4.2.1 Electric charge
- Syllabus
- 0625–2026–2027
- Topic
- 4.2.1
- Level
- —
Learning objectives
4.2.1.1There are positive and negative• State: there are positive and negative charges4.2.1.2Positive charges repel other positive• State: positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges4.2.1.3Simple experiments to show the• Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges4.2.1.4Charging of solids by friction• Explain that charging of solids by friction involves only a transfer of negative charge (electrons)4.2.1.5An experiment to distinguish between• Describe an experiment to distinguish between electrical conductors and insulators4.2.1.6A simple electron model to explain the• Recall/use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples4.2.1.7Charge is measured in coulombs• State: charge is measured in coulombs4.2.1.8An electric field as a region in which• Describe an electric field as a region in which an electric charge experiences a force4.2.1.9Direction of an electric field at a• State: the direction of an electric field at a point is the direction of the force on a positive charge at that point4.2.1.10Simple electric field patterns• Describe simple electric field patterns, including the direction of the field: (a) around a point charge (b) around a charged conducting sphere (c) between two oppositely charged parallel conducting plates (end effects will not be examined)
Distinguish positive, negative and neutral charge
Electric charge has two possible signs: positive (+) and negative (-). An object may have a net positive charge, a net negative charge or no net charge (neutral).
| Net state | Meaning |
|---|---|
| positive | positive charge exceeds negative charge |
| negative | negative charge exceeds positive charge |
| neutral | positive and negative charge balance |
Always state the sign as well as saying that an object is charged. Positive and negative are two types of electric charge, not descriptions of whether an answer is good or bad.
Neutral does not mean that the material contains no charged particles; it means that its positive and negative charges balance overall. Electric current is treated separately in the next Topic.
Predict attraction and repulsion between charges
Like charges repel and unlike charges attract. The rule applies to both positive and negative charge.
| Charge on object 1 | Charge on object 2 | Force |
|---|---|---|
| positive | positive | repel |
| negative | negative | repel |
| positive | negative | attract |
| negative | positive | attract |
The force is mutual: if two free like-charged objects repel, each moves away from the other; if two free unlike-charged objects attract, each moves towards the other.
The sign does not determine whether a charge always attracts or always repels. Compare both signs: same signs repel, opposite signs attract.
Produce and detect electrostatic charge
To produce electrostatic charge, rub a dry insulating rod firmly with a dry cloth while holding it by an insulating handle. Friction leaves the rod and cloth with electrostatic charge.
| Purpose | Simple method | Evidence |
|---|---|---|
| show production | rub two identical insulating strips or rods with the same cloth, suspend one and bring the other close | repulsion shows that both have acquired the same sign of charge |
| detect charge | bring the test object near a freely suspended charged strip or light neutral pieces without touching | movement shows an electrostatic force; repulsion is decisive evidence that the test object is charged with the same sign |
Begin with discharged objects, keep the surfaces dry, avoid touching the rubbed area and use an insulating support. Repeat from the same starting distance so that a movement can be compared with an uncharged control.
Attraction alone detects an electrostatic effect but does not identify the sign, because a charged object can attract a neutral light object. Repulsion between known like-charged objects is the clearer charge test.
Explain charging by friction as electron transfer
Charging solids by friction transfers electrons from one material to the other. Electrons carry negative charge; the positive charges in the solid do not transfer between the objects.
| Electron change of an object | Resulting net charge |
|---|---|
| gains electrons | negative |
| loses electrons | positive |
| gains and loses equal numbers | neutral |
If electrons move from a cloth to a rod, the rod becomes negative and the cloth becomes positive. If they move from the rod to the cloth, the rod becomes positive and the cloth becomes negative. Charge is transferred, not created.
A positively charged solid has lost electrons; it has not gained protons. A negatively charged solid has gained electrons; it has not lost positive charge.
