CIE iGCSE Co-ordinated Sciences-P4.2.1 Electrical charge- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P4.2.1 Electrical charge – Study Notes
CIE iGCSE Co-ordinated Sciences-P4.2.1 Electrical charge – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- State that there are positive and negative charges
- State that positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges
- Describe electrostatic charging by friction, and simple methods to determine if an object is charged
- Know that charging of solids by friction involves only a transfer of negative charge (electrons)
- Distinguish between electrical conductors and insulators and give typical examples
Supplement
- State that charge is measured in coulombs
- Describe an electric field as a region in which an electric charge experiences a force
- State that the direction of an electric field at a point is the direction of the force on a positive charge at that point
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Electric Charge
Existence of Charge:
- There are two types of electric charge: positive (+) and negative (–).
- Charges are a fundamental property of matter , for example, protons carry positive charge, and electrons carry negative charge.
Forces Between Charges:
- Like charges repel: – Positive repels positive. – Negative repels negative.
- Unlike charges attract: – Positive attracts negative.
- The strength of the force depends on: – The size of the charges. – The distance between them (closer charges feel stronger force).
Everyday Evidence of Electric Charge:
- When a plastic ruler is rubbed with a cloth, it becomes negatively charged by gaining electrons.
- The ruler can then attract small pieces of paper → evidence of attraction between unlike charges.
- Two balloons rubbed on hair both become negatively charged → they repel each other.
Example :
Two identical plastic rods are rubbed with the same cloth. When brought close together, they repel. Explain why.
▶️ Answer/Explanation
Step 1: Rubbing transfers electrons to both rods, giving them a negative charge.
Step 2: Since both rods carry the same type of charge (negative), they repel each other.
Final Answer: The rods repel because like charges repel.
Electrostatic Charging by Friction
Charging by Friction:
- When two different insulating materials are rubbed together (e.g. plastic rod and dry cloth), electrons are transferred from one material to the other.
- This leaves one object with an excess of electrons (negatively charged) and the other with a shortage of electrons (positively charged).
- The objects become charged because friction provides the energy needed for electrons to move.
Key Point – Only Electrons Move:
- Charging by friction always involves the transfer of negative charges (electrons).
- Protons are bound inside the nuclei of atoms and do not move between objects.
- So: – If an object gains electrons → it becomes negatively charged. – If an object loses electrons → it becomes positively charged.
Detecting if an Object is Charged:
- Electroscope: A simple device where a thin metal leaf or foil diverges when charged, because like charges repel.
- Attraction of small objects: A charged object can attract small pieces of paper, dust, or hair.
- Suspended test object: A light, charged object (like a pith ball) will be attracted or repelled by another charged object.
Everyday Examples:
- A plastic ruler rubbed with a dry cloth can pick up tiny paper pieces.
- Balloons rubbed on hair stick to walls due to charge transfer.
- Clothes in a dryer cling together because of friction charging.
Example :
A polythene rod is rubbed with a dry cloth and then attracts small pieces of paper. Explain how it became charged and why it attracts the paper.
▶️ Answer/Explanation
Step 1: Rubbing transfers electrons from the cloth to the polythene rod → the rod becomes negatively charged.
Step 2: The cloth, having lost electrons, becomes positively charged.
Step 3: When the charged rod is brought near the neutral paper, electrons in the paper are repelled, leaving the surface positively charged.
Final Answer: The rod attracts the paper pieces because of induced charges on the paper.
Electrical Conductors and Insulators
Electrical Conductors:
- Conductors are materials that allow electric charges (electrons) to move freely through them.
- This is because they contain a large number of free (delocalised) electrons that can drift under an electric field.
- Conductors usually have low resistance to current flow.
- Examples: Metals such as copper, aluminium, silver, gold, and graphite (a non-metal conductor).
- Applications: Used in wires, cables, and circuits to carry current safely and efficiently.
Electrical Insulators:
- Insulators are materials that do not allow electrons to move freely.
- They have very few or no free electrons → charges are tightly bound to atoms.
- They have high resistance to current flow.
- Examples: Rubber, plastic, glass, wood, ceramics, dry air.
