Electric charge

1. Overview

Electric charge is a fundamental property of matter that results in forces of attraction or repulsion. This topic explores how objects become charged through the movement of electrons and how these charges create "fields" that influence the space around them.

Key Definitions

• Electric Charge: A property of matter that causes it to experience a force when placed in an electromagnetic field. Measured in Coulombs ($C$).
• Electrostatics: The study of stationary electric charges.
• Conductor: A material (usually a metal) that allows electric charge to flow through it easily.
• Insulator: A material that does not allow electric charge to flow through it easily.
• Electric Field: A region in which an electric charge experiences a force.
• Electron: A subatomic particle with a negative charge that is responsible for the transfer of electricity in solids.

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Core Content

Types of Charge and Interactions

There are two types of electric charge: positive (+) and negative (-).

• Like charges repel: Two positive charges or two negative charges will push each other away.
• Opposite charges attract: A positive charge and a negative charge will pull toward each other.

Charging by Friction

When two insulating materials are rubbed together, electrons (negative charges) are transferred from one to the other.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.
• Crucial Note: Only electrons move. Protons are fixed in the nucleus and never move during charging by friction.

Common Experiment: Rubbing a polythene rod with a dry cloth. The rod gains electrons from the cloth and becomes negative. If you rub an acetate rod, it loses electrons to the cloth and becomes positive.

Detection of Charge

A Gold Leaf Electroscope can detect charge. When a charged object is brought near the metal cap, the gold leaf rises. This happens because the like charges in the leaf and the metal stem repel each other.

Conductors and Insulators

• Conductors: Contain "delocalized" electrons that are free to move through the structure (e.g., Copper, Iron, Graphite).
• Insulators: Electrons are bound tightly to atoms and cannot move freely (e.g., Plastic, Glass, Rubber).

Experiment to distinguish materials:

1. Set up a simple circuit with a battery and a bulb, but leave a gap between two crocodile clips.
2. Place the test material across the gap.
3. If the bulb lights up, the material is a conductor. If it stays dark, the material is an insulator.

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Extended Content (Extended Curriculum Only)

Charge Measurement

The SI unit for electric charge is the Coulomb (C).

Electric Fields

An electric field is a region where a charged particle experiences a force.

• Direction: The direction of an electric field at any point is the direction of the force on a positive charge placed at that point.
• Therefore, field lines always point away from positive and toward negative.

Electric Field Patterns

1. Point Charge: Radial lines. Pointing outward for (+), inward for (-).
2. Charged Conducting Sphere: Lines point radially away from the surface of a positive sphere.
3. Parallel Plates: A uniform field exists between two oppositely charged parallel plates. The lines are parallel, equally spaced, and point from the positive plate to the negative plate.

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Key Equations

• $Q = I \times t$
  • $Q$ = Charge (Coulombs, $C$)
  • $I$ = Current (Amperes, $A$)
  • $t$ = Time (seconds, $s$)

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that positive charges (protons) move when an object is charged.
  • ✓ Right: Only negative electrons move. A "positive charge" is simply a lack of electrons.
• ❌ Wrong: Thinking a charged rod can only attract another charged object.
  • ✓ Right: A charged object can attract a neutral object (like small scraps of paper) through induction.
• ❌ Wrong: Suggesting that conventional current is the flow of electrons.
  • ✓ Right: Conventional current flows from Positive to Negative, but electrons actually flow from Negative to Positive.
• ❌ Wrong: Drawing electric field lines pointing toward a positive charge.
  • ✓ Right: Field lines represent the force on a positive test charge, so they must point away from positive charges.

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Exam Tips

1. Static Electricity keywords: When describing how something becomes charged, always use the words "friction", "transfer", and "electrons". You will lose marks if you just say "charge moves."
2. Field Line Accuracy: When drawing field patterns, ensure your lines are perpendicular (at 90°) to the surface of the object they are leaving or entering.
3. Conductor vs Insulator: If asked why a metal is a good conductor, always mention "free electrons" or "delocalized electrons".