CIE IGCSE Physics (0625) Electric charge Study Notes - New Syllabus
CIE IGCSE Physics (0625) Electric charge Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Electric charge
Key Concepts:
- Electric charge
- Production of Electrostatic Charges
- Electrical Conductors and Insulators
- Charge and Electric Fields
- Electric Field Patterns and Directions
Electric Charge
Electric Charge:
There are two types of electric charge: positive and negative.
- Objects can become charged by gaining or losing electrons.
- Protons carry a positive charge \((+)\), and electrons carry a negative charge \((-)\).
Basic Charge Interactions:
- Like charges repel:
- Positive repels positive
- Negative repels negative
- Unlike charges attract:
- Positive attracts negative
Important Concept:
These forces between electric charges are called electrostatic forces. They act even without direct contact, through the electric field between the charges.
Example:
What happens when you bring a negatively charged balloon close to small bits of paper?
▶️ Answer/Explanation
The negatively charged balloon creates an electric field.
The electrons in the bits of paper are repelled, leaving the near side slightly positively charged.
This induced positive charge is attracted to the balloon’s negative charge.
So, the bits of paper are attracted and may jump toward the balloon.
The paper is attracted to the balloon due to opposite charges
Production of Electrostatic Charges by Friction
Production of Electrostatic Charges by Friction:
When two insulating materials are rubbed together, electrons are transferred from one material to the other.
- This causes one object to become negatively charged (gains electrons) and the other to become positively charged (loses electrons).
- Only electrons move; protons remain fixed in the nucleus.
Key Point: Charging by friction involves the transfer of negative charge (electrons) only.
Simple Experiment to Show Production of Charges by Friction:
- Take a plastic ruler and rub it with a dry cloth (e.g. wool or silk).
- Bring the ruler near small pieces of paper or your hair.
- You will see the paper pieces jump to the ruler or your hair get attracted.
Explanation: Rubbing causes electrons to transfer to or from the ruler, creating a charge imbalance. The charged ruler then attracts neutral objects via induced charge separation.
Simple Experiment to Detect Electrostatic Charge:
- Use a device called a gold-leaf electroscope or a metal leaf electroscope.
- Bring a charged object (like a rubbed balloon or comb) near the metal cap of the electroscope.
- The metal leaf diverges (spreads apart), indicating the presence of an electric charge.
Explanation: The charged object induces redistribution of electrons in the electroscope, causing like charges to repel within the leaves.
Example:
Explain why rubbing a balloon on your sweater allows it to stick to a wall.
▶️ Answer/Explanation
Rubbing the balloon transfers electrons from the sweater to the balloon (or vice versa), making it negatively charged.
When brought near the wall, the negative charge on the balloon repels electrons in the wall’s surface, leaving the surface slightly positively charged by induction.
The opposite charges attract, and the balloon sticks.
The balloon becomes negatively charged and sticks by attracting induced positive charges in the wall.
Electrical Conductors and Insulators
Experiment to Distinguish Between Electrical Conductors and Insulators:
- Set up a simple electric circuit using:
- A battery or cell
- An ammeter or bulb
- Two wires with crocodile clips at each end
- Insert the test material between the open ends of the circuit using the crocodile clips.
- Observe whether the ammeter shows a reading or the bulb lights up.
Interpretation:
If the bulb lights up or the ammeter shows current, the material is a conductor.
If not, the material is an insulator.
Electron Model Explanation:
- Conductors:
- Materials like metals (e.g. copper, aluminium) contain free electrons (also called delocalised electrons).
- These free electrons can move easily through the material, allowing electric current to pass.
- Insulators:
- Materials like plastic, rubber, wood, or glass have no free electrons.
- Electrons are tightly bound to atoms and cannot move freely.
- Hence, no current flows through them.
Example:
Why is copper used in electric wiring and plastic used as insulation around the wires?
▶️ Answer/Explanation
Reason for Copper: Copper is a metal and contains free-moving electrons that allow it to conduct electricity easily.
Reason for Plastic: Plastic is an insulator, meaning it doesn’t allow current to flow through. This protects users from electric shocks and keeps the current confined within the wire.
Copper is a conductor (free electrons); plastic is an insulator (no free electrons).
Charge and Electric Fields
Charge and Electric Fields:
Electric charge is a fundamental property of matter.
- It is measured in units called coulombs (C).
- There are two types of charge: positive and negative.
Electric Field:
- An electric field is the region around a charged object where it exerts a force on other charges.
- Any charge placed in this region will experience a force (attractive or repulsive depending on the type of charge).
Direction of Electric Field:
- The direction of the electric field at a point is defined as the direction of the force acting on a positive test charge placed at that point.
- This means:
- Electric field lines point away from positive charges
- Electric field lines point towards negative charges
Example
What is the direction of the electric field at a point near a positively charged object?
▶️ Answer/Explanation
The electric field is defined as the direction of the force on a positive test charge.
Since like charges repel, a positive charge will be pushed away from another positive charge.
The electric field points away from the positive object.
Example
Why do electric field lines always point toward a negative charge?
▶️ Answer/Explanation
Field lines show the direction of the force on a positive test charge.
Since opposite charges attract, a positive charge will be pulled toward a negative charge.
Because a positive charge would be attracted to the negative charge.
Electric Field Patterns and Directions
Electric Field Patterns and Directions:
- Electric field lines represent the direction a positive test charge would move if placed in the field.
- Lines start from positive charges and end on negative charges.
- The density of the field lines indicates the strength of the field (closer = stronger).
(a) Around a Point Charge:
- For a positive point charge, field lines radiate outward uniformly in all directions.
- For a negative point charge, field lines point inward toward the charge.
- This pattern is radially symmetric.
(b) Around a Charged Conducting Sphere:
- Field lines are similar to those around a point charge but originate from the surface of the sphere (not a single point).
- For a positively charged sphere, lines point outward from the surface.
- For a negatively charged sphere, lines point inward toward the surface.
- Field is stronger near the surface and weakens with distance.
(c) Between Two Oppositely Charged Parallel Conducting Plates:
- Field lines go straight from the positive plate to the negative plate, forming parallel and evenly spaced lines.
- This indicates a uniform electric field between the plates (ignoring end effects).
- The field strength is the same at all points between the plates.
Example
Which diagram best represents the electric field pattern between two oppositely charged parallel plates?
▶️ Answer/Explanation
The correct diagram will show:
- Straight lines from the positive plate to the negative plate
- Lines are equally spaced and parallel
Uniform parallel lines from + to – plate.
Example
What is the direction of electric field lines around a negatively charged sphere?
▶️ Answer/Explanation
Field lines always point in the direction that a positive charge would move.
Since opposite charges attract, a positive test charge would move toward the negative sphere.
Field lines point inward, toward the surface of the sphere.