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
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Current I = ΔQ/Δt
Current I = ΔQ/Δt — rate of flow of charge (coulombs per second).
19 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
19 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- PhETStart here · 1Java · best on a laptop9702 9.1 · IB B.5
Signal Circuit
Close a switch on a long loop of wire and watch every electron start moving at once while each one drifts slowly.
Why this one: Close the switch: every electron starts at once, yet each one creeps; drift velocity is tiny.
Try this
- Close the switch — the bulb lights immediately.
- Watch one electron — it creeps along even though the signal was instant.
- Open the switch — all the electrons stop together.
Look for The electric field is set up almost instantly; drift velocity is tiny, so current starts everywhere at once.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- PhETStart here · 2Java · best on a laptop9702 9.1 · 9.3 · IB B.5
Battery-Resistor Circuit
Vary the battery voltage and the resistance; watch electrons move through the resistor and the resistor heat up.
Why this one: Raise the voltage and the electron drift speeds up; collisions with the lattice heat the resistor.
Try this
- Raise the voltage — electrons move faster and the ammeter reads higher.
- Raise the resistance — the current falls and the atoms in the resistor jiggle more.
- Set the voltage to zero — electrons still move, but with no net drift.
Look for Current is the net drift of electrons; collisions with lattice atoms transfer energy and heat the resistor.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- SimuPhysicsStart here · 39702 9.1 · IB B.5
Current is the Same at Every Cross-Section
One wire with three thicknesses and a counter at each; squeeze the middle section and watch the carrier speed and the three counters
Why this one: Squeeze the wire and the carriers speed up so I = Anvq stays equal at every cross-section.
Try this
- Squeeze the middle section and watch the carrier speed.
- Compare the three counters.
- Widen the section and compare.
Look for Carriers move faster where the wire is thinner, so the current is the same at every cross-section.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- 3JCN PhysicsStart here · 49702 9.1 · IB B.5
Electron Charge via Electrolysis
Measure the electron charge from mass deposited in electrolysis
Why this one: Ions are the carriers here: the mass deposited is proportional to the charge passed.
Try this
- Run the electrolysis and read the mass deposited.
- Use the charge passed and the mass to compute the electron charge.
Look for The mass deposited is proportional to the charge passed, and each ion carries a whole number of electron charges.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
More simulations15 more on this topic — core ones first
- The Physics ClassroomCore9702 9.1 · 9702 9.2 · 9702 10.1
DC Circuit Builder
A virtual circuit board: add resistors, bulbs, wires and ammeters, use a voltmeter, and build series, parallel and combination circuits
Try this
- Build a series circuit and read the ammeter.
- Rebuild the same resistors in parallel and compare.
- Measure the voltage across each resistor with the voltmeter.
Look for Current is the same everywhere in series, and voltage is the same across parallel branches.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
- SimuPhysicsCore9702 9.1 · IB B.5
Charge Carriers in Different Materials
A metal, an electrolyte, a semiconductor and an ionised gas side by side, each with its own carriers moving inside
Try this
- Compare the carriers in the metal and the electrolyte.
- Compare the semiconductor and the ionised gas.
- Compare the direction of conventional current in all four.
Look for Conventional current points the same way in all four whatever carries the charge.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- SimuPhysicsCore9702 9.1 · 9702 11.2 · IB B.5
Charge is Quantised — Q = Ne
Dial in a charge and watch it divided by the elementary charge; a whole number shows that many electrons, a fraction shows what cannot exist
Try this
- Dial in a charge that gives a whole number.
- Dial in one that gives a fraction.
- Find the smallest charge that works.
Look for Any real charge is a whole-number multiple of e.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- SimuPhysicsCore9702 9.1 · IB B.5
Conventional and Electron Current Directions
Switch between the conventional picture and the electron picture and the four-cell table shows the direction inside and outside the source
Try this
- Switch to the electron picture and read the table.
- Switch to the conventional picture.
- Compare the direction inside the source with outside.
Look for Electron flow runs opposite to conventional current everywhere in the loop, including inside the source.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- SimuPhysicsCore9702 9.1 · IB B.5
Current, Charge and Time — the Tap and the Bucket
Two taps fill two buckets against two stopwatches; hold the time and double the charge, or hold the charge and double the time
Try this
- Hold the time fixed and double the charge.
- Hold the charge fixed and double the time.
- Compare the two currents.
Look for Current equals charge divided by time.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- SimuPhysicsCore9702 9.1 · IB B.5
Current Graphs — Slope and Area
Three graphs on draggable axes: the slope of Q–t, the slope of N–t and the area under I–t
Try this
- Drag the Q–t slope and read the current.
- Drag the N–t slope and compare.
- Read the area under the I–t curve.
Look for The slope of Q–t is the current and the area under I–t is the charge passed.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Tap a symbol — great for exam definitions
Tap a symbol — great for exam definitions
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)
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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
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- free electrons
In most metal wires, tiny particles called free electrons are the charge 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.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
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.
Lightning occurs when charge builds up in the atmosphere, creating a potential difference between the ground and the atmosphere.
During a lightning strike there is an average current of 3.3 × 10^4A for a time of 2.6 × 10^-5s.
Calculate the charge transferred during the lightning strike.
charge = ................................................................... C
The free electrons (charge carriers) in the wire have an average drift speed of 0.16 mm s⁻¹. Determine the number density of charge carriers in the metal.
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9702/22 · Q3(a) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
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