Static Electricity

When two insulators are rubbed together, electrons (which carry a small negative charge) move from the surface of one material to the other.

The material that LOSES electrons is left positive. The material that GAINS electrons becomes negative.

Which way electrons go depends on the material pair (the 'triboelectric series'):

• Rubbing a polythene rod with a duster → rod becomes negative (gains electrons).
• Rubbing an acetate rod with a duster → rod becomes positive (loses electrons).

Why only insulators? In a conductor (e.g. metal), free electrons immediately spread out and neutralise the charge — no static buildup. Insulators let the charge sit where it was deposited.

Worked example. A balloon is rubbed on a wool jumper. The balloon picks up extra electrons from the wool → balloon negative, jumper positive. The balloon then sticks to a wall (the balloon's negative charge induces an opposite positive charge on the wall's surface — attraction).

Like charges repel; unlike charges attract. This is shown by hanging two charged rods on threads — they swing apart (same charge) or together (opposite charge).

A charged object will also attract a neutral object by inducing opposite charges on its surface — this is why balloons stick to walls, and small bits of paper jump to a comb.

Sparks. If a large static charge builds up, the resulting potential difference between the charged object and a nearby earthed object can be huge (thousands of volts). When the air gap is small enough, the p.d. is enough to ionise the air molecules: this creates a brief, visible (and sometimes audible) spark.

Real-world examples:

• You touch a metal door handle after walking on a synthetic carpet — small spark, sometimes a 'click'.
• Lightning is a giant spark between cloud and ground.
• Fuel tanker drivers must earth their vehicles before refuelling — a static spark could ignite fuel vapour.

An electric field is a region of space where a charged object experiences a force. We represent it with field lines:

• Lines POINT in the direction a small positive test charge would move.
• For a positive point charge, lines radiate OUTWARDS.
• For a negative point charge, lines point INWARDS.
• Lines are perpendicular to the surface where they meet a charged object.

Field strength rules:

• Stronger field where lines are closer together.
• Field strength is higher near a small charge (close distance).
• Field strength is higher for a larger charge.

Between two parallel charged plates the field is uniform in the central region: parallel evenly-spaced lines from the positive plate to the negative plate. The field weakens (lines spread) near the edges.

This uniform field is exploited in:

• Inkjet printers (deflect charged ink droplets).
• Cathode-ray tubes (old TVs and oscilloscopes).
• Particle accelerators (start of beam acceleration).

A positive test charge placed in a field experiences a force in the SAME direction as the field arrow. A negative test charge experiences a force in the OPPOSITE direction.

This lets you predict:

• Two positive charges: each is in the other's outward-pointing field → they push each other away.
• A positive and a negative charge: the + is pulled into the −'s field; the − is pushed out of the +'s field — so they come together.
• A small neutral object near a charged rod: the rod's field induces an opposite-sign surface charge → attraction (this is why a balloon sticks to a wall).

Field strength factors:

• LARGER charge magnitude → stronger field everywhere around it.
• CLOSER to the charge → stronger field (force grows as you approach).