Electric Charge

Conductors — materials that contain free-moving charge carriers (usually electrons). Current flows easily.

• Metals: copper (used for wires), silver (best but expensive), aluminium (overhead power cables), gold (corrosion-resistant contacts in electronics).
• Carbon (graphite) — surprising conductor for a non-metal.

Insulators — materials with no free electrons. Current does NOT flow easily.

• Plastics: PVC, polythene, polystyrene.
• Other: glass, ceramic, dry wood, rubber, pure water (tap water is a poor conductor due to dissolved ions).

Why this matters for electrostatics. Insulators CAN be charged by friction because the transferred electrons can't escape (no free path). Conductors held in the hand CANNOT — any electrons transferred immediately leak through your body to the ground.

To charge a metal rod, hold it via an INSULATING HANDLE so the charge can't drain to earth.

Required practical (spec 2.23P). Rub a polythene rod with a woollen cloth (or an acetate rod with a silk cloth). Bring the rod near small pieces of paper or a suspended cork ball.

Observation. The polythene rod attracts the paper pieces (or repels a similarly charged object).

Why it happens. Friction between the two insulating materials transfers ELECTRONS from one to the other:

• Polythene + wool: electrons go FROM wool TO polythene. Polythene becomes NEGATIVE; wool becomes POSITIVE.
• Acetate + silk: electrons go FROM acetate TO silk. Acetate becomes POSITIVE; silk becomes NEGATIVE.

Which material gives up electrons depends on the so-called triboelectric series — not on the spec, but useful to know that the direction is determined by the materials, not the rubbing.

Spec 2.24P phrasing. Edexcel mark schemes use the EXACT phrasing "electrons are transferred from one material to the other". Avoid 'positive charges move' — protons are locked in the nucleus and do not migrate.

Conservation of charge (spec 2.26P). No new charge is created. The TOTAL charge of (rod + cloth) is the same after rubbing as before — zero. Equal amounts of positive charge appear on one and negative on the other.

Explaining electrostatic phenomena in general (spec 2.26P). Whenever Edexcel asks 'explain electrostatically...' your answer should:

1. Identify which object STARTS with electrons.
2. Describe the ELECTRON TRANSFER (direction, why).
3. Identify which object ends up POSITIVE / NEGATIVE.
4. Apply the like/unlike rule (spec 2.25P) for any force that follows.

The rule. Two charged objects:

• LIKE charges (both + or both −) → REPEL each other.
• UNLIKE charges (one +, one −) → ATTRACT each other.

Detecting charge sign. Use a known reference (a polythene rod = negatively charged after rubbing with wool; an acetate rod = positively charged after rubbing with silk).

If a mystery object is REPELLED by the polythene rod, the mystery object is NEGATIVE (like charges).

If it is ATTRACTED to the polythene rod, the mystery object is either POSITIVE or UNCHARGED (an uncharged object is attracted to any charged object due to induced polarisation). To distinguish: also test with the acetate rod. A POSITIVE object is repelled by acetate; an UNCHARGED object is still attracted.

This 'attract or repel?' logic is a favourite Edexcel multi-mark question.

Two named examples in the spec — fuel tankers and fuelling aircraft.

Scenario 1 — fuel tanker. As petrol flows through the delivery pipe, friction between the fuel and the pipe transfers electrons → the tanker truck becomes electrically charged. Without an escape path, the charge ACCUMULATES, and the VOLTAGE between the tanker and any nearby earthed object grows. Above a critical voltage, a SPARK jumps the gap. That spark can IGNITE petrol/air vapour → explosion.

Mitigation. A bonding wire is attached between the truck and an earthed point on the service station BEFORE unloading starts. The wire provides a low-resistance path that lets the charge leak harmlessly to earth, keeping the voltage low.

Scenario 2 — fuelling an aircraft. Same problem, larger scale. Aircraft are bonded to the refuelling truck (and the truck to the ground) so all three are at the same potential — no spark can form.

The chain of reasoning (Edexcel mark scheme).

1. Fuel flowing through pipe → friction → electrons transferred → object charged.
2. Charge accumulates → voltage rises.
3. Spark when air breakdown voltage is exceeded.
4. Spark provides activation energy → flammable fuel/air vapour ignites.
5. Bonding wire / earthing prevents accumulation → no spark → safe.

State each link explicitly. Vague answers about safety lose marks.

Photocopier. Uses electrostatic charge to transfer text/images from an original to paper.

1. A rotating DRUM inside the photocopier is given a uniform POSITIVE charge.
2. Bright light reflects off the WHITE areas of the original document onto the drum. Wherever LIGHT hits, the charge LEAKS AWAY.
3. The drum is left with a positively charged 'shadow image' of the document's DARK areas (text).
4. Negatively-charged TONER POWDER is dusted on the drum. It sticks where the drum is still positively charged (UNLIKE charges attract).
5. Paper pressed against the drum picks up the toner.
6. A HEATER fuses the toner to the paper permanently.

Inkjet printer. Uses electrostatic deflection to place tiny ink droplets precisely on the page.

1. Ink is forced through a fine nozzle producing a stream of tiny droplets.
2. Each droplet is CHARGED (positive or negative).
3. The droplets pass through an ELECTRIC FIELD between two parallel deflection plates. The field bends each droplet's path by an amount that depends on its charge and the plate voltage.
4. The varying deflection positions each droplet on the paper to build up the printed image.

Both examples are favourite 4-mark Edexcel descriptive questions.