Static electricity

Static electricity is electric charge produced, usually by friction between two different materials, that remains on the surface of an object rather than flowing through a circuit. When a plastic scale is rubbed with dry wool, or a balloon is rubbed with a woollen cloth, tiny negatively charged particles are transferred from one surface to the other. The material that gains extra negative particles becomes negatively charged; the material that loses them is left with a shortage of negative charge and becomes positively charged. Friction never creates charge from nothing — it only moves existing charge from one object to the other, so the two rubbed objects always end up with equal and opposite charge. There are exactly two types of electric charge, positive and negative, and one rule governs how they behave toward each other:

• Like charges repel — positive repels positive, negative repels negative.
• Unlike charges attract — positive attracts negative.

This rule alone explains flyaway hair (every strand picks up the same type of charge from the comb, so the strands repel one another and spread apart) and why two balloons rubbed with the same cloth swing away from each other when hung side by side (both carry the same type of charge). But the comb-and-paper trick needs one more idea, because the paper starts out completely uncharged. A charged object can still attract an uncharged one through induction: the charged comb, held close to a paper bit, repels like charge to the far side of the paper and pulls opposite charge to the near side, so the near side of the paper is left with a charge opposite to the comb's. Attraction between the comb and this induced near-side charge is what lifts the paper. Induction needs no contact and no prior charge on the paper at all — only proximity to a charged object. Whether an object can hold onto charge produced by friction, or induced by proximity, depends on whether it is a conductor or an insulator. Conductors — mostly metals, and the human body to a useful degree — allow charge to move freely through them, so charge delivered to a conductor spreads out and typically drains away (often through your hand or the ground) rather than staying put. Insulators — rubber, dry glass, dry plastic, dry wood — do not let charge move through them easily, so charge produced by friction stays localised exactly where it was produced. This is precisely why static-electricity demonstrations always use insulators: a metal rod rubbed with wool will not show any static effect, because whatever charge friction manages to produce runs straight off through the person holding it.

where: $Q_{\text{before}}$ and $Q_{\text{after}}$ are the charges the two rubbed materials carry on the object that loses negative charge before and after rubbing — the point being that whatever negative charge one material loses, the other gains exactly that amount, so total charge is conserved and the two final charges are equal in size and opposite in sign.

Question: A plastic scale is rubbed vigorously with a dry woollen cloth and becomes negatively charged. The scale is then (a) brought near a fine stream of water from a tap, and (b) brought near small bits of paper. Explain what happens in each case, and predict what would happen if two such rubbed scales were brought close to each other.

Step 1. Rubbing the plastic scale with wool transfers negatively charged particles from the wool onto the scale, since plastic tends to gain electrons from wool during friction, leaving the scale negatively charged and the wool positively charged by an equal amount.

Step 2. Bringing the negatively charged scale near the thin water stream induces a redistribution of charge within the water molecules — the near side of the stream is pulled toward a slight positive charge and repelled from the negative — so the attraction between the scale and this induced near-side charge visibly bends the stream toward the scale.

Step 3. Bringing the same scale near small bits of paper causes the identical induction effect: the near side of each paper bit is left with an induced positive charge, and the attraction between this induced charge and the negatively charged scale is strong enough to lift the light paper bits up onto the scale.

Step 4. If a second scale is rubbed with the same woollen cloth, it also becomes negatively charged (the same type of charge as the first scale), so by the rule that like charges repel, the two scales will push each other apart when brought close, rather than attract.

The water stream bends toward the scale, the paper bits jump up and stick to it, and two such scales repel each other because they carry the same type of charge.

Competitive stretch

If instead one scale were rubbed with wool and a second, different object were rubbed with silk (which is known to charge some plastics positively rather than negatively), the two objects could end up with opposite charges and would attract rather than repel — a reminder that whether two charged objects attract or repel depends entirely on what charge each one actually carries, not simply on the fact that both were charged by rubbing.

• Saying friction "creates" charge — it only transfers existing charge from one material to the other; one object ends up with a surplus of negative charge, the other with a matching deficit.
• Thinking any two rubbed objects show a strong static effect — the effect is clearest and longest-lasting with insulators; conductors let charge leak away almost immediately, often through the person holding them.
• Confusing "uncharged" with "cannot be attracted" — an uncharged paper bit can still be pulled toward a charged comb because the comb induces an opposite charge on the paper's near surface.
• Assuming two charged objects must attract — two objects charged by rubbing with the same material typically carry the same type of charge and therefore repel, not attract.

Ask the tutor: Curious why your hair sometimes stands up after pulling off a woollen sweater — ask the tutor to walk through that and other static-electricity examples with you.