Electric Charge and Electric Fields
PhysicsΒ· 4.2.1Β· 18 min read
1. 1. Core: Nature of Electric Chargeβ β ββββ± 4 min
Electric Charge
A fundamental property of matter that causes electrostatic force interactions. Neutral objects have equal numbers of positive protons and negative electrons; charged objects have an imbalance of electrons.
Example:
A plastic comb rubbed on hair gains excess electrons, giving it a net negative charge.
The two non-negotiable charge interaction rules are: 1) Like charges repel each other, 2) Opposite charges attract each other. Charged objects also attract neutral objects via temporary induced charge separation.
A student rubs an acetate rod with a duster, and the rod becomes positively charged. Explain the charge of the duster.
- 1
- Charging by friction only transfers electrons, not fixed protons.
- 2
- The acetate rod loses electrons to the duster, giving it a net positive charge.
- 3
- The duster gains the electrons lost by the rod, so it has a net negative charge.
2. 2. Core: Methods of Chargingβ β ββββ± 5 min
Friction: Rubbing two insulators transfers electrons between them, leaving one positive and one negative.
Contact: A charged conductor touches a neutral conductor, transferring electrons to give the neutral object the same charge as the original.
Induction: A charged object is held near a neutral conductor to separate charge, followed by earthing to remove repelled charge, leaving the conductor with opposite charge to the original.
Outline the steps to charge a neutral metal sphere negatively using induction, with no direct contact between the sphere and charged object.
- 1
- Hold a positively charged rod near the neutral sphere: free electrons in the sphere are attracted to the side closest to the rod, leaving the far side positive.
- 2
- Briefly earth the far side of the sphere: electrons flow from the ground to neutralise the positive charge on the far side.
- 3
- Remove the earth connection first, then remove the charged rod. The excess electrons spread evenly across the sphere, giving it a net negative charge.
Exam tip:
Always state you remove the earth connection before removing the charged object in induction questions: this is a mandatory marking point.
3. 3. Extended Only: Electric Fieldsβ β β ββExtended onlyβ± 5 min
Electric Field
A region around a charged object where any other charged object experiences a non-contact electrostatic force. Field lines show the direction a positive test charge would move if placed in the field.
Example:
A negative point charge has electric field lines pointing radially inwards, towards the charge.
Field line rules: Lines run from positive to negative charge, never cross, are closer together where the field is stronger, and meet charged surfaces at 90Β°.
Point charge patterns: Positive point charges have radial lines pointing outwards; negative point charges have radial lines pointing inwards.
Parallel plate pattern: Uniform electric field between two oppositely charged parallel plates, with straight, evenly spaced lines perpendicular to the plates.
Sketch the electric field pattern around a single negative point charge, and label the region of strongest field.
- 1
- Draw a small circle to represent the negative point charge, labelled with a minus sign.
- 2
- Draw 8-10 straight lines pointing radially inwards towards the circle, evenly spaced around the charge.
- 3
- Label the region closest to the point charge as the strongest field: field lines are closest together here.
4. 4. Core + Extended: Hazards and Uses of Static Chargeβ β ββββ± 4 min
Static charge has common real-world applications and hazards that are regularly tested in structured exam questions. Core candidates need to recall basic examples, while Extended candidates may be asked to explain their operation in detail.
Uses: Electrostatic precipitators (remove factory dust), inkjet printers, photocopiers.
Hazards: Spark risk when refuelling vehicles, static shocks from synthetic fabrics, damage to electronic components.
Explain why employees working with flammable chemicals often wear conductive shoes instead of insulating rubber shoes.
- 1
- Friction between clothing and work surfaces can build up static charge on the employeeβs body.
- 2
- A spark from this charge could ignite flammable chemical vapours, causing a fire or explosion.
- 3
- Conductive shoes earth the employee continuously, allowing excess charge to flow safely to the ground before it can build up to sparking level.
5. Common Pitfalls
Wrong move:
Stating protons move during charging of solid objects
Why:
Protons are fixed in atomic nuclei, only free electrons can move in solid materials
Correct move:
Always refer to electron transfer when explaining charging in solids
Wrong move:
Drawing electric field lines from negative to positive charge
Why:
Field lines show the direction a positive test charge moves, so they run from positive to negative
Correct move:
Add arrows pointing away from positive charges and towards negative charges on all field diagrams
Wrong move:
Removing the charged rod before the earth connection in induction charging
Why:
Separated charges will recombine if the rod is removed first, leaving the object neutral
Correct move:
Always remove the earth first, then the charged rod, for induction charging
Wrong move:
Claiming charged objects only attract other charged objects
Why:
Charged objects induce temporary charge separation in neutral objects, leading to attraction
Correct move:
Remember charged objects attract both oppositely charged objects and neutral objects
Wrong move:
Drawing unevenly spaced field lines between parallel plates
Why:
The electric field between oppositely charged parallel plates is uniform
Correct move:
Draw straight, parallel, equally spaced lines perpendicular to the plates for parallel plate fields
6. Quick Reference Cheatsheet
Concept | Core Requirement | Extended Requirement |
|---|---|---|
Charge Rules | Opposites attract, like repel; charged attracts neutral | Same as core + recall force is non-contact |
Charging Methods | Describe friction, contact, induction | Explain induction with electron movement and earthing steps |
Electric Fields | Not assessed | Draw field patterns for point charges and parallel plates, recall field line rules |
Static Hazards | Recall earthing prevents sparks | Explain how static causes hazards and mitigation methods |
7. Frequently Asked
Can a charged object attract a neutral object?
Yes. The charged object induces temporary charge separation in the neutral object, leading to a net attractive force between the two.
What particles move when an object is charged?
Only free electrons move in solid materials: protons are fixed in atomic nuclei and cannot be transferred during charging.
Do I need to learn electric field rules for Core tier?
No, electric field patterns and rules are only tested in Extended tier (Paper 2 and Paper 4) exams.
Going deeper
What's Next
Now that you have mastered electric charge and fields, you are ready to move on to current electricity, the next core topic in CIE IGCSE Physics 0625 Unit 4. This foundation will help you understand how charge flows in circuits, the relationship between current, voltage and resistance, and the operation of basic circuit components. For Extended candidates, your knowledge of electric fields will support later learning about how current flows in conductors. Make sure to practice drawing field patterns and explaining induction processes, as these are commonly tested in structured questions for both Paper 3 (Core) and Paper 4 (Extended).