Test electrical conductors and insulators
Build a low-voltage test circuit with a cell, lamp and a gap between two leads. Insert the candidate material so that it is the only connection bridging the gap.
| Step | Action and interpretation |
|---|---|
| 1 | close the gap with a known conductor; the lamp should light, confirming that the circuit works |
| 2 | replace it with the test sample using the same contact spacing and area |
| 3 | lamp lights: the sample conducts; lamp remains off after contacts are checked: the sample is an insulator under these conditions |
| 4 | repeat and test a known insulator as a negative control |
Use the same cell, lamp, lead positions and sample dimensions where possible. Clean, firm contacts matter: an open contact can make a conductor look like an insulator.
A dim lamp can indicate weak conduction rather than a perfect insulator. Use only a safe low-voltage source; do not test unknown materials with mains electricity.
Explain conductors and insulators with an electron model
In an electrical conductor, some electrons are free to move through the material. In an insulator, electrons are bound to atoms and are not free to move through the material.
| Property | Conductor | Insulator |
|---|---|---|
| mobile electrons | present | absent or not free to travel through the material |
| response to an electric force | charge can move through the material | charge stays localised |
| typical examples | copper, aluminium, iron, silver, graphite | plastic, glass, dry wood, rubber |
A metal object can lose excess charge through a conducting path to Earth, so it is supported on an insulator when charge must be retained. Plastic can retain charge where it was rubbed because electrons cannot move freely through it.
Insulators contain electrons; their electrons are not free to move through the material. Conductor does not mean that every electron is free, and metal does not mean magnetic.
Use the coulomb as the unit of charge
Electric charge is measured in coulombs, symbol C. The quantity symbol for charge is Q.
| Charge statement | Meaning |
|---|---|
| Q = +2 C | two coulombs of positive charge |
| Q = -2 C | two coulombs of negative charge |
| Q = 0 C | no net charge |
The numerical sign and the unit answer different questions: + or - gives the type of net charge, while C identifies the measured quantity as electric charge.
Do not use ampere, volt, ohm or watt as the unit of charge. Those units belong to different electrical quantities in later Topics.
Define an electric field
An electric field is a region in which an electric charge experiences a force.
| Item | Role |
|---|---|
| source charge | produces the electric field in the surrounding region |
| test charge placed in the region | experiences an electric force |
| space where the field is negligible | produces no detectable electric force on the test charge |
The field explains how a charged object can exert force without direct contact. The field exists around the source charge even before a test charge is placed there.
An electric field is the region and its directional effect; it is not the source charge itself and it is not a flow of charge. A magnetic-field definition uses a magnetic pole instead.
Use the definition of electric-field direction
The direction of an electric field at a point is the direction of the force on a positive test charge placed at that point.
| Test charge | Direction of its electric force |
|---|---|
| positive | along the electric-field direction |
| negative | opposite to the electric-field direction |
At a point on a field line, read the arrow or tangent direction. A positive charge accelerates with that direction if electric force is the only unbalanced force; a negative charge accelerates oppositely.
Field direction is defined using a positive charge even if the actual particle in a question is negative. Do not use an N magnetic pole or the direction in which electrons move as the definition.
Describe standard electric-field patterns
Electric field lines show field direction. They start on positive charge and end on negative charge; around an isolated charge they extend radially through the surrounding space.
| Source arrangement | Pattern and direction |
|---|---|
| positive point charge | straight radial lines directed outwards |
| negative point charge | straight radial lines directed inwards |
| positively charged conducting sphere | radial lines perpendicular to the surface, directed outwards |
| negatively charged conducting sphere | radial lines perpendicular to the surface, directed inwards |
| oppositely charged parallel conducting plates | straight, parallel, equally spaced lines directed from the positive plate to the negative plate |
Draw several lines distributed symmetrically and place consistent arrows on them. For the parallel-plate pattern, ignore end effects as required by the syllabus, so the central field is represented as uniform.
Field lines do not form circular orbits around a charge and do not cross. A conducting sphere has the same external radial form as a point charge, but the lines begin or end at its surface rather than continuing inside it.