CIE iGCSE Co-ordinated Sciences-P4.2.2 Electric current- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P4.2.2 Electric current – Study Notes
CIE iGCSE Co-ordinated Sciences-P4.2.2 Electric current – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Know that electric current is related to the flow of charge
- Know that electric current in metals is related to the flow of electrons
- Describe the use of ammeters (analogue and digital) with different ranges
- Know the difference between direct current (d.c.) and alternating current (a.c.)
Supplement
- Define electric current as the charge passing a point per unit time; recall and use the equation I=Q/t
- Describe electrical conduction in metals in terms of the movement of delocalised (mobile) electrons
- State that conventional current is from positive to negative and that the flow of electrons is from negative to positive
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Electric Current and Flow of Charge
1. Electric Current as Flow of Charge:
Electric current is the rate of flow of electric charge in a circuit.
The formula is:
$\mathrm{I = \frac{Q}{t}} $
where:
- \(\mathrm{I}\) = current (amperes, A)
- \(\mathrm{Q}\) = charge (coulombs, C)
- \(\mathrm{t}\) = time (seconds, s)
This means that 1 ampere = flow of 1 coulomb of charge per second.
Current in Metals:
- In metals, electric current is carried by electrons.
- Electrons are negatively charged and free to move through the metallic lattice.
- When a voltage is applied across a metal, electrons drift from the negative terminal to the positive terminal.
- This is called the electron flow.
Conventional Current vs Electron Flow:
- Conventional current: By convention, current is considered to flow from the positive terminal to the negative terminal of a battery.
- Electron flow: In reality, electrons move in the opposite direction — from negative to positive.
- Both descriptions are used, but conventional current is the standard used in circuit diagrams.
Everyday Examples:
- A 2 A current in a torch bulb means that 2 coulombs of charge flow through the bulb every second.
- In copper wires, this charge is carried by moving electrons.
Example :
A current of 3 A flows through a wire for 20 seconds. How much charge passes through the wire?
▶️ Answer/Explanation
Step 1: Use formula \(\mathrm{Q = I \times t}\).
Step 2: Substitute: \(\mathrm{Q = 3 \times 20 = 60 \, C}\).
Final Answer: 60 coulombs of charge flow through the wire.
Use of Ammeters
An ammeter is an instrument used to measure the electric current flowing in a circuit.
General Features:
- An ammeter must always be connected in series with the component, so that the same current flows through both.
- It has a very low resistance, so it does not significantly reduce or alter the current being measured.
- Units: current is measured in amperes (A).
Analogue Ammeters:
- Contain a moving pointer (needle) that deflects across a scale to show current value.
- Available with different ranges (e.g. 0–1 A, 0–5 A, 0–10 A).
- When using, the chosen range should be slightly larger than the expected current to avoid damaging the meter.
- Analogue ammeters show continuous movement, useful for observing small variations or fluctuations in current.
Digital Ammeters:
- Display the current as a number on a digital screen.
- Often automatically switch between different ranges, or allow manual selection.
- Provide a more precise and easier-to-read measurement than analogue ammeters.
- Common ranges may include milliamps (mA) for small currents and up to tens of amperes for larger circuits.
Choosing the Correct Range:
- If the range is set too low, the ammeter may be overloaded or damaged.
- If the range is set too high, the reading may be less accurate (small currents are harder to detect).
- Good practice: always start with the highest range, then switch down to the lowest suitable range for accuracy.
Everyday Uses:
- Measuring current through a lamp in a simple circuit (usually < 1 A).
- Checking battery current draw in appliances.
- Industrial circuits may require ammeters with higher ranges.
Example :
A student wants to measure the current through a lamp. The expected current is about 0.8 A. Which ammeter range should be chosen?
▶️ Answer/Explanation
Step 1: The expected current = 0.8 A.
Step 2: Always choose a range slightly above the expected current to avoid overload.
Step 3: Possible ranges: 0–1 A (too close), 0–5 A (safe and suitable).
Final Answer: The student should select the 0–5 A range.
Direct Current (d.c.) and Alternating Current (a.c.)
Direct Current (d.c.):
- Current flows in one direction only.
- Produced by sources such as batteries, solar cells, and d.c. power supplies.
- Voltage is constant over time.
- Commonly used in electronic devices (phones, laptops, torches).
Alternating Current (a.c.):
- Current reverses direction periodically.
- Produced by a.c. generators and supplied through the mains electricity grid.
- Voltage varies with time in the form of a sine wave.
- In most countries, the frequency is 50 Hz (current changes direction 100 times per second).
- Used for household electricity and long-distance transmission because it can be transformed easily to higher or lower voltages.
