Static electricity · Model

Forces between charges

Like charges push apart, unlike charges pull together — and a charged object also pulls on something with no charge at all, which is the case people forget.

Start here

Two balloons, one jumper, two opposite results.

Rub two balloons on the same jumper and hang them side by side on threads: they swing away from each other and refuse to touch. Take either one and hold it near the wall and it sticks there.

How can the same balloon push one thing away and pull another thing in?

• ABoth balloons got the same charge, so they repel; the wall has no charge, and a charged object attracts an uncharged one
• BThe wall is oppositely charged because the room itself is positively charged overall, so the balloon is pulled to it
• CThe balloon that sticks has lost its charge, so it is held to the wall by ordinary friction and nothing electrical
• DBalloons repel other balloons and attract everything else, because that is simply how rubber behaves once rubbed

Two charged objects push or pull on each other without touching, and the rule is short: like charges repel, unlike charges attract. Two positives push apart. Two negatives push apart. A positive and a negative pull together. The two forces are always equal in size and opposite in direction — whichever object is smaller or more lightly charged, it feels exactly the same pull as the other one.

The force gets rapidly weaker as they move apart. Doubling the separation does not halve it: it cuts it to about a quarter. That is why static effects are dramatic at a centimetre and undetectable across a room.

Then the case that surprises people. A charged object also attracts a completely uncharged one. Bring a negative rod near a scrap of paper and the rod's charge pushes the paper's own electrons to the far side, leaving the near side slightly positive. The paper is still neutral overall — nothing has been added or removed — but its near side is now oppositely charged and closer, so the pull wins over the push. This is induction, and it is the whole reason a rubbed rod picks things up.

At the bench · two light spheres on insulating stands

Charge them. Move them.

Change a control to begin

Each sphere can be left neutral or given a positive or a negative charge of the same size. The arrows show the force on each one and how strong it is.

Commit first. One sphere is charged negative. The other is left completely neutral. What happens?

• ANothing — a neutral object has no charge, so there is nothing to push or pull on
• BThey repel, because the neutral one has as many negatives as the charged one
• CThey attract weakly, because the charged one pushes the neutral one's electrons aside
• DThey attract strongly, exactly as two opposite charges would

The figure

Every combination there is

Two objects, three states each, nine cases. Only one of the nine produces nothing at all — which is worth noticing, because the usual summary of this topic mentions two.

Left / right	Positive	Neutral	Negative
Positive	Repel — like charges	Attract weakly — induction	Attract — unlike charges
Neutral	Attract weakly — induction	Nothing	Attract weakly — induction
Negative	Attract — unlike charges	Attract weakly — induction	Repel — like charges

This gives you a test worth remembering. Attraction proves nothing — an object that is pulled towards a charged rod might be oppositely charged, or might have no charge at all. Only repulsion is proof, because nothing but a like charge can push.

!

Think again

“The rod picks up the paper, so the paper must be charged.”

The paper came out of a drawer and nobody rubbed it. It is neutral, and it stays neutral the whole time — its total charge never changes. What the rod does is push the paper's own electrons to the far side of it, so the near face becomes slightly opposite to the rod and the far face slightly the same. Both faces feel a force, but the near one is closer, and the force weakens so fast with distance that the near face wins. The paper is pulled in as a whole. Attraction is never proof of charge; only repulsion is.

“They have to be touching, or there has to be air in between to carry it.”

Neither. The spheres never touch, and the force is undiminished in a vacuum — two charged objects in an evacuated jar push and pull on each other exactly as they did in air. There is nothing in the gap doing the carrying. That is such a strange claim that physics gives it its own name and its own lesson: the charged object fills the space around it with an electric field, and the field is what the other object responds to.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Predict

A negatively charged rod is held near a hanging metal-coated ball on a thread. The ball swings towards the rod. What can you conclude about the ball?

• AIt must be positively charged, because unlike charges attract
• BNothing certain — it could be positively charged, or it could be neutral
• CIt must be neutral, because a charged ball would have been pushed away
• DIt must be negatively charged, because the rod is negative

Rung 2 · The one that catches people

Two identical spheres carrying the same charge repel with a certain force at 5 cm. They are moved to 10 cm apart. What happens to the force?

• AIt falls to about half, because the distance doubled
• BIt stays the same, because neither charge has changed
• CIt falls to about a quarter on the nearer sphere and stays the same on the further one
• DIt falls to about a quarter of what it was

Rung 3 · Explain

A balloon rubbed on a jumper sticks to a wall that nobody has charged. Explain how a charged object can be attracted to an uncharged one.

Your explanationComplete

Rung 4 · Take it somewhere new

You are given two identical hanging balls, one of which you know is charged, and no other apparatus. Describe how you would find out whether the second ball is charged too, and explain why one possible result would tell you nothing.

Your answerComplete

Key note

Charged objects push and pull on each other without touching. Like charges repel, unlike charges attract, and the two forces are always equal in size and opposite in direction. The force weakens quickly as the separation grows — doubling the distance cuts it to about a quarter. A charged object also attracts a neutral one, because it moves that object's own electrons to one side and the closer, opposite face wins. So repulsion is proof that an object is charged, and attraction is not.

Going further

The electric force is staggeringly strong compared with gravity. Between two protons it beats their gravitational attraction by a factor of about ten thousand million million million million million million. You never notice, because matter is so precisely balanced: every object you have ever picked up has its positive and negative charges matched to an extraordinary accuracy, and the tiny imbalance a duster can produce is enough to lift paper against the pull of the whole Earth.

Induction is quietly useful. Electrostatic paint spraying gives the paint droplets a charge and the car body the opposite one, so the paint is pulled onto the metal and wraps round edges instead of drifting past; a power-station precipitator charges the smoke particles and collects them on plates, taking out most of the ash before it reaches the chimney. Both work on the case in the middle of the table — charge one thing, and something uncharged comes to you.

The bench is a teaching model. Both charges are treated as equal in size and as sitting at the centre of each sphere, and the strength is reported as a relative figure with the closest fully charged case set to 100 — no force in newtons is given anywhere on the bench, because the equation for it is beyond this stage and any number in newtons here would be invented rather than measured. Induced attraction on a neutral sphere is reported in relative words only, never as a figure, because the size of that effect is a chosen coefficient rather than a measurement. The relative figure falls as the square of the separation for two charged spheres, which is the real relationship; the much weaker attraction to a neutral sphere is modelled as falling faster still, which is the right behaviour but the coefficient is chosen to be readable rather than measured. Real spheres are not points, so at the closest separations the true force is somewhat larger than the model gives. The induced charges drawn on a neutral sphere are indicative, and the sphere's total charge stays zero throughout.

Lesson content © MrBadmusAI.