- Applications: Used to cover and protect wires, prevent electric shocks, and support electrical components.
Key Differences:
- Conductors let current flow easily; insulators block current flow.
- Conductors are used where charge movement is required (wiring); insulators are used for safety and protection.
Everyday Applications:
- A copper wire carries current, while its plastic coating insulates and protects users from electric shock.
- Overhead power lines use aluminium conductors for electricity and ceramic insulators to hold them safely.
- Rubber gloves are used by electricians as insulators when working with electrical systems.
Example :
Why is copper used in household wiring while the wires are coated with plastic?
▶️ Answer/Explanation
Step 1: Copper is a good conductor → allows current to flow with low resistance.
Step 2: Plastic is a good insulator → prevents accidental shocks and short circuits.
Final Answer: Copper wires carry the current efficiently, while the plastic covering ensures safety.
Charge and Its Unit
Electric charge is a fundamental property of matter that causes it to experience a force in an electric or magnetic field.
- The SI unit of charge is the coulomb (C).
- One coulomb is defined as the amount of charge carried by a current of 1 ampere flowing for 1 second.
Mathematically:
$ \mathrm{Q = I \times t} $
where:
- \(\mathrm{Q}\) = charge (C)
- \(\mathrm{I}\) = current (A)
- \(\mathrm{t}\) = time (s)
Key Notes:
- A single electron carries a very small negative charge of \(-1.6 \times 10^{-19} \, \mathrm{C}\).
- The coulomb is a large unit, so everyday electrical charges often involve very large numbers of electrons.
Example :
A current of 2 A flows through a circuit for 30 s. How much charge passes through the circuit?
▶️ Answer/Explanation
Step 1: Use formula \(\mathrm{Q = I \times t}\).
Step 2: Substitute: \(\mathrm{Q = 2 \times 30 = 60 \, C}\).
Final Answer: The total charge that passes is 60 C.
Electric Field
An electric field is a region of space where an electric charge experiences a force.
Key Features of Electric Fields:
- A positive test charge placed in an electric field will feel a force.
- The direction of the field is defined as the direction of the force on a positive charge.
- The strength of the field depends on:
- The size of the charge producing the field (bigger charge → stronger field).
- The distance from the charge (closer → stronger field).
- Fields can be produced by single charges, multiple charges, or between charged plates.
Electric Field Lines:
- Field lines represent the pattern of the electric field.
- They are drawn starting at positive charges and ending at negative charges.
- The spacing of field lines shows field strength: – Close together = strong field, – Far apart = weak field.
- Field lines never cross each other.
Examples of Electric Fields:
- Single positive charge: Field lines radiate outwards in all directions.
- Single negative charge: Field lines point inwards towards the charge.
- Between parallel charged plates: Field lines are straight, parallel, and evenly spaced → indicating a uniform electric field.
Example :
A small positive test charge is placed near a negatively charged sphere. Describe the direction of the force on the test charge.
▶️ Answer/Explanation
Step 1: By definition, field direction is the force on a positive test charge.
Step 2: Opposite charges attract → the test charge is pulled towards the negative sphere.
Final Answer: The force (and hence the field direction) is towards the negative sphere.
Direction of an Electric Field
The direction of an electric field at any point is defined as the direction of the force that would act on a positive test charge placed at that point.
Explanation:
- By convention, electric field lines always show the path a positive charge would follow.
- Therefore:
- Field lines point away from positive charges (since they repel).
- Field lines point towards negative charges (since they attract).
- This convention ensures that the field direction is always consistent, even though negative charges (like electrons) would actually move in the opposite direction.
Examples:
- Near a single positive charge, the field radiates outward.
- Near a single negative charge, the field radiates inward.
- Between a positive and negative charge, field lines run from the positive charge to the negative charge.
Example :
A positive and a negative charge are placed near each other. In which direction will the electric field lines point?
▶️ Answer/Explanation
Step 1: Field direction = direction of force on a positive charge.
Step 2: A positive charge would be repelled by the + charge and attracted towards the – charge.
Final Answer: The field lines point from the positive charge towards the negative charge.