Comparison Table
| Feature | Direct Current (d.c.) | Alternating Current (a.c.) |
|---|---|---|
| Direction of Current | One direction only | Reverses direction periodically |
| Source | Batteries, solar cells | Mains electricity, generators |
| Voltage | Constant | Varies as sine wave |
| Frequency | 0 Hz (no reversal) | 50 Hz (in most countries) |
| Uses | Electronics, batteries, vehicles | Household power, transmission |
Example :
A phone is powered by a battery, but it charges from the mains supply. Explain the types of current involved.
▶️ Answer/Explanation
Step 1: The battery provides current in one fixed direction → d.c.
Step 2: The mains supply provides current that reverses direction 50 times per second → a.c.
Step 3: A charger circuit converts the mains a.c. into d.c. to charge the phone battery.
Final Answer: The phone uses d.c. from the battery, but the mains provides a.c.
Electric Current
Electric current is defined as the rate of flow of charge. It is the amount of electric charge passing a point in a circuit per unit time.
Equation:
$\mathrm{I = \frac{Q}{t}}$
- \(\mathrm{I}\) = current (amperes, A)
- \(\mathrm{Q}\) = charge (coulombs, C)
- \(\mathrm{t}\) = time (seconds, s)
Explanation:
- 1 ampere (1 A) means 1 coulomb of charge passes a point in 1 second.
- In metals, this charge is carried by moving electrons.
- Current can be direct (d.c.) or alternating (a.c.), but in both cases it describes the movement of charge per unit time.
Everyday Understanding:
- A current of 2 A means that 2 coulombs of charge pass through a point in the circuit every second.
- This is why higher current often means brighter bulbs or stronger heating effects in resistors.
Example :
A current of 5 A flows through a wire for 3 minutes. How much charge passes a point in the wire during this time?
▶️ Answer/Explanation
Step 1: Use formula \(\mathrm{Q = I \times t}\).
Step 2: Convert time: \(\mathrm{3 \, min = 180 \, s}\).
Step 3: Calculate: \(\mathrm{Q = 5 \times 180 = 900 \, C}\).
Final Answer: The total charge is 900 coulombs.
Electrical Conduction in Metals
Metals conduct electricity because of the presence of delocalised (mobile) electrons that can move freely through their structure.
Structure of Metals:
- A metal is made up of a lattice of positive metal ions arranged in a regular pattern.
- These ions are held together by a “sea” of delocalised electrons that are free to move throughout the lattice.
- The delocalised electrons come from the outer shells of metal atoms, which have been released from their parent atoms.
How Conduction Happens:
- When a potential difference (voltage) is applied across a metal, the delocalised electrons experience a force.
- They drift through the lattice towards the positive terminal of the supply.
- This movement of electrons constitutes the electric current in the metal.
- The metal ions themselves remain fixed in place and do not move with the current.
Direction of Flow:
- Electron flow: Actual movement of electrons is from the negative terminal to the positive terminal.
- Conventional current: By convention, current is considered to flow in the opposite direction — from positive to negative.
Why Metals are Good Conductors:
- They have a high density of delocalised electrons available for conduction.
- This is why copper, aluminium, silver, and gold are widely used in electrical wiring.
Example :
Why is copper used for household wiring instead of plastic?
▶️ Answer/Explanation
Step 1: Copper contains many delocalised electrons, which can move freely under an electric field.
Step 2: This makes copper a very good conductor with low resistance.
Step 3: Plastic has no free electrons → it is an insulator and cannot conduct.
Final Answer: Copper is used because of its excellent conductivity, while plastic is used as an insulator covering for safety.
Conventional Current and Electron Flow
Conventional Current:
By definition, conventional current is the flow of charge from the positive terminal of a power supply, through the circuit, and back to the negative terminal.
- This convention was established historically before electrons were discovered.
- It is still used in circuit diagrams and calculations for consistency.
Electron Flow:
- In reality, in metallic conductors, current is carried by electrons.
- Electrons are negatively charged, so they are repelled by the negative terminal and attracted to the positive terminal.
- This means the actual flow of electrons is from the negative terminal to the positive terminal.
Key Point:
- Conventional current: Positive → Negative.
- Electron flow: Negative → Positive.
Example :
In a torch, the battery is connected to a bulb. In which direction does (a) conventional current, and (b) electron flow occur?
▶️ Answer/Explanation
Step 1: Conventional current is always taken as flowing from the battery’s positive terminal to the negative terminal.
Step 2: Electrons, being negative, flow from the negative terminal to the positive terminal.
Final Answer: (a) Conventional current: Positive → Negative. (b) Electron flow: Negative → Positive.