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CIE IGCSE Physics (0625) Current Study Notes

CIE IGCSE Physics (0625) Current Study Notes - New Syllabus

CIE IGCSE Physics (0625) Current Study Notes

LEARNING OBJECTIVE

  • Understanding the concepts of Current  

Key Concepts: 

  • Electric Current and Flow of Charge4
  • Use of Ammeters (Analogue and Digital) with Different Ranges
  • Conventional Current vs Electron Flow

CIE iGCSE Physics (0625)  Study Notes – All topics

Electric Current and Flow of Charge

Electric Current and Flow of Charge:

Electric current is the rate at which electric charge flows through a conductor.It is caused by the movement of electrons in a wire or conductor.

  • By convention, electric current is defined as the flow of positive charge, even though electrons (which are negative) move in the opposite direction.

Definition of Electric Current:

Electric current is defined as the amount of electric charge passing a point per unit time.

\( I = \dfrac{Q}{t} \)

  • \( I \) = current in amperes (A)
  • \( Q \) = charge in coulombs (C)
  • \( t \) = time in seconds (s)

So, a current of 1 A means that 1 coulomb of charge passes through a point every second.

Example:

A charge of \( 12\,\text{C} \) flows through a wire in \( 4\,\text{s} \). What is the current in the wire?

▶️ Answer/Explanation

We are given:

  • \( Q = 12\,\text{C} \)
  • \( t = 4\,\text{s} \)

Use the formula: \( I = \dfrac{Q}{t} \)

\( I = \dfrac{12}{4} = 3\,\text{A} \)

Final Answer: \(\boxed{3\,\text{A}}\)

Use of Ammeters (Analogue and Digital) with Different Ranges

Use of Ammeters (Analogue and Digital) with Different Ranges:

An ammeter is an instrument used to measure electric current in a circuit.

  • It must always be connected in series with the component through which the current is to be measured.
  • Ammeters are designed to have very low resistance, so they do not significantly affect the current in the circuit.

Types of Ammeters:

Analogue Ammeter:

    • Uses a pointer (needle) to show current on a scale.
    • May have multiple ranges like 0–1 A, 0–5 A, etc.
    • Less precise for small current changes compared to digital.

Digital Ammeter:

    • Displays current as a number on a screen.
    • Often auto-ranges or allows manual range selection.
    • More accurate and easier to read.

Use of Ranges:

  • Choosing the correct range is important to avoid overloading the meter.
  • Start with the highest range and move to lower ranges if the reading is small.
  • This avoids damaging the meter or getting an “off-scale” reading.

Electrical Conduction in Metals (in terms of free electrons):

  • Metals contain free (delocalised) electrons that are not bound to any one atom.
  • These electrons can move freely throughout the metal lattice.
  • When a potential difference (voltage) is applied across the metal, these free electrons drift in a direction, creating an electric current.
  • The positive metal ions stay fixed in place while the electrons move.

This explains why metals are good electrical conductors:

  • They have many free electrons that respond quickly to electric fields.

Example

A digital ammeter is used in a circuit with a maximum current of 0.8 A. Which range should be selected on the ammeter?

▶️ Answer/Explanation

The current is 0.8 A. To avoid going off scale or damaging the meter, we should choose a range slightly above this value.

Answer: \(\boxed{\text{0–1 A range}}\)

Example

Explain why copper is a better conductor of electricity than rubber.

▶️ Answer/Explanation

Copper is a metal with many free electrons that can move easily and carry electric charge.

Rubber is an insulator; it has tightly bound electrons and no free electrons to move.

Copper conducts due to free electrons; rubber does not

Conventional Current vs Electron Flow

Conventional Current vs Electron Flow:

Conventional Current is defined as the flow of positive charge from the positive terminal to the negative terminal of a power supply.

  • This was established before electrons were discovered, and the convention is still used in diagrams and theory today.
  • Electron Flow is the actual physical flow of charge in metallic conductors, where free electrons move from the negative terminal to the positive terminal.

Direct Current (d.c.) and Alternating Current (a.c.):

Direct Current (d.c.):

    • Current flows in only one direction.
    • Produced by cells, batteries, and d.c. power supplies.
    • Examples: torch battery, mobile phone charger (output side).

Alternating Current (a.c.):

    • Current continuously reverses direction.
    • Typically changes direction in a regular pattern (e.g., sinusoidal waveform).
    • Used in mains electricity (e.g., 50 Hz in many countries).
    • Examples: household sockets, electric ovens.

Example

In a simple electric circuit with a battery, in which direction does conventional current flow?

▶️ Answer/Explanation

Conventional current flows from the positive terminal of the battery, through the external circuit, to the negative terminal.

Positive to Negative Terminal

Example 

Which type of current is used in home wall sockets, and what is its key feature?

▶️ Answer/Explanation

Home wall sockets supply alternating current (a.c.), which changes direction periodically (usually 50 or 60 times per second).

Alternating Current (a.c.), reverses direction

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