Electric Charge and Electric Fields

Electric charge is a property of matter carried by electrons and protons; transferring electrons is what makes a rubbed balloon stick to a wall and why fuel tankers carry earthing straps. Cambridge tests it because it underpins the whole circuits topic: current is moving charge. Questions reward exact electron-transfer language, so this page fixes the wording first.

What actually happens when an object is charged?

Only electrons move. Rubbing a polythene rod with a cloth transfers electrons from the cloth to the rod. The rod gains electrons and becomes negative; the cloth loses electrons and becomes positive. Protons never move during charging. Charge is measured in coulombs (C).

Quantity	Symbol	Unit
Charge	$Q$	coulomb (C)
Current	$I$	ampere (A)
Time	$t$	second (s)

Charge links to current through one equation. In words: charge equals current multiplied by time. In symbols: $Q = It$. One coulomb is the charge passing when one ampere flows for one second.

Two charge rules mirror magnetism. Like charges repel; unlike charges attract. An electric field is the region where an electric charge experiences a force. Field lines show the direction of the force on a positive charge: away from positive charges, towards negative charges. For 0625 you must draw the field around a point charge, around a charged conducting sphere, and between two oppositely charged parallel plates (straight, parallel, evenly spaced lines from + to −).

How do conductors and insulators differ?

Conductors, such as copper, contain electrons that are free to move, so charge flows through them. Insulators, such as polythene, have no free electrons, so charge stays where friction puts it. That is why you can charge a plastic rod by rubbing but not a metal rod held in your hand, because the charge leaks through you to earth. The simple test: connect the material into a circuit with a cell and lamp; the lamp lights only for a conductor.

Worked Exam Question

A plastic rod is rubbed with a dry cloth and becomes positively charged. (a) Explain, in terms of particles, how the rod becomes positive. (2 marks) (b) Charge of 0.60 C flows from a charged dome to earth in 0.30 s. Calculate the average current. (2 marks)

Solution. (a) Friction transfers electrons from the rod to the cloth. The rod loses electrons, so it carries a net positive charge. (b) Equation: $Q = It$, so $I = \dfrac{Q}{t}$. Substitute: $I = \dfrac{0.60}{0.30}$. Answer: $I = 2.0\ \text{A}$.

Mark scheme:

• B1: electrons transfer from rod to cloth (not protons, not “charge moves”).
• B1: loss of (negative) electrons leaves the rod positive.
• M1: $I = \dfrac{Q}{t}$ selected and values substituted correctly.
• A1: 2.0 A with unit.

Common Mistakes

• Saying protons or positive charges move. Fix: in solids only electrons transfer. Mark schemes reject anything else.
• “The rod gains positive charge.” Fix: say the rod loses electrons. Cambridge wants the electron-transfer mechanism.
• Field arrows pointing the wrong way. Fix: arrows show the force on a positive test charge, out of +, into −.
• $Q = It$ unit slips. Fix: time must be in seconds. A question giving 5 minutes means $t = 300\ \text{s}$.
• Confusing electric and magnetic fields. Fix: electric fields act on charges; magnetic fields act on poles and moving charges. Name the right one.

Exam Technique Tip

For any “explain how it becomes charged” question, write three short sentences in a fixed order: what moves (electrons), which direction (from X to Y), and the resulting charge on each object. That template matches the mark scheme one-to-one and takes under a minute. The same structure earns the physics marks inside 6-mark extended responses on electrostatics hazards.

How This Is Examined

This is a CS subtopic. Core candidates (Papers 1 and 3) cover charging by friction, conductors and insulators, and attraction/repulsion. Extended candidates (Papers 2 and 4) add electric field definitions and the three required field patterns, and meet $Q = It$ in calculations. Paper 4 often pairs part (a) explanation marks with a part (b) calculation, exactly as in the worked question above. Alternative-to-practical papers can describe a gold-leaf electroscope or charged-strip demonstration. In Malaysia’s humid climate, electrostatics demonstrations fail more often than the textbook suggests, a detail examiners have used in “suggest why” questions about damp air conducting charge away.