
positiveCharge is conserved: the rod’s negative charge and the duster’s positive charge are equal and opposite.
WE 2
A metal sphere sits on an insulating stand. Describe how it can be given a positive charge using a negatively charged rod, without the rod ever touching the sphere.
Step 1 — bring the rod near
Hold the negative rod close to the sphere (no contact)
Step 2 — charge separates
Electrons in the sphere are repelled to the far side:
near side positive, far side negativeStep 3 — earth the sphere
Connect it to earth; the repelled electrons flow away to earthStep 4 — disconnect in the right order
Remove the earth first, then remove the rod
sphere left positive (opposite to the rod)Take the earth off after the rod and the electrons flow back — the sphere ends up neutral. Order matters.
💡 Top tips
Explain with electrons, always. Gained electrons → negative; lost electrons → positive. Never say an object “gained positive charge”
Induction vs contact: induction (no touch) leaves the opposite sign; contact (touch) leaves the same sign. Learn which is which
Order in induction: earth it, remove the earth, then remove the rod — the wrong order gives a neutral object
“Electrostatic” ≠ “electromagnetic” induction — same word, totally different topic; don’t mix them up
⚠ Common mistakes
Saying an object “gains positive charge” — it becomes positive by losing electrons; protons can’t move
Mixing up induction (near, no contact, opposite sign) with contact (touching, same sign)
Getting the induction order wrong — removing the rod before the earth leaves the sphere neutral
Confusing electrostatic induction with electromagnetic induction, or forgetting that friction charges insulators
You can now charge things up and drain them safely. Up next: Coulomb’s Law — putting a number on the force between two charges, and seeing how neatly it mirrors Newton’s law of gravitation (with one twist: this force can push as well as pull).
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