In simple terms
A friendly intro before the formal notes — no formulas yet.
Electric current
Cambridge 9702 Paper 2 — Electric current (9.1). Senpai Corner diagram-backed pilot with premium structure and live visuals.
- 1
I is the electric current in Amperes (A).
- 2
ΔQ is the change in charge (amount of charge that flows) in Coulombs (C).
- 3
Δt is the time interval in seconds (s).
- 4
1 Ampere is equivalent to 1 Coulomb per second (1 A = 1 C s⁻¹).
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 9.1.1
Understand that an electric current is a flow of charge carriers
- 9.1.2
Understand that the charge on charge carriers is quantised
- 9.1.3
Recall and use
- 9.1.4
Use, for a current-carrying conductor, the expression $I = Anvq$, where n is the number density of charge carriers
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
What is Electric Current?
Electric current is defined as the rate of flow of electric charge. It measures how much charge passes a specific point in a circuit per unit time. Without current, our devices wouldn't work! It's a fundamental quantity in all electrical systems.
I is the electric current in Amperes (A).
ΔQ is the change in charge (amount of charge that flows) in Coulombs (C).
Δt is the time interval in seconds (s).
1 Ampere is equivalent to 1 Coulomb per second (1 A = 1 C s⁻¹).
Quantization and Conservation of Charge
Two fundamental principles govern electric charge. First, charge is quantized, meaning it exists in discrete packets. The smallest unit of free charge is the elementary charge, 'e' (1.60 × 10⁻¹⁹ C). Any amount of charge in a system is an integer multiple of 'e'. Second, charge is conserved. In any closed system or circuit, the total amount of electric charge remains constant. It cannot be created or destroyed, only moved around. This is the basis for Kirchhoff's First Law, which states that the total current entering a junction must equal the total current leaving it.
Charge Carriers and Drift Velocity
In most metal wires, tiny particles called free electrons are the charge carriers. They move randomly, but when a voltage is applied, they gain a net directional movement. This average speed is called the mean drift velocity. Different materials have different numbers of these carriers.
I = Anvq
A = cross-sectional area of the conductor (m²).
n = number density of charge carriers (number of carriers per unit volume, m⁻³).
v = mean drift velocity of the charge carriers (m s⁻¹).
q = charge of a single carrier (e.g., elementary charge 'e' = 1.60 × 10⁻¹⁹ C).
Higher A, n, or v lead to a larger current.
Conventional Current vs. Electron Flow
Historically, before the discovery of electrons, current was imagined as the flow of positive charge. This is called conventional current and moves from the positive terminal to the negative. However, in metals, it's actually negatively charged electrons that move, flowing from negative to positive. Remember, these are opposite directions!
Always remember that conventional current is defined as the direction positive charges would flow, even though in most metallic conductors, it's the negatively charged electrons that are actually moving in the opposite direction. Be careful with this distinction in exam questions!
Charge Carriers in Different Media
While free electrons are the charge carriers in metallic conductors, other materials have different carriers:
- Electrolytes: In liquids like salt solutions or molten salts, current is carried by the movement of positive and negative ions.
- Semiconductors: In materials like silicon, current is carried by both electrons (negative carriers) and holes (which behave as positive charge carriers).
- Gases: Under certain conditions (e.g., high voltage), gases can be ionised, and current is carried by ions and electrons.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A wire has a cross-sectional area of 2.0 × 10⁻⁶ m² and carries a current of 4.0 A. If the number density of free electrons is 8.5 × 10²⁸ m⁻³, calculate the mean drift velocity of the electrons. (Elementary charge e = 1.60 × 10⁻¹⁹ C)
- 1
Identify the given values:
A current of 250 mA flows through a resistor for 4.0 minutes. Calculate (a) the total charge that passes through the resistor, and (b) the number of electrons that pass through the resistor in this time. (Elementary charge e = 1.60 × 10⁻¹⁹ C).
- 1
First, convert all units to SI units.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is the SI unit for electric charge?
Coulomb (C)
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
I is the electric current in Amperes (A).
- ✓
ΔQ is the change in charge (amount of charge that flows) in Coulombs (C).
- ✓
Δt is the time interval in seconds (s).
- ✓
1 Ampere is equivalent to 1 Coulomb per second (1 A = 1 C s⁻¹).
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9702/23 · Q4(b)(i)
Determine the cross-sectional area of wire X.
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Checkpoint
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Reading it isn’t knowing it — prove it.
Before you move on: do 9702/23 · Q4(b)(i) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.